ANATOMY & PHYSIOLOGY SERIES · the pulmonary circulation. Then the oxygenated blood is returned to...

1 Lecture 4 – Cardiovascular and Lymphatic Systems

ANATOMY & PHYSIOLOGY SERIES

LECTURE 4 – A&P

THE CARDIOVASCULAR & LYMPHATIC SYSTEMS

Lecture 4 – Cardiovascular and Lymphatic Systems 2


CONTENTS

INTRODUCTION

THE CARDIOVASCULAR SYSTEM

INTRODUCTION TO THE HEART AND CIRCULATORY ROUTES

Structure of the Heart, Cardiac Cycle Conduction System of the Heart, Blood Pressure

Blood Vessels and Systemic Circulation, Structure of Veins and Arteries,

Specialised Circulation, Foetal Circulation Summary

INTRODUCTION TO THE BLOOD

Composition of Blood, Red Blood cells, Erythopoiesis, White Blood cells, Platelets

Blood Groups, Conclusion, Insight

PATHOPHYSIOLOGY OF THE CARDIOVASCULAR SYSTEM

INTRODUCTION TO THE LYMPHATIC SYSTEM

The Lymphatic Vessels, Function of Lymph Vessels, Lymph and Interstitial fluid, Lymphatic pump mechanisms,

Structure of Lymph Nodes, Location and Functions of Lymph nodes and Lymph Tissue

The Lymphatic Drainage of the Breast, Lymph Lacteals in the Digestive Villi, The Tonsils,

The Thymus, The Spleen, Conclusion

PATHOPHYSIOLOGY OF THE LYMPHATIC SYSTEM


INTRODUCTION

& READ CHAPTERS 18, 19, 23, 24 IN THIBODEAU & PATTON

Today we are going to be taking a look at the Cardiovascular System having previously explored the Respiratory System. Each of these systems works very closely with the other to maintain the body’s internal balance called homeostasis. After looking in detail at the cardiovascular system, we shall draw the two systems together and see how intrinsically linked they are, working in unison to maintain homeostasis and ultimately life.

The cardiovascular system encompasses three main components. First we have the heart. Secondly we have the vast vascular network formed by the blood vessels. Arteries carry high pressure blood that is pumped away from the heart to supply the capillary beds that deliver the blood to close proximity to the body cells. The veins then carry the low pressure blood that leaves the capillaries back to the heart. The blood vessels fully supply every area of the body with all the necessary nutrients, water and oxygen, while simultaneously removing the metabolic waste products and any excess material. The heart and the blood vessels really only form the transport system, for the third anatomical element of the system is the transporter proper, it is in fact the universal transporter. It is described as a ‘liquid tissue’ and is equally responsible for delivering all the materials required for life to the tissues and cells, and collecting all the materials detrimental to life from the tissues and cells. The universal transporter is, of course, the blood.

The heart has two halves, left and right, and is a double pump. The left side pumps blood into the arteries that supply all the bodily regions and organs apart from the lungs – the blood then returns to the right side of the heart. We call this the systemic circulation. The right side pumps blood to the lungs that is then reoxygenated and returned to the left side of the heart. This loop is called the pulmonary circulation.

The cardiovascular system and blood are also involved in the transportation of water and nutrients, but for the purpose of these lectures we shall concentrate on the structure and function of the two systems and their relationships with the exchange of gases.

THE CARDIOVASCULAR SYSTEM

To start with, please turn to your text book and browse for a moment over a pictorial diagram of the flow of blood around the body. You will notice that in your text books blue is often used to relate to blood that is deficient in oxygen (unsaturated), as it has given up some of its oxygen to the tissues and cells and collected carbon dioxide, the main gaseous waste product of cellular respiration. In life, unsaturated or deoxygenated blood is a darker red than the bright red colour of saturated or oxygenated blood. Note that the terms saturated/unsaturated and oxygenated/deoxygenated are shorthand – at rest the blood leaving the left side of the heart going to the body is around


97% saturated while the blood returning to the right side of the heart from the body is unlikely to be less than 70% saturated with oxygen – it is only during strenuous exercise that the blood may drop below 25% saturation.

So we find blue coloured vessels taking blood back to the right side of the heart – these are the systemic veins – and then blue coloured vessels leaving the right side of the heart to go to the lungs – the pulmonary arteries. Then red coloured pulmonary veins take the oxygenated blood leaving the lungs to the left side of the heart which then pumps this out into the systemic arteries – also coloured red. The capillary beds between the arteries and veins are often coloured purple to represent the transition in blood gas levels that occurs here.

This colour coding may sound confusing to those taught the mantra at school: veins blue, arteries red – but this only applies to the systemic circulation – the reverse applies in the pulmonary circulation. I have laboured the point above because of this commonplace misleading idea.

By classical definition ‘an artery always carries blood away from the heart’ and ‘a vein always carries blood towards the heart’. As with many of these descriptive classifications there are exceptions to the rules, and we will look at those ‘rule breakers’ later on. But for now, let us take these ‘rules’ as guide lines as we concentrate now on the whole of the cardiovascular system.

Why do systemic veins in the skin look blue when the blood is actually dark red – it is because they and the skin above them contain fibrous tissue that scatters blue light – the blue tinge is made visible by the dark blood providing a background.

As we explore more of this system you will find that there are seemingly endless veins and arteries, in ever decreasing size. These are always described by their location or where they are heading to, so do not panic, it’s quite simple to work out with the aid of a good book.

INTRODUCTION TO THE HEART AND CIRCULATORY ROUTES

Let’s consider the pulmonary and systemic circulations in a little more depth. The heart as we have said is considered to be a double pump, in the sense that it is solely responsible for pumping the blood around the systemic or general circulatory system and the pulmonary circulatory system (where ‘pulmonary’ appertains to lungs).

The systemic circulatory system supplies oxygenated blood to the whole body, whereas the pulmonary circulatory system is responsible for circulating deoxygenated blood around the lungs so that it can be reoxygenated. The deoxygenated blood then can return to the heart where it rejoins the systemic circulatory system. The systemic circulatory system is much larger than the pulmonary circulatory system and so the heart has one pump larger than the other, as we will now see.

Blood from the systemic circulatory system that has been around the entire body, delivering oxygen and collecting the waste products from cellular respiration, (ie carbon dioxide) is eventually returned to the heart by the


superior vena cava (‘hollow vein above’ the heart) and the inferior vena cava (‘hollow vein below’ the heart). Two things are now needed. Firstly, the blood must be able to release these poisonous gases and secondly, it must replenish its oxygen so that the cycle may continue. This blood is then poured into the smallest upper chamber found on the right of the heart called the right atrium. The right atrium pumps the deoxygenated blood to a larger pump found below it called the right ventricle. This side of the heart is smaller than the left because it only needs to pump blood up and across to the lungs, the great gas exchangers, where carbon dioxide and oxygen exchange takes place. The right ventricle contracts very quickly as the blood shoots up the blood vessels and then separates equally, travelling to both the left and right lung via the pulmonary arteries. In the lungs, the blood exchanges carbon dioxide for oxygen and becomes oxygen rich, turning the blood bright red, and then returns via the two pulmonary veins. The blood, now fully oxygenated, is returned into the larger, left half of the heart called the left atrium, this pumps it down to the largest part of the heart which is the left ventricle, here it is then pumped up the main artery, the aorta, to be circulated around the whole body.

As the blood circulates, the oxygen diffuses out and into the extracellular fluid and then diffuses once more into the cells. Conversely, carbon dioxide diffuses in the opposite direction. In this way the blood becomes de oxygenated once more. Finally it is delivered back into the right half of the heart, where it begins the whole cycle once more.

SUMMARY These are really the first points to take on board. First the heart is in fact two separate ‘pumps’. It pumps deoxygenated blood from its smaller right side to the lungs where it is reoxygenated through the pulmonary circulation. Then the oxygenated blood is returned to the larger left side of the heart where it is pumped, once more, in a continual cycle around the whole body, by the systemic circulation.

STRUCTURE OF THE HEART

The heart itself is a muscular organ made up of three distinct layers. First we have the epicardium (meaning ‘on the heart’), the outer layer, which is really two tough layers of connective tissue – the two pericardial membranes. One layer attaches itself to the major blood vessels above it, while the other layer actually envelops the heart itself. In between the two layers we have the pericardial space, full of a lubricating serous fluid. This has the effect of reducing significantly the friction of the constantly moving heart. This is occasionally clinically significant where fluid accumulates in the pericardial space and may interfere with normal heart contraction – this is called cardiac tamponade.


Next is the myocardium, the middle, bulky layer of cardiac muscle (remember that myo refers to muscle tissue), responsible for the contractions of the heart. Cardiac muscle cells are specialised and so can continue to contract without becoming fatigued. Another, rather specialised feature is that many cardiac muscle cells are electrically coupled into single functional units called a syncytium – this is a group of cells that are joined by special junctions with pores that result in a continuous cytoplasm. Because they have this ability, muscle cells can pass an electrical potential quickly along the heart wall, I always think of it as the ‘syncytium’ keeping the heart contractions ‘in sync’ so to speak! In this way a network of cells penetrates the myocardium that form preferential pathways for the spread of the wave of contraction that is the heart beat – this network is called the conduction system. When the heart muscle is deprived of its blood supply (or more specifically its oxygen supply) it can be permanently damaged resulting in a heart attack known technically as an MI – a myocardial infarct. Infarction is you remember from the respiration notes is cell death due to a lack of oxygen (hypoxia).

The third and final layer is the endocardium and is simply a lining of endothelium membranous tissue that lines all blood vessels, including the heart and it forms specialised folds that go to making the functional parts of the major valves found in the heart but more about them later! While simple, this inner layer can be considered as an endocrine gland as it secretes a wide range of factors directly into the blood that influences the blood cells. This includes ‘nonstick’ molecules that prevent blood cells sticking and forming a clot. The endocardium in capillaries also influences the movement of immune cells across the capillary wall – in inflamed areas the endothelium displays special receptors on its surface that cause white cells to stick (like Velcro) rather than flowing past – and once stuck the white cells squeeze between the endothelial cells to defend the tissue cells in that area.

As we have already seen, the heart, as well as being two separate pumps, has each pump separated into two distinct chambers. The two upper chambers, where blood is received from the veins, are called atria (singular, atrium), whereas the two lower chambers are called ventricles, and depending on what side of the heart they are decides whether they are the right or left atrium or ventricle.

The left and right chambers are separated by the septum, an extension of the heart wall. A special set of valves separates the upper and lower chambers, called atrioventricular or cuspid valves, acting very much like a lock gate in a canal. Another set of valves, the semilunar valves (‘half moon shaped’ valves), are located in the ventricles and regulate the flow to and from the aorta and the pulmonary artery. So the deoxygenated blood from the systemic veins enters from the right atrium into the right ventricle through the


right atrioventricular valve, called the tricuspid valve. At the same time, oxygenated blood that has just returned from the lungs is re entering from the left atrium into the left ventricle through the left atrioventricular valve, the bicuspid valve. It is said that the heart is ‘at rest’ at this point, and no noise will be heard through a stethoscope. Coronary arteries branch off from the ascending aorta here, and you could say that the heart itself, rightfully so, receives the freshest, most oxygenated blood directly from the lungs as it is squeezed out of the left ventricle up the ascending aorta.

It is these coronary arteries that can become clogged by fatty deposits (atheroma) and calcified (arteriosclerosis hardening) – if simultaneous these two processes are termed atherosclerosis. Atheroma can predispose to blood clots forming coronary thrombosis – and pieces of thrombus or atheroma may break off blocking a smaller vessel downstream – a coronary embolism, causing the cardiac muscle cells to become ischemic, or deprived of oxygen as a result of decreased blood supply to the tissues. This leads to the heart’s metabolic functioning becoming impaired and possible tissue death. Myocardial infarction may be the result – as mentioned above, with severe damage to the myocardial tissue and a heart attack is said to have occurred. There are two coronary arteries that come off the root of the aorta, one of which quickly branches into two. When coronary artery bypass operations are carried out, the number of bypasses typically describes how many of these three large branches are obstructed. Slowly worsening obstructions in smaller coronary arteries are able to be bypassed by the new growth of blood vessels (termed collateral vessel formation) over 23 days but rapid obstructions or obstructions in the larger coronary arteries are not able to be naturally bypassed.

Cardiac veins, as in all things, somewhat mirror the coronary arteries and return the used up deoxygenated blood directly back to the right atrium of the heart via the coronary sinus.

CARDIAC CYCLE

A complete heart beat is known as a cardiac cycle and there are 5 phases involved in this cycle. The main events are split into time intervals, and basically describe the contraction (systole) and relaxation (diastole) of both the atria and the ventricles. We have briefly seen how the blood flows from one part of the heart to the next, and this has been given ‘formal’, rather long names, but don’t be put off this cycle is used to identify how well the heart is functioning, and each phase can be recorded on an ECG (electrocardiogram – EKG in the USA) and so the ‘experts’ can identify any problems!


ECG Pulsewave

STAGE 1 THE ATRIAL SYSTOLE

The atria on both sides contract and empty the blood into the ventricles, which are relaxed. At this time, the AV valves are open and the semilunar valves are closed. This part of the cycle is shown as the P wave on an ECG. If the wave was ‘out of sync’, then an ECG may reveal a problem was occurring with the atrium and valves!

STAGE 2 ISOVOLUMETRIC VENTRICULAR CONTRACTION

What a great name, which simply means ‘having the same measured volume’. This phase is very brief and is the period just before the ventricles contract. Like a bit of a pressure cooker really because the pressure is increased greatly inside the ventricles but it has nowhere to go yet! (This is represented by the R wave on an ECG).

STAGE 3 EJECTION

So the pressure has built up in the ventricles and when the gates open (or the semilunar valves), the pressure in the ventricles is greater than the arteries and the ventricles contract ejecting the blood. Not all of the blood is eliminated from the ventricles, and in the case of a heart attack, much more blood is left in the ventricle than is eliminated from it due to the weakened heart muscle. This phase is characterised by the T wave on the ECG.

STAGE 4 ISOVOLUMETRIC VENTRICULAR RELAXATION

I’m getting to like these names now! This is the 1st stage in the cycle of relaxation (diastole) of the ventricles. It is the very brief period between the closing of the semilunar valves and the opening of the AV valves. The blood has just departed the ventricles remember, and so the pressure in the ventricles is now much less and the AV valves will not reopen until the pressure in the atrial chambers is greater than the relaxed ventricles.

STAGE 5 PASSIVE VENTRICULAR FILLING

Now, the veins are returning the blood back to the atria, increasing the pressure to such a point that the AV valves are forced open and it can flush down into the relaxing ventricles. Venous blood is still entering the atria, so


after the initial flood (lasting all of 0.1 seconds), there is a slower movement of blood into the ventricles that takes a whole 0.2 seconds!

Blood (in black) fills the ventricles and is pumped out

Such an important function requires a lot of control and organisation, for this reason, there are many feedback loops constantly sending information to the brain.

CONDUCTION SYSTEM OF THE HEART

One more important point is to be learned at this stage about the heart. The heart is controlled by its own unique electrical impulse system via four structures forming the conduction system. These structures start with the sinoatrial node, known as the SA node or more commonly as the pacemaker. It is here that the electrical signal is sent to initiate the contractions of the heart – it is innervated by nerves originating in the autonomic nervous system and is sensitive to various hormones including adrenalin (epinephrine in American textbooks). In this way the pacemaker is sensitive to regulatory influences that match its rate to the body’s needs.

The pacemaker is situated in the right atrium and sparks off a signal down its network, exciting the atrial myocardium, until it reaches the atrioventricular node, or AV node, just near the entrance to the right ventricle. The muscle of the atria and ventricles is separated by a fibrous sheet preventing spread of the impulse other than by way of the AV Node that sits on a hole in the fibrous sheet. From the AV Node the impulse travels down its branches to the ventricular myocardium by way of the atrioventricular bundle, or AV bundle, which separates into two bundles that supply the two ventricles of the heart. The bundles continue as the Purkinje fibres, which deliver the impulse to muscle of ventricles causing it to contract. All this occurs in just one electrochemical pulse, or message, that orchestrates one beat of the heart – ensuring that each part beats at the right time and will coincide perfectly with the filling of the chambers.


BLOOD PRESSURE

Finally, in order to keep the blood moving smoothly through this closed circuit of vessels, arteries need to be able to maintain a high pressure, imagine what might happen if they ballooned out or became slack or saggy! If there is an increase in the volume of blood, which would mean that more blood was flowing through the system, then it stands to reason that will exert a greater pressure against the walls of the arteries. So an increase in blood volume directly relates to an increase in arterial blood pressure, and vice versa if there was a decrease in blood volume. There are 2 levels of blood pressure, the higher systolic blood pressure, which refers to the force of blood pushing against the arterial wall when the ventricles are ‘contracting’, and the lower diastolic blood pressure that is the force against the artery walls when the ventricles are ‘relaxed’. Without going into too much detail here, suffice to say that maintaining the appropriate pressure is so important to sustaining the life of every single cell in the body. This pressure is being constantly checked and measured by various systems of the body, and several mechanisms are in place to control and regulate blood volumes. All of these mechanisms have an impact on water, this makes perfect sense because if blood volume (and so blood pressure) drops, then more water needs to move into the blood plasma, therefore increasing the volume and raising the pressure. Conversely, if the blood volume (and so blood pressure) is too high, water needs to leave the blood plasma and so reduce the blood volume with me so far?

The control of blood pressure takes place between the heart, kidneys, central nervous system and endocrine system. See the diagram below.

There are basically three ways to alter the blood pressure in the large systemic arteries (this is the pressure measured when a sphygmomanometer cuff is used to take your blood pressure):

1. To increase the heart rate and/or the amount of blood pumped out each beat (the stroke volume) – the amount of blood pumped out each minute is the cardiac output which is calculated by multiplying the heart rate by the stroke volume. So anything that increases the cardiac output can increase the blood pressure.

2. To increase the peripheral resistance. This strange sounding term is based on the fact it is much harder to force fluid through a small tube than it is through a larger tube – half the diameter and it takes 16 times as much effort. So it is relatively easy to get blood moving through the large arteries but a huge effort to get it through the small arteries that supply the capillary beds. This means the small arteries cause a backlog


of pressure behind them in the large arteries – and because we can open and close the small arteries we can control this backpressure. Open lots of small arteries and the pressure will fall in the large arteries, close lots of them and the pressure will rise. Coupled with the control of the cardiac output, these first two methods maintain the blood pressure moment by moment. Note that exercise causes a dramatic increase in cardiac output but an equally dramatic fall in peripheral resistance as loads of small arteries open up in the skeletal muscles. If you are fit this is coordinated perfectly with little increase in blood pressure. If you are not there can be transient peaks and troughs as the system adjusts.

3. Control of blood volume. Essentially this is driven by thirst, regulating fluid intake and urine formation, regulating fluid output. Like a sink with the tap on and the plug out – the balance of inputs and outputs will determine the fluid level – and too much fluid causes the whole system to ‘burst’ with pressure – too little and you face circulatory collapse (termed shock) as there is not enough blood returning to the heart to be pumped out (if you lost all your blood you would have – for a little while – a high heart rate but no cardiac output).

We have evolved ways of communicating what action is needed on a minutetominute basis to maintain homeostasis. One mechanism involves the hypothalamus in the brain. This secretes a hormone called Anti Diuretic Hormone (ADH). As the name implies, this hormone opposes a diuretic effect by instructing the kidneys to reabsorb more water from the urine before it is excreted. Similarly, Renin is released by the kidneys when it senses the blood pressure is too low, this leads to the formation of an intermediary substance called Angiotensin (that stimulates thirst and causes blood vessel contraction) and in turn stimulates Aldosterone secretion by the adrenal glands. Aldosterone stimulates sodium to be retained, as water follows the sodium, it causes water to be retained so having the effect of increasing the blood volume and blood pressure.

If blood pressure is too high, ANH (Atrial Natriuretic Hormone, sometimes ANP: Atrial Natriuretic Peptide) is triggered by the action of the atrial walls of the heart being ‘overstretched’. This hormone promotes the loss of water from the blood plasma and so decreasing the blood volume and thus the lowering the blood pressure very clever isn’t it? This is actually achieved by telling the kidneys to excrete more sodium, and what will follow sodium? Yes, water of course!


BLOOD PRESSURE CONTINUED…….

It is interesting to observe that there are more mechanisms in place to conserve water than to excrete it, and we will be looking at all of this again in more detail when we study the urinary system and the CNS and endocrine systems, so there’s no need to worry if you haven’t grasped it all! For now, have a good look again at what we discussed so far, then return to your textbook and study in more detail chapters 18 and 19. There is a hierarchy of organs when it comes to blood pressure regulation – those that are essential for escaping danger or fighting (brain, heart, lung, muscles) are favoured at the expense of those that are not essential to immediate survival (skin, most of the digestive system, but not the liver, and the kidneys). Blood flow to the nonvital organs will reduce significantly if there is blood loss – this explains why our skin develops pallor and goes cold in such circumstances – the skin blood vessels have constricted maintaining the pressure in the large arteries so that the brain and heart etc. can still be supplied.

& LOOK AT CHAPTER 19 Here there are some excellent diagrams that make it very easy to follow these mechanisms. Take time to study these to the point of understanding how the cardiovascular system works hand in hand with the CNS, once you get a feel for this, we will be looking at the CNS in greater detail later on in this series of booklets, and we will build on your understanding then.

SUMMARY The heart is a threelayered muscular pump that is divided into two distinct and separate halves. The smaller right half acts as a pump for the pulmonary circulation. It receives de oxygenated blood in the right atrium through the superior and the inferior vena cava, pumps blood through the right ventricle and into the lungs through the pulmonary artery to oxygenate it and returns it to the left atrium. From the left atrium, the blood enters the left ventricle, where it is pumped into the systemic circulation through the aorta and the network of arteries around the whole body, with the exception of the lungs. The deoxygenated blood finally returns to the heart via the network of veins, and collects again in the right atrium to start a new cycle. Each half of the heart is divided into an upper and lower chamber. The upper chamber, called atrium always receives the blood into the heart. From the upper chambers the blood flows through the cuspid valve into the lower chambers, or ventricle. Both the bicuspid (left side) and the tricuspid (right side) valves snap shut when the lower chambers or ventricles contract. There are two more valves and they are situated at the entrance of the pulmonary artery and of the aorta, called respectively the pulmonary semilunar valve and the aortic semilunar valve. Both sets of valves have the vital function of regulating blood flow in and out of the atria and the ventricles: they will simultaneously prevent backflow and ensure forward flow. The heart has its own unique electrical or conduction system, which consists of the sinoatrial node or pacemaker, the atrioventricular node, the atrioventricular bundle and finally the Purkinjie fibres. These control the rhythmical contractions of the heart, initiated by the pacemaker.


BLOOD VESSELS AND SYSTEMIC CIRCULATION

The blood vessels are a continuous network of three different types of vessels: arteries, veins and capillaries. The basics are as any plumbing system: to maintain adequate pressure for movement of the fluid and to fully supply all areas in a continuous cycle. Hence it is necessary for the vessels, or pipes if you like, to become narrower as they branch off to smaller areas. The main supply vessel for oxygenated blood is the aorta and this travels up from the left ventricle as the ascending aorta, then curves around as the arch of the aorta and down behind the heart to form the main descending aorta. Three main arteries branch from the aortic arch: first the brachiocephalic artery which further subdivides to supply one side of the neck, with the right common carotid and the right arm, with the right subclavian artery. Then we have the left common carotid artery which supplies the left side of the neck and finally we find the left subclavian artery which supplies the left arm. Again, do not be frightened by these new long words, but instead find out what they are telling you, for instance the left subclavian artery is on the left underneath (sub) the clavicle bone.

The aorta then continues down behind the heart, liver, stomach and intestines to the level of the ‘belly button’ or umbilicus. Here it divides into two branches, left and right, and proceeds to travel down both legs, where it branches off into differently named arteries depending on its location. As you can see, we find no separate distinctive blood vessels but only a complete ‘pipe’ system with a mains pipe being the aorta and the supplies branching off to supply the organs, muscles and tissues.

An important point to remember concerning vessels classification is that the largest blood vessels carrying blood away from the heart are called arteries, the largest of which is the aorta. Arteries then become arterioles, or small arteries (at what point this is achieved is unclear answers on a postcard will be appreciated). In the case of arteries and arterioles, blood vessels are only tubes containing blood, with the purpose of transporting materials using blood as the carrier. When arterioles become microscopic, so that only one blood cell can travel along at a time and the walls of the blood vessel are only one cell thick they have become capillaries. The fibrous layers and muscle layers that give arteries their strength have now been lost – we just have the endothelial layer in the capillaries – thin enough to allow substances to pass across its wall. It is here, in massive networks of capillaries called capillary beds, that the transfer of gases, nutrients and other materials takes place from the interstitial fluid into the blood and from the blood into the interstitial fluid surrounding and bathing the cells. Goods will be dropped off and goods will be picked up along with waste products and debris for its new journey and destination. No surprise then, that these tiny, yet crucial vessels, are so vast in number, that somebody (with no life!) has calculated that if you joined all of the capillaries together, they would extend to somewhere in the region of 62000 miles (lost for something to do on a rainy afternoon try laying these out endtoend) you just have to be impressed!


Capillaries are not only the dropoff and pickup point of goods. It is at this level, and as a result of the ‘drop off scheme’, that capillaries form the ‘missing link’ between the arterial and venous circulatory networks. From the single cell walled tiny capillaries, where all exchanges occur, the blood drains and collects into slightly larger venules, or small veins, these in turn pool and collect into larger veins which all finally reenter the main vein that runs parallel with the aorta, the vena cava.

STRUCTURE OF VEINS AND ARTERIES

Arteries and veins are very similar in design, but as structure governs function, we find a few subtle differences. Take a look at a diagram of an artery, you will see it has three main layers and a fatty insulating layer. The outermost layer is called the tunica adventitia and as usual comprises of a tough connective tissue that surrounds and holds the other layers in place. Next, we have the middle layer, the tunica media, which in the arteries is a thick muscular layer of smooth muscle controlled by the autonomic nervous system. This enables the artery to alter its diameter and/or its length, and so affect blood pressure and blood flow. Beneath this layer is a thin insulating layer of fatty tissue and then the innermost layer, the tunica intima, which once again is just an endothelial lining. Arteries are generally referred to as ‘distributors’, whereas arterioles perform a very important function, because they offer resistance, which is very important in the maintenance of blood pressure and circulation as discussed above. Depending on the circumstances, arteries and arterioles will contract or dilate to alter the blood pressure and flow to suit the needs of the tissues or of the body as a whole. For example, when in very cold conditions, the blood vessels in the extremities and near the surface of the skin will contract, altering blood pressure and directing blood flow towards the core of the body and the vital organs to maintain core body temperature and thus sustain life. On the other hand, in extreme heat conditions, the blood vessels will dilate to obtain the opposite result.

Furthermore, arteries are under pressure from the beating heart and so need to resist the force of the blood from blowing them up like balloons. Veins, however, are under a great deal less pressure and so do not require such extensive muscle to control it or prevent ballooning as the pressure is just not there. This though, has its problems, and so to help the pooling blood travel up against gravity back to the heart, two simple but ingenious mechanisms are in place.


STRUCTURE OF VEINS AND ARTERIES CONTINUED…..

Firstly, we have a skeletal muscular pump, this comes into action as the major leg muscle groups contract when walking for example, exerting a pressure on the blood vessels themselves and therefore squeezing the blood upwards and onwards with the help, of course, of the faint and distant pulse of the heart. As the blood moves up with the contraction of the heart and of the local muscles, it forces open the valve above and shunts the blood onwards. As the pressure lapses, gravity causes the blood to pool back, creating back pressure: this closes the valve and prevents back flow. You can see these valves working in your arm veins – run your finger over a vein in the direction of blood flow and it stays filled behind your finger – but go against the flow (towards your finger tips) and the vein empties until you pass a junction where the valves are found. Do this on a bus to keep the seats next to you free (unless it is a biology teacher trip out).

The legs have a deep venous network that comprises veins running deep in the legs – it is these that are squeezed by the muscle pump. The superficial venous network runs under the skin – this is at a lower pressure as it does not much benefit from muscular squeezing. Communicating valves allow blood to enter the deep veins from the superficial veins. The valves in these communicating veins stop backflow of relatively higher pressure blood in the deep veins coming into the superficial veins – but lack of exercise or movement combined with long periods of standing (and obesity which further stretches the valves) leads to the pooling of blood within the veins and excessive pressure on the communicating vein valves – and finally the valves can collapse, causing varicose veins.

Next, we have the increasing and decreasing abdominal and thoracic pressure, the result of the ‘respiratory pump’ that squeezes blood out of the abdomen as the diaphragm contracts during inspiration and pushes blood into the heart as the pressure in the chest cavity increases during expiration. If you feel your pulse as you breath you will notice a change in pulse rate and volume with the breath – this is due to the action of the respiratory pump changing the rate of blood return to the heart.

So veins collect blood to return it to the heart, and because they have the ability to ‘stretch’, they can also act as reservoirs pooling blood and therefore, helping the body manage varying amounts of blood without adversely effecting blood pressure.


SPECIALISED CIRCULATION: HEPATIC PORTAL SYSTEM

Before we leave the cardiovascular system proper, let’s look briefly at what is described as a separate circulation system within the system, and represents in fact an exception to the circulatory system routes. In the hepatic portal system, blood is filtered before returning to the heart, by means of a venous secondary capillary network where blood from the spleen, stomach, pancreas, gallbladder and intestines is filtered by the liver. Blood sugar control and toxicity reduction are the main reasons for this detour. There is a network of capillaries that surround the small intestine and colon, ever ready to absorb any nutrient that can freely pass through the intestinal mucosa. They all ultimately lead to the hepatic portal vein and into the liver, the body’s ‘chemical factory’. This whole network will be encased within the protective mesentery, and it is useful to hold this picture in your mind so as to understand and to explain why enemas, particularly coffee enemas work so effectively upon the liver. Anything introduced into the rectum that is absorbable through the lumen will be transferred directly to the liver very, very quickly, where it can effect change. A key concept though, is that this hepatic portal drainage runs from the lower oesophagus down to the upper part of the rectum. The capillaries from the mouth, pharynx, oesophagus, anus and lower rectum drain into veins that enter the general circulation as normal – this bypasses the liver. Hence the logic of taking certain drugs sublingually or rectally as a suppository. Drugs which would be broken down by the liver are able to bypass the liver, reducing the dose required. Many medicines are still administered this way in Europe, where a philosophically sound reason for doing something is more important than social niceties of not sticking pills up your bum.

If the portal circulation becomes blocked (e.g. cirrhosis of the liver) then small communicating arteries between the oesophageal and rectal veins (specifically the haemorrhoidal veins) and the portal circulation open up resulting in haemorrhoids and, in the lower oesophagus, oesophageal varicies. The latter are potentially life threatening as if they are cut open as they bulge out into the oesophagus (perhaps by a sharp crisp) there can be catastrophic bleeding only revealed by a falling blood pressure and blood in any vomit.


FOETAL CIRCULATION

Finally, mention should be made of foetal circulation, as it differs from postbirth circulation in some fundamental ways, because the foetal blood has to get its oxygen and nutrients from the mother’s blood. Of course, after the baby is born, it will get its oxygen from its own lungs and its nutrients from its own digestive tract. Before birth however, six important structures counteract the absence of pulmonary circulation and digestion in the foetus as well as providing an interface for the exchange of blood, nutrients, gases and wastes between the foetus and the mother. The nutrients dissolved in the placental sac enter the body through the umbilical vein, which forms, with the two umbilical arteries, the umbilical cord. The mother’s blood and the foetus’ blood are kept largely separate as oxygen and nutrients are absorbed from the mother’s blood through the placenta (foetal cells do pass over the placenta and seem to regulate the mother’s immune system preventing rejection of the foetus amongst other activities). Therefore the placenta acts as the interface for the exchange of nutrients, gases and wastes between the foetus and the mother. Most of the foetus’ blood returning from the placenta bypasses the foetus’ liver by means of the ductus venosus, a continuation of the umbilical vein, which in turn empties in the inferior vena cava. The foetus does not use its lungs and therefore does not need the right side of the heart to pump blood to its lungs. As a result of this, a temporary hole connecting the right and left atrium exists, called the foramen ovale (the oval opening). This allows the incoming blood to bypass the foetus’ absent pulmonary circulation by entering straight into the left atrium and so allowing it to be pumped around the foetus. Some blood does enter the pulmonary artery but another temporary vessel, the ductus arteriosus, diverts most of this back into the aorta. The returning systemic blood leaves the foetus from where it came, that is through two umbilical arteries that leave through the umbilicus, where they exchange once more with the mother’s blood through the placental membrane.

When the umbilicus is cut and the placenta delivered, several changes should begin to take place in the newborn baby. The umbilical vein eventually becomes fibrous and becomes the round ligament of the liver. The ductus venosus becomes the ligamentum venosum of the liver and the ductus arteriosus becomes the ligamentum arteriosum. The umbilical arteries become the umbilical ligaments, but most importantly the foramen ovale, which allowed the blood to pass from the left atrium into the right atrium and ventricle, should seal itself closed, so as to allow the heart and its pulmonary circulation to begin. If this fails to occur the baby tends to turn blue and is said to have a ‘hole in the heart’. Surgery can alleviate this problem quite easily by fusing the opening closed and thus allowing the pulmonary circulation to begin functioning properly.


SUMMARY

To summarise then, the cardiovascular system is a network of blood vessels consisting of arteries, arterioles, capillaries, venules and veins. As a rule of thumb the arteries carry blood away from the heart and veins carry blood to the heart. With the exception of the capillaries, the other blood vessels are only involved in supplying a network for the universal transporter, blood, to travel through with no exchange taking place with the cells or tissues. It is only in the one cell thick membrane of the capillaries that any exchange takes place of either gases, nutrients, wastes, by products of cellular metabolism or fluids.

The blood flow and pressure is very much regulated by the heart which is a ‘double pump’ also acting as a flexible series of valves. The left side of the heart controls the systemic circulation with the aid of the muscular walled arteries which can alter their diameter to maintain, decrease or increase blood flow to a region of the body. The less muscular veins rely upon the contractions of the respiratory pump and of the skeletal muscular pump to aid venous blood flow to the heart. The presence of small semilunar valves, located about every six inches along the veins, will also prevent back flow.

The right side of the heart is smaller as it only has to pump de oxygenated blood across to the lungs where gaseous exchange can take place, expelling carbon dioxide and replenishing oxygen. The reoxygenated blood then returns to the heart and enters the left side, to join the systemic circulation. This smaller circulation of blood is called the pulmonary circulation


Introduction to the blood

& READ CHAPTERS 17 & 24 IN THIBODEAU & PATTON

Blood: such a noble, life giving and sustaining substance. The ‘stuff’ that people faint at the sight of, the essential additive to films that have the inability to otherwise create pictures in one’s mind and so its presence brings a surrogate reality to the silver screen.

In this session I would first like to separate blood into what the microscope can observe and define certain constituents of blood and their functions. Secondly I shall discuss the relationship between oxygen and carbon dioxide with the blood, and their exchange within the cells and lungs.

COMPOSITION OF BLOOD

It is estimated that blood constitutes approximately 8% of our body’s total weight, although it is equally recognised that the fatter one is the less blood one has proportionally. Extracting blood and placing it into a test tube that is then centrifugally whizzed around will separate the blood into its constituent parts. At the bottom of the test tube we find that a deep red coloured concentration has collected and above this we have a straw coloured region. The red part is known as the formed elements, a term used to describe the various blood cells found in blood, and normally accounts for about 45% of the blood (of this, white blood cells and platelets make up less than 1% of the total blood volume). The formed elements comprise:

• the red blood cells (RBC), or erythrocytes (‘erythro’ = red, ‘cyte’ = cells), the cells that actually carry the oxygen and carbon dioxide within the blood;

• the white blood cells (WBC), or leukocytes (‘leuko’ = white, responsible for cellular and immune defense mechanisms;

• the platelets, or thrombocytes (‘thrombus’ = clot), partially responsible for mending wounds and ruptures to the blood vessels and surrounding tissues.

The straw coloured portion of blood is called the plasma (the colour is from the breakdown products of haemoglobin, Mainly bilirubin, it stains plasma, urine and faeces yellow when fresh – brown when oxidised) and makes up about 55% of the blood volume. It is in the plasma that nutrients, a small amount of the respiratory gases, waste products and byproducts of cell metabolism are dissolved and transported. Plasma is the fluid portion of blood and is in fact about 91% water. Dissolved within the water are approximately 7% proteins and 2% solutes, which we will discuss later.


RED BLOOD CELLS

First let us delve deeper into the formed elements of blood. Red blood cells (RBC), or erythrocytes (meaning red cells), are present in vast numbers within the blood: a drop of blood will contain about 1015 million erythrocytes. Red blood cells vastly outnumber white blood cells and platelets for a very good reason. Erythrocytes are shaped as an ‘ice hockey puck’: they are disk shaped with the surfaces concave like a holed doughnut that has not quite penetrated all the way through. This shape ensures that the internal volume remains the same whereas the surface area is maximised, and as a result more erythrocytes can be packed in the blood. This is vital to an erythrocyte as its purpose is to pickup and transport to the cells the life sustaining gas molecules oxygen, while at the same time removing the poisonous waste product of cell metabolism, carbon dioxide. In fact, if someone was to open out every erythrocyte in their body and lay them side by side, they would cover an area greater than a football pitch! Now, apart from the fact that the person attempting this very silly experiment would be dead from boredom long before even covering the semicircular quadrant of the corner flag, the point of this example is that this is the total area available to the blood for the exchange of gases with the billions of cells around the body and the alveoli of the lungs.

Erythrocytes are unique and peculiar cells, so much so that there is still some disagreement amongst A&P literature on their structure. Your textbook, Thibodeau & Patton, states that a mature erythrocyte, meaning a RBC that has entered the circulation for about four days, contains no nucleus, mitochondria, ribosomes and no organelles, believed to having been expelled before entering the blood stream. However, you may come across other literature which may dispute this and state that, although a nucleus may not be present, a relatively small number of mitochondria may be present and glucose would be absorbed and used for energy – to some extent this depends on which point a RBC is considered mature. Immature RBCs that still possess a nucleus are called reticulocytes, increased numbers are an indication of increased release of RBCs from bonemarrow, perhaps due to recovering from blood loss or a more rapid turnover if defective RBCs are being produced. For the rest of its 120 day life, the erythrocyte uses its stores of enzymes, proteins and RNA to carry out its needs.


ERYTHROPOIESIS

This rather impressive name simply refers to the process of RBC formation. The process of making a new RBC takes about 4 days and begins in the red bone marrow. Here you will find specialised ‘blood forming stem cells’ called hematopoietic stem cells. These have the ability to keep the stocks of RBC at a fairly constant level depending on the needs of the body. In order to make RBC, the bone marrow needs certain ingredients such as vitamin B12, folic acid, iron, zinc, amino acid plus cobalt & copper are also required to act as a catalyst. The kidneys play a key role in the feedback loop that regulates the amount of RBC being produced. If the amount of oxygen reaching the tissues decreases, the kidneys secrete a hormone called erythropoietin. This effects the bone marrow by stimulating it to produce more RBC, therefore, increasing the ability of the blood to carry more oxygen to its tissues.

As RBC only have a life span of 105120 days, and because they are so vast in their numbers, there needs to be an efficient way of breaking them down once they have become ‘worn out’. Their parts need to be recycled and any ‘unwanted bits’ can then be excreted. This takes place by macrophages (phagocytic cells capable of engulfing aging RBCs) found in the lining of the blood vessels (especially in the liver and spleen). Here, haemoglobin is broken down into amino acids, iron and bilirubin. Iron is saved and sent back to the bone marrow for reuse, bilirubin is excreted by the liver in the bile and the amino acids are used to provide energy or for the building of new proteins.

Rather wonderfully, RBCs also have receptors on their surface that bind various immune proteins. These proteins in turn latch onto foreign proteins – e.g. antibodies latching onto the proteins on a bacterium or virus particle. RBCs get covered in immune proteins bound to these foreign proteins, effectively clearing them from the blood as the macrophages mentioned above ‘slurp’ over the RBCs testing to see if they are too old and as they do so clearing the bound proteins from the RBC surface.

WHITE BLOOD CELLS – LEUKOCYTES

White blood cells (WBCs) are also formed in the hemopoietic stem cells in the bone marrow and also in the lymphatic tissues. They are classified in 2 ways, so they are either granular (called granulocytes), and consist of neutrophils, eosinophils and basophils, or nongranular (also called agranulocytes), which are the lymphocytes and monocytes. Unlike RBCs, white blood cells do contain a nucleus and their life span varies depending on the different types, ranging from hours to months.


WHITE BLOOD CELLS – LEUKOCYTES CONTINUED…….

v Neutrophils function: defends the body by destroying small pathogenic microorganisms. Can engulf particles through phagocytosis.

v Basophils function: helps the inflammatory response by secreting heparin and histamine. They may convert to mast cells in the tissues – but this is highly controversial. Mast cells release inflammatory substances such as heparin and histamine in the tissues and are responsible for certain allergic responses – allergic asthma and anaphylactic shock are examples.

v Eosinophils function: defend the body by destroying larger pathogenic microorganisms such as parasitic worms and protozoa, and release antiinflammatory substances.

v Lymphocytes function: 2 main types – T cells engage in cell tocell contact killing virus infested or tumour cells while B cells defend the body by secreting antibodies. The B and T cells are fundamental to the acquired immune response. Natural killer cells (NK cells) are similar to lymphocytes but work in a different way, giving another means of attacking virus infested and tumour cells.

v Monocytes function: can enter the tissues and here transform to macrophages or dendritic cells – the former have a powerful defence role as they are capable of ingesting bacteria, cellular debris and cancer cells by phagocytosis, the latter collect bits of foreign cells, viruses, proteins and take them to lymph nodes to see if any lymphocytes are ‘interested’ in them.

We will meet all these guys again in the lecture specific to immune function, so this level of information will serve as a useful introduction as ever though, there is so much more to know!

PLATELETS

Platelets (thrombocytes), are tiny tiny little cell fragments that look like irregular discs or spindles with a relatively large nucleus. They are also formed in the red bone marrow as well as the lungs and to a lesser extent the spleen, and they ‘live’ for about 7 days.


PLATELETS CONTINUED……

Platelets perform a function called hemostasis, which means they can help stop the inappropriate flow of blood. If a capillary has burst or been damaged, then blood cannot be allowed to pour out into the surrounding tissue. Because platelets have the gift of being able to become ‘sticky’, they can form a plug that will prevent the blood haemorrhaging. They also secrete chemicals such as fatty acids (arachidonic acid or AA), thromboxane and adenosine diphosphate (ADP) at the site of injury. These chemicals help to reduce blood flow to the area and so it helps to reduce blood loss.

Furthermore, platelets need the presence of calcium and vitamin K in the blood to help them go through the various chemical pathways that need to take place before the platelets can clot. Of course, anything interfering with the amount of these substances in the blood, will have an adverse impact on clot formation. Clot formation may be instigated by tissue damage or damage to a blood vessel and when this is the case the ability to clot will save your life! Atherosclerosis (plaque forming on the artery wall) or immobility and inactivity may also lead to the production of blood clots (thrombosis), which can cause a heart attack or stroke.

Incidentally, aspirin is often used because it breaks the chemical pathway that helps in the formation of a blood clot i.e. the production of AA. If however, enough Omega 3 essential fatty acid is present, this would have the same effect because it inhibits the production of AA and so, naturally helps to prevent clotting.

BLOOD GROUPS

Definition of an Antigen: Substance that when introduced into the body, causes formation of antibodies (or a specific immune response) against it. Allergen is an equivalent term, immunogen is the latest version.

Definition of an Antibody: Substance that is produced by the B cells that binds to (and usually destroys or inactivates) a specific substance (Antigen) that has entered the body.

Keep referring to the definitions above to help yourself out here. When we talk about blood groups, what we are actually referring to is the type of antigen present on RBC membranes. The three main types of antigens found on RBC membranes, for the purpose of this discussion, are antigens ‘A’, ‘B’ and ‘Rh’. Each one of us belongs to one of the following four blood types:

v Type A Antigen A on RBC membrane v Type B Antigen B on RBC membrane v Type AB Both Antigen A and B on RBC Membrane v Type O Neither Antigen A or B on RBC Membrane


BLOOD GROUPS CONTINUED………

These antigens, A or B, are proteins found on the RBC surface whose presence is determined by having the necessary genes. The O type is actually the result of having a nonfunctioning gene that makes no protein. As we have 2 copies of the genes, one copy inherited from each parent, we can have the following combinations: AO, AA, AB, BO, BB, OO. The AO and BO types are still A and B blood group as they have one copy of a productive gene – enough to cover their RBCs in A or B antigen. The A and B antigens are normal body constituents – but these substances are also found on common bacteria. It is therefore inevitable that we will form immune responses against the blood antigens that are foreign to us as we will have been trained to do so by common infective organisms. Why do the organisms share these substances? Probably this is historical – a previous attempt of these organisms to fool our ancestors into thinking they were normal cells by coating themselves in a molecular camouflage of normal cell surface substances. Whatever the reason, the result is that A type people make antiB antibodies, B type make antiA antibodies while AB make neither antiA or antiB (as they would attack themselves) and blood group O makes both antiA and antiB. Given that a normal transfusion involves placing a relatively small amount of blood in someone’s circulation, the presence of antibodies in the donated blood is not usually that significant as they are diluted out. However, it is critical not to give donated blood to someone with antibodies to the donated cells as these will destroy the donated cells and the ‘corpses’ of these cells can block the kidney capillaries.

It is important to remember though, that even if your RBCs do not contain any antigens (i.e. Type ‘O’) or only contain one antigen (Type ‘A’ or ‘B’) then the blood plasma will contain antibodies against the antigens that are not present. What this all means is that if you have a blood group ‘A’ person (RBC’s contain antigen A), then they would not have antibodies to attack ‘A’ antigens. They would however, have anti ‘B’ antibodies, and this would be OK because the RBC’s do not have antigen B on their membrane so won’t be attacked. Therefore, donating blood from a type ‘A’ to a type ‘A’ would be fine, however donating to a type ‘B’ wouldn’t be so clever!

Because blood group ‘O’ does not contain either antigen ‘A’ or ‘B’, it should be safe to give to any other blood group, and it is for this reason that it is generally called the Universal Donor. Although, in reality the blood is always matched with the recipients blood in the laboratory to check that their plasma does not contain any other antibodies that may react to it. Similarly, blood group ‘AB’ does contain antigens ‘A’ and ‘B’, therefore, it does not contain antibodies ‘A’ or ‘B’ and explains why it is referred to as the Universal Recipient (although the same cross matching principles applies as for blood group ‘O’).


BLOOD GROUPS CONTINUED………

This brings us to the third common type of antigen found on RBC’s that being the ‘Rh’ factor. If the ‘Rh’ antigen is present on RBC’s, then the blood is said to be ‘Rh’ positive – written as Rh + or D + (as the Rhesus antigen is also known as the D antigen). If it is not present then the blood is said to be ‘Rh’ negative. An important point here is that there is not usually an antibody found in the blood plasma against the ‘Rh’ factors – never in ‘Rh’ positive and only in ‘Rh’ negative when exposure to ‘Rh’ positive blood has occured. The main issue here is in pregnancy or when we give a ‘Rh’ negative person a blood transfusion of ‘Rh’ positive blood. In pregnancy, if the mother is ‘Rh’ negative and the baby has taken on board the ‘Rh’ positive trait of the father, the mother may then produce anti ‘Rh’ antibodies that are then present in her blood plasma. This becomes a problem if she gets pregnant again and the baby is ‘Rh’ positive. The mother now has antibodies to attack the ‘Rh’ antigen not a good sign for the baby! This should also be considered if there has been a termination or a miscarriage previously, because the ‘Rh’ antibodies may well be present. The woman is treated with RhoGam (an antiD antibody concentrate) which prevents her from forming anti ‘Rh’ antibodies, so she is of no risk to any future ‘Rh’ positive offspring. It does so by taking out foetal ‘Rh’ positive RBCs from the mother’s circulation before she has had time to mount an immune response.

BLOOD CONCLUSION

When just talking about the gases and gaseous exchange qualities of blood we have to say that there are many, many other chemicals that quite happily bind to haemoglobin to make it dysfunctional. Lack of exercise or physical movement consistently lead to a decrease in the production of red blood cells. Poor posture, on the other hand, would prevent full and adequate respiration from taking place. All of the above systematically deprive the body’s cells and tissues of life sustaining oxygen, while at the same time preventing the efficient and full excretion of the waste gas carbon dioxide. When cells and tissues are deprived of oxygen they will inevitably die if the lack is sustained. It is important to be clear that the physiological terms for energy and metabolic toxins overlap with but are different from the naturopathic concepts of vital energy and toxins. The latter terms include psychological, subtle energetic and spiritual aspects that are beyond the scope of physiological terms. The naturopathic concepts that bodily health is achieved through adequate nutrition, elimination and vitality resonates with the role of the circulatory system to provide nutrients to the cells, remove wastes for elimination and be vitalised by adequate exercise, proper posture and relaxation. There is often considerable attention paid to oxygenation – but at rest or when undertaking moderate activities and in reasonable health, most tissues are fully


oxygenated so that taking oxygenenriched air (or crazily, hydrogen peroxide or ozone – neither of which provides any significant quantities of oxygen but are powerfully carcinogenic) would have little effect. Indeed, high oxygen levels cause oxidative damage to brain cells. It may be that the undoubted health improving effects of a vigorous circulation is being confused with the oxygen carrying function of blood.

INSIGHT But before you go on to take a peek at this ‘Transilvanian Claret’, let us first wander to a place that I recently visited, the Sahara desert. There we have an abundance of sand that may appear as an everincreasing expanse of lifeless dust slowly encroaching inwards towards areas of vegetation and life. Inch by inch the moving desert claims more and more of the fertile land as a cancer eats at healthy tissue. Travel back in time and stories will be told of how once this desert was a fertile land but how the rivers dried up and, once water had left, death was inevitable to most life forms: slowly vegetation died and finally the unbound soil was blown away. Nature very slowly began its process of disassembling highly organised forms of life back into their original, indestructible components and then scattering them in the winds to find conditions for life again. It is a wellknown fact that if the water supply is insufficient or polluted then the land will not oblige us with its gifts of food. Firstly, water should be in reality nothing, the ultimate universal medium that binds all things together without having anything directly to do with these processes. It is uniquely malleable and will unarguably ‘bend’ any way to accommodate. It is believed to be ‘alive’ when it is flowing freely within rivers. It is selfcleansing and charged, and as life cannot exist without water many say that it is life. If you have ever drank from a mountain spring where the flow is turbulent and alive, you will know what live water is compared with the dead fluorinated and chlorinated waters of modern times. I recently drank from a waterfall high up in the Atlas mountains of Morocco, and never have I felt so charged and alive after drinking water! As with the waters of the Atlas mountains, all water rushes through the rocks that form its river bed, breaking down and carrying the minerals with it, which are ultimately deposited in the soil and then taken up by the plants. The land that is watered by ‘purified’ mains water or by underground streams that have been poisoned by chemical fertilisers and pesticides, is devoid of the natural essential nutrients for life and also of the charged capacity of live water. The soil is currently so over farmed that more and more chemical fertilisers are needed to grow ever reduced albeit intensive crops. The soil is slowly dying because the lifeblood, water, has been polluted, because man has tampered with the natural flow of rivers, generally straightening them and cutting down woodlands for more agricultural land.


INSIGHT CONTINUED…. What has this to do with blood? What I am trying to do here is to put across certain aspects of A&P, whilst at the same time fitting A&P in the greater picture, showing you the simplicity and the reflections in the whole picture. The land problems are a symptom of the direct action of man against the earth’s blood and its ‘circulatory system’. If these were to be restored to their original natural balance then in time other related problems would be hopefully equally solved. Like water, blood is the life force of all animals: it flows continuously through ‘meandering’ vessels that cleanse and feed our internal land, composed by the cells and tissues. When the integrity of the blood is altered then so too are our cells and tissues. When our cells and tissues cannot cleanse and are equally not properly nourished, they begin to become stagnant and infertile. As long as our life force, the blood, is polluted the rot continues until the individual plots of land, the cells, become so stagnant, polluted and devoid of nutrients that they simply follow the laws of nature and release lysosomes to start decomposing these toxic cells back into their fundamental and indestructible chemical elements. In many ancient medicine practices, breathing and diet hold an important, if not paramount, role. Conversely, blood is viewed as sacred, as the medium to cleanse the body and the spirit. Blood is regarded as sacred to these ancient cultures as the rivers are to their lands. Rudolph Steiner has always said “if you wish to heal yourself, first heal the earth”. Restoring our rivers will restore our ‘live’ water supplies, the soil will slowly cleanse itself with the aid of the rivers and thus real food will be grown once more. We can then indulge in these life sustaining gifts, providing our internal land, with all the essential nutrients carried effortlessly by the most sacred of all rivers, our blood, is equally unpolluted! It is fortunate that evolution has equipped us with such robust, selfhealing systems that are able to withstand and recover from the insults our modern life throws at it given the right conditions.

PATHOPHYSIOLOGY

Using the websites reviewed in the Respiratory System lecture, explore the following conditions:

v Circulatory disorders: understanding the pathological processes that result in many different conditions Atheroma, arteriosclerosis, thrombosis, embolism, infarction, shock, haemorrhage, oedema, organ failure, clotting disorders.

v Symptoms and signs relating to diseases of the heart and lungs: chest pain, breathlessness/dyspnoea, wheezing and pleural signs, cough with sputum (with or without haemoptysis), palpitations, cyanosis and clubbing of fingers.


v Symptoms and signs relating to diseases of the heart and blood vessels: angina, myocardial infarction, heart failure, hypertension, abnormal heart rhythms, peripheral vascular diseases.


INTRODUCTION TO THE LYMPHATIC SYSTEM

In this lecture we will be looking at the extremely important and yet so often neglected Lymphatic System.

& READ CHAPTER 20 IN THIBODEAU & PATTON

Although the lymphatics are indeed part of the Circulatory System, they possess specific differences and functions to warrant a separate chapter of discussion. There are two main functions of the Lymphatic System: maintaining the fluid balance in the internal environment of the body and supporting the body’s immunity.

Being connected to, or perhaps more correctly, with the Circulatory System, the Lymphatic System must therefore possess a network of vessels that enable a circulating fluid to travel within the body: these are the lymphatic vessels and the fluid is simply called the lymph.

THE LYMPHATIC VESSELS

The lymphatic vessels share certain similarities with the blood vessels – particularly the veins with which they generally run, but, as structure governs function (at least for this discussion – it is just as true in reverse), we find some specific structural differences to aid the very specific functions of this system. Take a look at the typical structure of a lymphatic capillary. On first observing the vessel you might notice its similarities to a vein, with its pocket or semilunar valves lining the interior walls of the vessel (this just refers to the shape – similar shaped valves are found in the heart). This is, of course, to maintain lymph movement in only one direction, and as the lymphatic system is not pumped by the heart, it must therefore be subject to similar reverse flow pressures as experienced in the venous system. The venous system does maintain a regular, if not decreasingly weak pressure to drive the blood upwards; the lymphatics, conversely, do not enjoy the advantage of this pressure, and so the possibility of reverse flow is greater. It is for these very reasons that we find the first distinction between lymphatic and circulatory vessels: the lymphatic vessels are aided by many more ‘lock gate like’ semilunar valves. These are present every few millimetres in the larger vessels and even more so in the smaller ones, and control effectively fluid movement and direction.

A typical lymphatic vessel is very similar to a typical vein with the thick, connective tissue outer covering called the tunica adventitia, followed by the smooth muscular layer in the middle called the tunica media, and finally the innermost endothelium layer, the tunica intima, the vessel proper where the true cells of the vessel are to be found.


THE LYMPHATIC VESSELS CONTINUED……

Blood vessels, if you recall, are merely ‘transport vessels’, where blood is transported around the body and no exchange between the tissues takes place. In short they are simply specialised tubes. The lymphatic vessels are no different. However, as the blood vessels reduce in diameter and the two outer coverings become less prominent, they become single cell thick vessels called capillaries. It is the capillaries that are the ‘exchange vessels’ of the cardiovascular system: as they are just one cell thick, fluid and substances can move through and between the cells in and out of the tissues surrounding the capillaries. The lymphatic capillary is very similar, except that it is formed by a very large and flat, single layer of endothelial cells which overlap one another leaving rather large clefts or openings. This structural difference allows a greater degree of movement of fluids, or even larger molecules dissolved or carried within the fluids, to pass in or out of the lymph vessels.

Generally, you will find lymphatic capillary and blood capillary networks lying side by side each other.

The final point about the lymphatic vessels to mention here is that they do not form a closed circuit, or a circulatory ring as the cardiovascular vessels do. Instead they begin as microscopic, ‘dead end’ lymphatic capillaries, positioned between the intercellular or interstitial spaces of the soft tissues, where they act like ‘drains’ collecting excess tissue fluid. They eventually return the interstitial fluid to the venous blood supply via the subclavian vein, where it rejoins the main blood supply. The fluid is returned to the main systemic circulation via the major lymphatic vessel, called the thoracic duct, which runs almost parallel to the returning inferior vena cava, one of the major veins of the venous system. Lymph from the upper right quadrant is emptied by the right thoracic duct into the right subclavian vein. So the dead ended lymphatic vessels join to form larger vessels and these then merge to form the 2 main lymphatic trunks which as we have seen above, are the thoracic duct and right thoracic duct.

SUMMARY

In summary then, lymphatic vessels are structured in a similar way to veins, except they:

have thinner walls

contain more valves

contain lymph nodes at intervals along the vessels


FUNCTION OF LYMPH VESSELS

As I said, structure determines function, and in this section we will explore how the lymphatic vessels are perfectly designed to perform their vital functions. Acting like a ‘secondary circulatory system’, these vessels can absorb substances that either cannot, or have not, been collected by blood capillaries. Because lymphatic vessels are more permeable, tissue fluids, proteins, fats and other matter can be taken up and transported back to the blood. Lymphatic vessels also play a key role in digestion, as the microscopic villi, found in the small intestine contain a ‘lacteal’, which is a blind ended lymph vessel that absorbs fat and other fat soluble nutrients (more about this in the digestion lecture). These are dumped into the left neck vein. Finally, each lymphatic vessel contains lymph nodes along its route, and it is these lymph nodes that help the lymphatic system perform one of its crucial functions that of defence and protection.

LYMPH AND INTERSTITIAL FLUID

Like so many other areas of A&P, lymph and interstitial fluid are denominations of the same or similar fluids in different areas of the body. Lymph is the clear, watery fluid found in the lymphatic vessels. Interstitial fluid is a complex and organised material which, in some tissues is a semifluid ground substance, the organic matrix that makes up the structural network of bone and cartilage. In other areas it is bound water in a gelatinous ground substance. Both substances, though, are very similar and are, or should be, fluids that bathe and surround the cells. You might say here that these fluids could be classified as extracellular fluids and you would be partially correct. Extracellular fluid, though, is the fluid outside the cells, and is a combination of the watery plasma, found in the blood vessels, and of the interstitial fluid, surrounding the cells. Conversely, intracellular fluid is the water component found inside the cells. What about the lymph then? Well, lymph is also considered as an extracellular fluid, in so far that it is outside rather than inside the cells, although in terms of composition lymph and interstitial fluid are so similar that they could be regarded as the same material. To confuse matters further, lymph and interstitial fluids are also very similar in composition to blood plasma. However blood plasma contains a higher percentage of proteins so as to maintain a high osmotic pressure in the blood, which will always tends to draw water back into the plasma and hence in the capillaries. The exception to this rule is the lymph in the thoracic duct, which tends to contain twice the concentration of proteins compared to most interstitial fluid elsewhere. This is due to the fact that the lymph flowing into the thoracic duct has been enriched with proteins as a result of absorption, whilst flowing through the small intestines.


LYMPHATIC PUMP MECHANISMS

The lymphatic system, as mentioned earlier, is not subject to the constant pressure that is provided to the cardiovascular system from the heart and muscular blood vessels. However, this system does take full advantage of the other ‘pump mechanisms’, described in the venous return system, for example, the breathing pump and the musculoskeletal pump. The first pump mechanism, the breathing pump, allows for the maximum influx of lymph into the thoracic duct, and then on into the general circulatory system, at the moment of maximum inspiration. This is due to exactly the same reasons found in the venous return system, and follows the primary law of fluid flow, that ‘a fluid will move from a high to a low pressure zone’. As a result of the diaphragm descending during inspiration, intraabdominal pressure rises because the diaphragm compresses the abdominal organs and raises the internal pressure. Conversely and simultaneously, the pressure in the thoracic cavity has fallen, due to the fact that the thoracic cage has expanded. The intraabdominal increase in pressure raises the pressure of the abdominal region of the thoracic duct and thus a pressure gradient has been achieved. Following the laws of fluid dynamics, the lymph will flow slowly but surely ‘uphill’, along a lymph pressure gradient. The amount of valves found along each vessel, certainly aids this process, thus preventing the backflow of lymph and ensuring continuous movement in the right direction.

The second most important pump to the lymphatic system is the musculoskeletal pump which acts in exactly the same way, depending upon every contraction of skeletal muscle. The effect here is to literally ‘milk’ the lymph upwards and it is for this reason that so many semilunar valves are needed to control the direction of flow.

Of course, any postural movement or physical pressure applied to the body will influence lymph flow in some way, but it is these two main pumps, and the musculoskeletal pump in particular, that maintain the average flow of 3 litres per day of lymph back into the general circulatory system.

Massage is particularly good at speeding lymphatic flow when done with the direction of lymph drainage – and increased lymph flow means increased clearance of wastes in the interstitial space and so improved cell function.


STRUCTURE OF LYMPH NODES

Definition of phagocytosis: ingestion and digestion of particles by white blood cells.

Along the length of lymph vessels, you will find several clusters of lymphatic nodes. Lymphatic nodes, or glands, are the biological filters of the lymphatic system and are oval in shape or bean shaped. They vary in size from about the size of a pinhead up to the size of a large kidney bean. Look at the structure of a lymph node in detail. Note firstly that there are several afferent lymph vessels entering the lymph node, each one provided with a semilunar valve to ensure a ‘oneway system’ of lymph flow. Through the afferent lymph vessels the lymph flows into the node, where it then slows down and percolates through the sinuses, where ‘foreign’ material is consumed by phagocytosis. The ‘cleaner’ lymph then continues onwards until it collects at the hilus end of the gland where it exits the node via the single efferent lymph vessel. The lymph then continues its journey to reenter the lymph circulation and ultimately the general circulation. An artery and a vein will also enter and exit at the hilus. Each lymph node is encapsulated by a tough, fibrous membrane called the capsule. The trabeculae, or fibrous septa, extends from the capsule into the gland and splits the gland into compartments. The compartments that are formed are called cortical nodules and are packed full of lymphatic cells or lymphocytes. During an ‘infection’, germinal centres form, and it is here that new lymphocytes are produced and released, after reaching maturity. The centre of the node, or medulla, is composed of sinuses and medullary cords, lined with reticuloendothelial cells, or in other words fixed macrophages, which are capable of phagocytosis or consumption of ‘foreign’ material.

LOCATIONS AND FUNCTIONS OF LYMPH NODES AND LYMPH TISSUE

Most people have at some or other time been more than aware of where some of their lymph glands are situated, due to the fact that they have become enlarged and painful. There are six main clusters of these glands and each is responsible for a specific area of drainage and filtration of body fluids.

v Preauricular lymph nodes located just in front of your ears and responsible for draining the lateral sides of the superficial tissues and skin of the head and face.

v Submental and submaxillary nodes situated in the floor of the mouth, to collect and drain the lymph from the nose, lips and teeth.


LOCATIONS AND FUNCTIONS OF LYMPH NODES AND LYMPH TISSUE CONTINUED..

v Superficial cervical lymph nodes these quite often feel quite lumpy to an Osteopath or a Chiropractor as they are situated in one of the most complained muscular areas of the body, that of the sternocleidomastoid muscle, that runs up the side and front of the neck. These are secondary filters for the lymph draining from the whole of the neck and head.

v Superficial cubital or supratrochlear lymph nodes located just above the bend in the elbow and so drain and filter the lymph from the hands and forearm.

v Axillary nodes probably the most examined of all lymph nodes, these are a cluster of 20 or 30 nodes that reside deep under the arm in the axilla or armpit and in the upper chest regions. They drain the remainder of lymph from the hands and arm but also from the thoracic wall and the breasts.

v Inguinal nodes situated in the pit of the groin along and around the inguinal ligament and draining the lymph from the leg and from the external genitals.

Lymph nodes have two main functions. The first one is haematopoiesis, which is to provide a site for the final maturation of some lymphocytes and monocytes. The second function of lymph nodes is in providing support to the body’s defence mechanisms. This function is achieved by means of the structure of the sinus channels in lymph nodes, which, by curbing lymph flow, allows the sinus channels lining cells (the reticuloendothelial cells) to phagocytose foreign or toxic microorganisms. If, however, the number of these microorganisms overcomes the phagocytes, an infection of the node, called adenitis, will develop. In other cases, cancer cells that have become detached from a malignant tumour may drift in the lymphatics and eventually attach themselves to lymph nodes. There they may lead to new cancerous growths and hence congestion of the local lymphatic drainage system, thus creating a vicious circle.

THE LYMPHATIC DRAINAGE OF THE BREAST

There is an extensive network of lymphatic vessels and nodes that are associated with the breast. The breast, or mammary gland, is drained by two sets of lymphatic vessels.

v One set is superficial and drains lymph from the skin and surface areas of the breast, with the exception of the areola and nipple, and converge to form the cutaneous lymphatic plexus.

v The other set drains the breast proper, including the areola and nipple. This set is connected to the deeper subareolar plexus, or plexus of Sappey, situated under the areola, and is joined by the cutaneous lymphatic plexus in working together to drain the whole of the breast.


THE LYMPHATIC DRAINAGE OF THE BREAST CONTINUED……

Over 85% of the breast lymph will drain into the lymph nodes of the axilla, the remainder draining around the sternum or breast bone. Part of the breast tissue, going under the name of axillary tail of Spence, actually comes into contact with many of the larger axillary nodes, and it is maintained by allopathic medicine that this is the reason why breast cancer may spread to adjacent areas in both the lymphatics and tissue regions.

LYMPH LACTEALS IN THE DIGESTIVE VILLI

The lymph lacteals in the villi are worthy of a mention before we move on. Recall once again a single villus with a lymph lacteal, or specific lymph vessel, running up its centre. It is here that all fat and other substances are absorbed into the lymph and then later into the blood. Fat molecules are surrounded by bile salts in the GastroIntestinal lumen, where they are chemically digested further into fatty acids, monoglycerides and glycerol. The broken down fats are then absorbed into the cells of the villi where they are repackaged as chylomicrons. These are then passed out of the cell by a process known as exocytosis to enter the lymph lacteal and then be transported into the blood.

THE TONSILS

The famous tonsils are masses of lymphoid tissue situated underneath a protective ring in the membranes of the mouth and throat. Three sets of tonsils exist, the first are a pair of tonsils situated on either side of the throat and are named palatine tonsils. The second set are higher up towards the posterior opening of the nasal cavity and are called the pharyngeal tonsils, also known as the adenoids when they become swollen. The third and final set are the less known lingual tonsils, at the base of the tongue. All tonsils are considered to be the body’s first lines of defence against exterior ‘invasions’, and are often subjected to inflammations, named tonsillitis, which often leads to the disputable customary antibiotic treatment or even their removal. We have now seen that there are lymphatic vessels, lymph, lymphatic nodes or glands and even clusters of lymphatic tissue, but there are also two very important lymphatic organs to discuss.

THE THYMUS

The thymus is still a largely misunderstood organ of the body. However it has now been identified as a primary organ of the Lymphatic System, as well as being defined as an endocrine gland. The thymus is situated within the central area of the chest cavity between the lungs the mediastinum, resting upon the top of the heart.


THE THYMUS CONTINUED……..

The function of the thymus is similar to that of the lymph nodes, but its structure is quite different. The thymus consists of two pyramidshaped lobes, each subdivided into small lobules, composed of an outer cellular cortex and an inner medulla, which contains large corpuscular structures called thymic corpuscles or Hassall’s corpuscles. Although the lobules are still composed of lymphocytes, they are in this case enmeshed in an epithelial framework that is quite different from the ordinary lymphatic node. It is within these corpuscles that you will find densely packed lymphocytes in the lobules outer cortex, while not so densely packed lymphocytes, in the medulla of the corpuscle.

As mentioned earlier on, little is still known about the thymus and it is said to be ‘one of the body’s bestkept secrets’. However, it is believed to perform two important functions. Firstly, it is the site where final maturation of the T lymphocyte cells takes place in the foetus, as the foetus can only produce immature lymphocytes in its bone marrow.

After birth, the thymus also becomes active as an endocrine gland and begins secreting thymosin, a hormone that enables lymphocytes to develop into mature killer T cells which seek out and attack foreign bodies and abnormal cells. The T cells also serve to regulate the immune system, and so the thymus is not only part of the lymphatic and immune system proper but also helps to regulate it. To conclude, I would like to mention one interesting, and as yet not fully explicable point about T cell production: approximately 90% of all T cells produced in the thymus ‘die’ while still in the organ – some of these are T cells that would react against the body’s own cells – but this is not the complete answer.

THE SPLEEN

The spleen is located in the left side of the body, just below the diaphragm and above the left kidney, tucked neatly under the fundus of the stomach. Unlike other glands of the lymphatic system, only two vessels enter ?? only one vessel enters and one vessel leaves ?? the spleen. These are the splenic artery and splenic vein respectively. The splenic artery leads into the hilum of the spleen and very quickly spreads out into a large, dense capillary network, having the effect of engorging the spleen with blood. Once more, we find that as the blood is filtered out into the spleen, and its dense lymphatic nodules packed full of lymphocytes, it may undergo several changes before being collected in the venous sinuses and finally returning to the general circulation via the splenic vein.


THE SPLEEN CONTINUED…..

The spleen has many important functions, namely defence, hematopoiesis and red blood cell and platelet destruction. Finally last, but not least, the spleen provides a huge blood reservoir. As we will be discussing the Immune System later on, I have kept the role of the Lymphatic System concerning defence to a minimum. However, it is worth mentioning that the spleen’s role in the immune system is one of ‘eating’ foreign and damaged cells. The macrophages, or reticuloendothelial cells, that line the venous spaces in the interior of the spleen, remove and literally ‘eat’ any passing microorganism in the blood by the process of phagocytosis. It also seems to act as another site for maturation of the Immune System cells, the socalled leukocytes. The spleen can be viewed as the blood’s own lymph node. There are two types of leukocytes, simply defined as granular leukocytes, or leukocytes with visible granules in their cytoplasm, and nongranular leukocytes, or leukocytes without any visible granules in their cytoplasm. It is the nongranular leukocytes that are found to mature in the spleen (hematopoiesis) and it is these cells that are mostly responsible for phagocytoic activities. Hematopoiesis is further accomplished in the spleen by the formation of red blood cells before birth, and in very rare circumstances after birth

We find that the macrophages, lining the spleen’s sinusoids, also ‘eat’, or destroy, worn out and damaged red blood cells and platelets. The macrophages draw the platelets and red blood cells into themselves by phagocytosis, and then set about breaking up the valuable haemoglobin molecules, hence salvaging the precious iron. Once this process has been fulfilled, the macrophages release these materials back into the blood for transportation and storage in the liver and bone marrow.

The final major function of the spleen is a vital and often life saving one, consisting in providing a blood reservoir. The spleen can hold up to about 350 ml of blood: if for any reason there is a drop in the volume of the blood then the spleen, via sympathetic stimulation from the autonomic nervous system, will produce marked constriction of the smooth muscular capsule, resulting in the spleen giving up most of its blood into the general circulation. This may appear as a small amount, but it can prove to be a possible life saver in extreme cases, like for example, if a haemorrhage was to occur, you can give yourself a blood transfusion (at least you know where its been)!


CONCLUSION

The Lymphatic System is primarily responsible for two very important functions: fluid balance and defence. As part of the fluid balancing function, the lymphatic vessels act as drains that create a huge network for collecting any excess fluid that surrounds the tissue cells. Within the lymph, large fat and protein molecules may be carried to be either: returned to the general circulatory system or broken down in the lymphatic glands and organs. The lymphatic glands and organs in general act as biological filters, able not only to filter out debris and microorganisms, but also to destroy them by engulfing them in the numerous macrophagic cells that line these glands and organs.

The lymph flows slowly but surely around the body maintaining a constant balance of fluids that bathe the cells. It maintains its flow by the use of the same ‘pump mechanisms’ that are to be found in the venous return system, the respiratory pump and the musculoskeletal pump.

Without the adequate drainage facilities that the Lymphatic System offers, and the equally efficient filtration and cleansing functions of its glands and organs, the blood and then ultimately the cells would find themselves quite literally in ‘deep water’!

PATHOPHYSIOLOGY

Using the websites reviewed in the Respiratory System lecture, explore the following conditions:

v Lymph Vessels: Lymphoedema (lymphedema in USA), lymphangitis

v Lymph Nodes & other Lymphatic organs: Lymphoma (Hodgkin’s & nonHodgkins lymphoma), tonsillitis, splenomegaly

ANATOMY & PHYSIOLOGY SERIES · the pulmonary circulation. Then the oxygenated blood is returned to...

Documents

Transcript of ANATOMY & PHYSIOLOGY SERIES · the pulmonary circulation. Then the oxygenated blood is returned to...