Physiology, Lecture13, CVS 1

8/8/2019 Physiology, Lecture13, CVS 1

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Cardiovascular system

Shadi karkabi


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The cardiovascular system is composed of 3 parts:

1)

H eart : as a pumping organ.2)

B lood vessels : there are the arteries taking the blood from the heart to the tissues andthe veins bringing the blood from the tissues to the heart.

3)

Capillaries : where the exchange between the blood and the tissue will take place.

If you take the system again it is composed of two units: one unit is the respiratory circulation(or pulmonary circulation) which is connecting the heart to the lungs and the other one is the

systemic circulation , which is connecting the heart to the peripheral tissue.

I m going to explain the systemic circulation, the pulmonary circulation is going to be

explained by Dr Naif, now let s take one by one, heart , blood vessles and then the capillaries.

Heart:

A natomical view: You have two atria, and two ventricles, the two atria actsimultaneously as one unit , the two ventricles also contract simultaneously as one unit.

They are separated by the atrioventricualr valves ( AV valves) , and there are two valves, one isthe mitral valve that separates the left atrium from the left ventricle and the right one, thetricuspid valve , seperates the right atrium from the right ventricle

Concerning the Mitral valve, you have two cusps, two chordate tendinae and papillary musclesand the same thing here for the tricuspid valve , the flow of the blood is always normally in

one direction , from the inferior and superior vena cava to the right atrium, from the rightatrium to the right ventricle and the same thing from the pulmonary veins to the left atrium

then to the left ventricle to the aorta

You have two great arteries, one is the aorta (red) and the other one is the pulmonary artery(blue). The blood going from the right ventricle to the pulmonary circulation through thepulmonary artery and the aorta from the left ventricle to the aorta to the peripheral tissues

and these two great vessels are separated from the ventricles by valves and these valves are

called semilunar valves .

AV valves separate the atria from the ventricles

Semilunar valves separate the great vessels from the ventricles


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Physiological view: anatomically you have 4 chambers which are two atria and twoventricles, but physiologically the heart is divided into two units , the right and the left sides,the right side is the right atrium + the right ventricle, and the left side is the left atrium + the

left ventricle, anything happening in the right is the same thing happening in the left , but

there is one exception which is in the left side it is the side of high pressure and the right sideis the side of low pressure because the left ventricle is pumping the blood to the peripheral

circulation, the distance on the left side is much more than the distance on the right side that s

why it has higher pressure.

you keep in mind from now on, when we talk about the heart, like when we say for example

the function, cardiac output, systolic or diastolic, always we relate that to the left ventriclelike when we say the blood pressure, we relate that to the left ventricle.

If you look at the anatomy of the heart you will see the thickness is much more in the left side

that s why the force generated by the left ventricle is much more than the force generated by

the right ventricle

so simply when you look to the heart in here, the right atrium, the blood comes from the

inferior and superior vena cava then passes through the tricuspid valve then to the right

ventricle, from the right ventricle to the pulmonary artery across the semilunar valve which

then goes to the lungs, while in the left side, the blood comes through the pulmonary veins to

the left atrium and then to the left ventricle, to the aorta, finally going to the peripheralcirculation.

H istological view: the histology of the cardiac muscle, as you can see in here, eachmuscle fiber in the heart is divided and then they are united again, so the difference between

the cardiac muscle fibers and the skeletal muscle fibers is that the skeletal muscle fibers areadjacent to each other, so when you want to increase the power of contraction, it depends on

the weight , so the more you have weight the more you will have fibers , in the cardiac muscle

it is a different story, each fiber is divided and united as we said so what s happening here is

all the fibers are as a one unit , they are as a single skeletal fiber, either there is stimulus andcontraction or there is not , so there is no way to increase the contraction by increasing the

number of fibers because actually there is no numbers, it is only one unit, it is ALL or NONE

law, either there is contraction or there is no contraction, that s why there are called sensitial,


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sensitial means unit, so both atria they are atrial sensitial, both ventricles they are ventricular

sensitial, one unit.

So how the contraction is going to be increased? In the skeletal muscles we said it s thenumber of fibers, in the cardiac muscle it depends on the length of the fibers, that is thefrank-starling law , exactly if you imagine the elastic thread , if you hold it from one side and

then you stretch it, the more you stretch it the more it will hit your finger, so the more youstretch it the more force, if you stretch it too much, you will destroy the elasticity, this is what

happens in heart failure . Normally, within physiological limits, the more you stretch themyocardium (the cardiac muscle) the more you have contraction; if you stretch it too much

you ll have heart failure.

So the contraction here depends on the length , a good example of that is during exercise moreblood is coming, so more blood is pumped, so more blood more stretching more

contraction. On the other hand during sleeping , the metabolism decreases less blood willcome less contraction less force.

The lines you see in the picture are called the intercalated discs , for excitation they have verylow ressitance for transporting the electricity (the excitation in the heart is electricity) so theyallow the electricity to pass easily, that s why at any time we stimulate the heart, all the atria

will stimulate together and then the impulse will go to the ventricles and both ventricles will

stimulate together.

H ow is the heart stimulated? Where is the pacemaker?

The heart is composed of 3 types of fibers:

1) V entricular fibers : which are contractile fibers.2) A trial fibers : which are also contractile fibers, the difference between the two is the force of contraction.

3) Conductive fibers : they are present in the SA node (sinoatrial node), in the AV node(atrioventricular node) and in between those two nodes we have the internodal fibers , afterthe AV node we have the AV bundle or bundle of H is, then we have two branches, the leftbundle branch and the right bundle branch and then the last part of this system is thepurkinje fibers.


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So all of

them, the

SA node, AV

node,internodal

fibers, AV

bundle,

right and

left

branches

and the

purkinje

fibers, wecall them

the

conductive system , they are neither muscle because they are not contractile, nor nerves,because they don t transmit the impulse as fast as in the nerves, in the nerves the

transmission of the impulse is very high, in those fibers there is transmission of impulse but is

not as fast as in the nerves and they are not contractile as the muscles, that s why they are

specialized fibers , they have the ability to generate the impulse and to transmit it. Again, you

have only one way direction , the impulse is started at the SA node going to the AV node, tothe AV bundle to the 2 branches and finally to the purkinje fibers, the impulse cannot go back. The separation between the two atria and the two ventricles is by a fibrous tissue which is

called atrioventricular fibrous ring or tissue , that tissue is electrically resistant , doesn t allowelectricity to pass, it is the opposite of intercalated disc, so there is no way for the current or

for the impulse to pass from the atria to the ventricles except through the AV node.

The excitation starts in the SA node which is the pacemaker of the heart, but why?

The resting potential is as you know -90 or -80, the resting potential of the SA node is higher

than that, it is -60 , it is higher than any other resting potential in the body, and the structure of the SA node it is leakage to Na ( ) it allows the Na to get in, so it increases the resting


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potential gradually until it reaches to the threshold which is 4 0 (he showed that on the slideand explained the slow depolarization and during this period the Na is leaking and will get inuntil the resting potential will increase and reach to 40 and then the action potential will start)

So the SA node is the pacemaker of the heart for 2 reasons:

1) H igher resting potential

2) Leakage to Na .

That s why it generates impulses higher than all the other parts of the conductive system, all

parts of the conductive system can generate impulses, but the higher rate is in the SA node.

What will happen now to the impulse? It will start in the SA node and will go to the atria and

the AV bundle, it will stimulate the two atria as one unit, now in the AV node, the impulse will

now cross straight away to the AV bundle, it will be delayed, the velocity will be less to give

time for the atria to contract, because if there no delay the impulse will pass to the ventriclesand all the chambers will contract together, so when the impulse reaches the AV node the

velocity will be decreased and that will give time for the atria to contract,

Now the question is why the velocity is decreased at the AV node? The fibers in the AV node

are very few in number, and they are very small , so they need time to generate voltage, that swhy there is a delay

once it reaches the AV bundle, it will go to the two branches and then to the purkinje fibers, as

you can see the numbers of the purkinje fibers are too many that s because the two ventricles

are two large masses so we need a system to increase the velocity without that the ventricleswill need very long time to be stimulated, that s why we have too many purkinje fibers and the

velocity of transmission in the purkinje fibers is very fast, so it is exactly the opposite of the AV

node, they are Many in number, and Large in size

So after reaching the purkinje fibers there is the resting potential, now what will happen to the

next beat? The time between one beat and the next beat is the time where the heart will rest

and this is called the resting potential and in this period where we have the leakage of Nawhich will lead to the next beat.

you have numbers on this slide, the numbers start from zero time which is the SA now, once

the impulse starts in the SA node it will spread to both atria reaching to the AV node, by the

time from zero until it reaches to the AV node it is only 3 0 ms (millisecond) and for both atria


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to be excited it takes 60 ms (the Dr you said you don t have to know the exact numbers butthere is something else he wants you to know) the delay in the AV node is 13 0 ms, so how longthe impulse will take from the SA node until it reaches to the ventricles? 30+130 = 160 ms, sonow the impulse is in the ventricles, from 160 until all the chambers of the ventricles to be

excited it takes 60 ms, so from the zero time until the ventricles get excites it is 22 0 ms. this isthe normal impulse


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The action potential in skeletal muscles is composed of depolarization (leakage of Na) and

then repolarization (opening of K channels) this is the simple excitation which develops in the

skeletal muscles , that s why the skeletal muscle is very fast, but in the heart it is not the same,

the action potential of the heart is different from the action potential of the skeletal muscle, in

the action potential of the cardiac muscle, you have the depolarization and then by the timeall the Na channels are open the repolarization starts, it cannot continue because there is

another channel that opens which is the Ca, and the calcium channels will remain opened and

this is what we call the plateau (same line), during this plateau we will have the contractionand then the repolarization.

y

Skeletal muscle action potential = depolarization and repolarization

y

Cardiac muscle action potential = depolarization, plateau and repolarization

The heart rate is affected by both the Na and the K, if you have too much Na you ll havevery fast rate, if you have too much K, you ll have less rate.


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The heart contraction is affected by the Ca, the more Ca the more contraction and viceversa.

Electrocardiogram (ECG) it is the recording of the action potential, when you want to recordelectricity you will need two electrodes and volt meter, concerning the two electrodes one has

to be positive and the other negative, now it depends on the current, the direction of current,

if the current is going towards the positive electrode it will cause positive deflection, if the

current is going towards the negative electrode, it will give negative deflection, that is thesame for the potential, now imagine that on the heart, when the excitation starts in the SA

node which is located in the right side of the heart, and when we connect two electrodes one

on the right arm and one on the left arm (right

hand ve and the left hand +ve) always keep in

mind that the right side is negativebecause always excitation starts in the right

side, our body is composed of tissues that

contains fluids and ions all over, and it is good

for conduction of electricity anywhere in thebody, the electricity will reach the surface where

the electrodes are, but keep in mind that the

voltage recorded is less than the actual voltage

Einthoven triangle: the man who first recorded the ECG is called Einthoven, he assumedthat the heart is in the chest

cavity and the chest cavity he

assumed it as a collateral

triangle and the heart is at thecenter of that triangle, so the

right side is negative and the left

is positive and he connected the

right arm with the left leg, and

this is called lead II, so lead I is


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between right arm and left arm, and lead II between right arm and left leg, lead III connects

the left arm to the left leg

In lead I the direction is from right to left so you have a positive deflection

In lead II the direction is from the right arm to the left leg again you have a positive deflection

In lead III the direction is from the left arm to the left leg and the direction of the current is

again towards the positive.

Summary:

Always keep in mind, the current which is going towards the positive electrode gives

positive deflection, the current which is going away from the positive electrode towards

the negative electrode gives negative deflection

always remember the SA node is the source of excitation

always the right side is negative because excitation starts there

always the left side is positive

always keep in mind that the SA node is the pacemaker of the heart because of higher

resting potential and leakage of Na,

the delay in the AV node is because of the fibers are few in number and small in size,

the fast velocity in purkinje fibers is because the fibers are many in number and large in

size

Where is the least resistance? (the less the resistance the more the velocity) it is in the

purkinje fibers

Where is the highest resistance it is in the AV node

Do ne By:

Karkabi Shadi.

Physiology, Lecture13, CVS 1

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