Section A: Applied Anatomy and Physiology

9. Structure and function of the heart

Syllabus • Internal and external structure of the heart, to include the heart

chambers and valves, all blood vessels attached to the heart, the heart wall and pericardium

• Conduction system of the heart, cardiac cycle • Definitions and relationship between cardiac output, stroke

volume, heart rate: differences in values at rest and during exercise • Regulation of heart rate to include, neural, hormonal and intrinsic

factors• Measurement of heart rate response to varying intensities of

workload• Heart rate response during recovery, with a graphical

representation of data

Section A: Applied Anatomy and Physiology

10. Function of the vascular system

Syllabus

• Pulmonary and systemic circulatory systems • Factors linked with venous return • Distribution of cardiac output at rest and during

exercise, to include the vascular shunt mechanism, the role of precapillary sphincters and the role of the vasomotor centre

• Blood flow, blood velocity, blood pressure and the effects of exercise on blood pressure

• Oxygen and carbon dioxide transport

Location of the Heart

• Size of a closed fist • Rests on the diaphragm • Near midline of thoracic cavity • Lies in the MEDIASTINUM – Between the lungs– From sternum to vertebral column

Pericardium

• Pericardium (Fibrous)– Prevents overstretching – Provides protection – Anchors the heart

Layers of the Heart Wall

• Epicardium (slippery texture of outermost surface)– External layer

• Myocardium (cardiac muscle)– Middle layer

• Endocardium (covers valves)– Inner layer

External Structure

Internal Structure

Electrical System of the Heart

• Auto-rhythmic fibres– Cardiac muscle fibres are self-excitable

– They act as a pacemaker – They form the conduction system

The Conduction System Sequence

1. Sino-Atrial node 2. Atrioventricular node3. Bundle of His4. Right and left bundle branches 5. Purkinje fibres

Control of the Heart

• Two mechanisms that help to control the heart rate

– Neural – Hormones

Autonomic Regulation of Heart Rate

• Cardiovascular Centre (Brain) – In the MEDULLA OBLONGATA

• Receives input from sensory receptors and higher brain centres

• Direct appropriate output by increasing or decreasing frequency of nerve impulses

• Cardiac acceleratory centre (sympathetic)• Cardiac inhibitory centre (parasympathetic)

Autonomic Nervous System Divisions

Sympathetic Nervous System• Increases HR by releasing

adrenaline and noradrenaline

• Adrenaline increases strength of contraction

• Noradrenaline aids the spread of the impulse throughout the heart

Parasympathetic Nervous System

• Slows HR by releasing acetylcholine

• Acetylcholine slows the spread of impulses and therefore reduces the heart rate

Cardiac Function of Autonomic Nervous System

Sympathetic Function • Increased HR• Increased strength of

contraction • Vasodilation of arteries

supplying muscles • Some vasoconstriction of

arteries of abdomen etc

Parasympathetic Function• Decreased HR• Decreased strength of

contraction • Vasoconstriction of arteries

supplying muscles • Vasodilation of arteries of

abdomen etc

Neural Factors

• Proprioceptors– Found in muscles, tendons, joints– Informing where movement is occuring

• Chemoreceptors– Located in aorta and carotid arteries– Informing of composition of blood – CO2

• Baroreceptors– Found in aorta, atria and vena cava– Inform of changes in blood pressure

Chemical Regulation of Heart Rate

• Chemical influence– Hypoxia (lowered oxygen levels)– Low OR High pH – ALL DEPRESS CARDIAC ACTIVITY

• Hormones– Epinephrine and norepinephrine increase HR and

contractility

Other Factors affecting Heart Rate

• Temperature– Blood viscosity

• Age – Younger people tend to have higher heart rates

• Gender – Women tend to have higher heart rates

Cardiac Dynamics

• The performance of the heart is largely dependent on two variables:

• Stroke volume• Heart rate

• Multiplying these two variables will provide you with an individual’s CARDIAC OUTPUT

CO = SV × HR

• An average person has a resting heart rate of 70 beats/minute and a resting stroke volume of 70 mL/beat.

• The cardiac output for this person at rest is:

Cardiac Output = 70 (beats/min) X 70 (mL/beat) = 4900 mL/minute.

Cardiac Output Facts

• Your entire blood volume flows through your pulmonary and systemic circulations EACH MINUTE

• MILD EXERCISE– HR (100bts.min) and SV (100ml.beat)

• INTENSE EXERCISE (not maximal)– HR (150bts.min) and SV (130ml.beat)

Stroke Volume

• The volume of blood pumped out the heart per beat

• Usually refers to the blood ejected from the left ventricle

• Typically the resting value is 75ml (non-trained individual)

Stroke Volume: Determining Factors

• Venous return – Volume of blood returning to right atrium

• Elasticity of cardiac fibres (Starlings Law)– Also known as pre-load– Degree of stretch prior to contraction

• Contractility of cardiac tissue

Cardiac Dynamics during Exercise

• Heart Rate Response– Extent of increase is dependent on extent of intensity

• Steady state • Anticipatory rise

• Stroke Volume Response– At 40-60% of maximum effort it plateaus – Able to increase for 2 reasons

• Increased venous return (due to muscle pump)• Frank-Starling mechanism (stretch more/contract more)

• Cardiac Output Response – During maximum exercise, Q may reach values of 8 times that

of resting

Stroke Volume Response to Exercise

Resting Stroke Volume

Submaximal Exercise

Maximal Exercise

Trained 80-110ml 160-200ml 160-200mlUntrained 60-80ml 100-120ml 100-120ml

Cardiac Output Response to Exercise

Resting Cardiac Output

Submaximal Exercise

Maximal Exercise

Trained 5L/min 15-20L/min 30-40L/minUntrained 5L/min 10-15L/min 20-30L/min

A Trained Heart

• Cardiac hypertrophy– Endurance athletes tend to display larger ventricular cavities– High-resistance strength training will display thicker ventricular

walls

• Bradycardia– Reduction in the resting heart rate that accompanies training

• Ejection fraction – Proportion of blood actually pumped out of the left ventricle per

contraction – SV divided by EDV (end diastolic volume)

Blood

• Consists of cells and cell fragments surrounded by plasma

• Average male has between 5-6L

• Average female has between 4-5L

Functions of Blood

• Transportation of nutrients (glucose and oxygen)

• Protection and fighting disease through interaction with lymphatic system

• Maintenance of homeostasis (temp regulation, enzyme and hormone action, pH balance)

Blood Viscosity

• The relative thickness of blood

• Ratio of blood cells to plasma (dehydration)

• Training brings an increase in the total blood volume and therefore an increase in the number of red blood cells. Plasma volume increases more facilitating flow!

Arteries & Arterioles

• High pressure vessels

• Constant subdivision decreases diameter

• As network subdivides, blood velocity decreases

• Arterioles have more smooth muscle than elastic tissue in the tunica media– Allows vasodilation and vasoconstriction – Regulate blood pressure – Enable efficient delivery and exchange of gases

Functions of Arteries & Arterioles

• Act as conduits

• Cushion and smooth out the pulsatile flow of blood from the heart

• Help control blood pressure

Veins and Venules

• Low pressure vessels

• Possess less smooth muscle and elastic tissue

• Veins gradually increase in thickness the nearer the heart

• Thinner walls often distend and allow blood to pool in them (pocket valves)

• Up to 70% of blood volume is found in venous system at any one time at REST

Capillaries

• Functional units of vascular system

• Single layer of endothelial cells

• Pre-capillary sphincters – controls and regulates volume of blood entering capillary bed

Venous Return Mechanism

• Venous return – blood returning to right side of heart via veins

• Several mechanisms aid this;– Muscle pump – Pocket valves – Respiratory pump– Smooth muscle – Gravity

Blood Pressure

• The force exerted by the blood against the walls of the vessels

• BLOOD PRESSURE = CARDIAC OUTPUT MULTIPLIED BY RESISTANCE

• Systolic pressure – heart pumps• Diastolic pressure – heart relaxing/filling

Exercise and BP

• Steady aerobic exercise– Systolic increases as a result of increase cardiac output – Diastolic remains as blood feeds into muscles due to

increased ateriole dilation

• High-intensity isometric and anaerobic exercise – Both rise due to increased resistance of blood vessels– Increased peripheral resistance (veins) – Increased intra-thoracic pressure due to contraction of

abs

Q at Rest and during Exercise ORGAN AT REST (CM3) % BLOOD

FLOW MAX EFFORT (CM3)

% BLOOD FLOW

Skeletal muscle 1000 20 26000 88

Coronary vessels

250 5 1200 4

Skin 500 10 750 2.5

Kidneys 1000 20 300 1

Liver/gut 1250 25 375 1.25

Brain 750 15 750 2.5

Whole body 5000 100 30000 100

Transport of Oxygen

• Approx. 97% of oxygen is carried by RBCs (haemoglobin)

• Each molecule of haemoglobin can combine with 4 molecules of oxygen (1.34ml)

• Concentration of Hb is about 15g per 100ml• Each 100ml can transport up to 20ml of oxygen

• Haemoglobin + Oxygen = Oxyhaemoglobin Hb + O2 = HbO2

Transport of Carbon Dioxide

• Approx. 8% is dissolved in blood plasma

• Up to 20% combines with Hb to form carbaminohaemoglobin

• Up to 70% is dissolved in water as carbonic acid