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 node 3.Bundle of His 4.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 Trained80-110ml ml Untrained60-80ml ml

Cardiac Output Response to Exercise Resting Cardiac Output Submaximal Exercise Maximal Exercise Trained5L/min15-20L/min30-40L/min Untrained5L/min10-15L/min20-30L/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 ORGANAT REST (CM3)% BLOOD FLOW MAX EFFORT (CM3) % BLOOD FLOW Skeletal muscle Coronary vessels Skin Kidneys Liver/gut Brain Whole body

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