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heart receives oxygenated blood from the lungs via the Mammalian Heart Structure The left side of the heart receives oxygenated blood from the lungs via the PULMONARY VEINS The heart is the major organ of the circulatory system Superior VENA CAVA Inferior AORTA It is a fist-sized muscular pump consisting of four chambers The left side of the heart pumps oxygenated blood out into the body’s arteries via the AORTA The human heart recirculates the entire blood volume (5 dm3) every minute when the body is at rest PULMONARY ARTERY Deoxygenated blood returns to the right side of the heart via the VENA CAVA The ability of the heart to perform such work is due to the presence of specialised cardiac muscle in its walls CORONARY ARTERIES Deoxygenated blood is pumped to the lungs via the PULMONARY ARTERY The job of the heart is to pump blood around two separate circuits Heart muscle receives its own supply of blood from the CORONARY ARTERIES

Mammalian Heart Structure Aorta Pulmonary artery Vena cavae Pulmonary veins Semilunar valves Left atrium Right atrium Bicuspid valve Tricuspid valve Right ventricle Left ventricle Septum (dividing wall) Cardiac muscle

Mammalian Heart Structure The mammalian heart is a muscular pump that consists of four chambers Two upper chambers, called the atria, are thin walled cavities that receive blood from veins Two lower chambers, called the ventricles, are thick walled cavities that receive blood from the atria and pump blood away from the heart The cavity of the heart is divided completely by a partition called the SEPTUM Right atrium Left The muscular walls of the heart are referred to as the myometrium and consist of specialised cardiac muscle cells Right ventricle Left Septum The thicker walled structure of the left ventricle is a consequence of the distance over which it is required to pump blood

The direction of blood flow through the heart is maintained be valves Between the right atrium and the right ventricle is the TRICUSPID VALVE Right atrium Left ventricle Aorta This valve prevents the backflow of blood from the right ventricle to the right atrium Pulmonary Artery Between the left atrium and the left ventricle is the BICSUPID VALVE OR MITRAL VALVE This valve prevents the backflow of blood from the left ventricle to the left atrium Semilunar valves Bicuspid or Mitral valve The bicuspid and tricuspid valves are collectively known as the ATRIO-VENTRICULAR VALVES or AV valves Tricuspid valve Pocket-shapes valves known as SEMILUNAR VALVES are located at the base of the arteries responsible for transporting blood away from the heart

The average human heart rate at rest is 72 beats a minute The Mammalian Heart The average human heart rate at rest is 72 beats a minute Each heart beat lasts for approximately 0.8 of a second at rest Each heart beat involves a series of events referred to as THE CARDIAC CYCLE

THE CARDIAC CYCLE is the sequence of events taking place during ONE COMPLETE HEARTBEAT A single heartbeat may be divided into two major phases known as SYSTOLE AND DIASTOLE The Cardiac Cycle Systole describes periods when the heart is contracting Diastole describes periods when the heart is relaxing

Pressure Changes during the Cardiac Cycle Throughout the cardiac cycle, pressure changes take place in the atria, ventricles and arteries Pressures in the right and left atrium, right and left ventricle, aorta and pulmonary arteries can be recorded and illustrated in graphical form The graph on the next slide shows pressure changes in the left side of the heart and the aorta A similar graph can be drawn for the right side of the heart and the pulmonary arteries Such a graph is similar in shape to that obtained for the left side of the heart but all the pressures readings are of a lower value

aortic pressure left ventricular left atrial Pressure Changes in the Left Side of the Heart W A X Y A Z Period Z to A represents the phase of Passive Filling of the ventricles when the AV valves are open and the semi-lunar valves are closed aortic pressure left ventricular left atrial Period A to W represents the phase of Atrial Systole when the atria contract and the ventricles are filled to full capacity Period W to X represents the first phase of Ventricular Systole when the ventricles contract in an isometric fashion; the greatest rise in ventricular pressure occurs during this phase and the ventricular volume remains constant Period X to Y represents the second phase of Ventricular Systole when ejection of blood takes place and pressure in the aorta rises Period Y to Z represents relaxation of the ventricles (diastole) when the ventricular pressure drops sharply

Period Z to A represents the phase of Late Diastole when all chambers of the heart are relaxed, atrial and ventricular pressures are low and the aortic pressure is falling The AV valves open at the beginning of this phase and the semi-lunar valves are already closed Passive filling of the ventricles takes place during this phase

Period A to W (Atrial Systole) begins as the atria contract filling the ventricles to their full capacity Both the atrial and ventricular pressure curves rise slightly at this time, as additional blood is forced into the left ventricle At the end of atrial systole, the increased blood pressure in the ventricles forces the AV valves to close

Period W to X (the first phase of Ventricular Systole) begins as the ventricles contract in an isometric manner Both the AV and semi-lunar valves are closed and the steeply rising pressure in the left ventricle reflects the increasing muscle tension created by the ventricular muscles No blood enters or leaves the ventricles during this phase and the aortic semi-lunar valve is forced open at the end of this phase

Period X to Y represents the second phase of Ventricular Systole when blood is ejected from the left ventricle As the semi-lunar valves open, blood is ejected into the aorta and pulmonary arteries (right side) The ventricular and aortic pressures are the same throughout this period and both pressures reach their highest value (around 120 mm Hg)

Period Y to Z represents the phase of Ventricular Relaxation (Diastole) Backflow of blood from the aorta closes the semi-lunar valve at the beginning of this phase The ventricular pressure drops sharply as the ventricle relaxes and the ventricular blood volume remains constant as the AV and semi-lunar valves are both shut The AV valve opens at the END of this phase as the atrial pressure is slightly greater than that in the ventricle

As the AV valve opens, passive filling of the ventricle starts during phase Z to A and the cycle begins again

Summary of Valve Movements during the Cardiac Cycle W A X Y A Z aortic pressure left ventricular left atrial The AV bicuspid valve opens at the beginning of Phase Z to A (Passive filling of the ventricles) Aortic semi- lunar valve closes The AV valve opens as the pressure in the atrium is slightly greater than that in the ventricle Aortic semi- lunar valve opens The AV valve closes at the end of atrial systole (Period A to W) The aortic semi-lunar valve opens at the end of Isometric Ventricular Systole following a steep rise in pressure in the left ventricle AV bicuspid valve opens The aortic semi-lunar valve closes at the end of Ventricular Ejection due to a slight backflow of blood from the aorta AV bicuspid valve closes As the ventricle relaxes during diastole, the pressure falls slightly below that of the atrium and the AV bicuspid valve opens again

Ventricular Volume during the Cardiac Cycle relaxation (diastole) Ventricular Systole (isometric phase) Atrial systole Ventricular ejection (systole) Passive filling (late diastole) 100% 70% 0% % Capacity Time (seconds) Ventricular volume decreases sharply as blood is ejected into the arteries Ventricular volume rises sharply and then levels off as ventricles fill to 70% of their capacity Ventricular volume rises sharply as ventricles fill to capacity Ventricular volume remains constant as all valves are closed Ventricular volume remains constant as all valves are closed

The Origin of the Heartbeat The mammalian heart is special in that the electrical stimulation necessary for contraction of its muscles originates from within the heart itself Within the heart there is a network of specialised cardiac muscle cells designed for initiating each heart beat and for the rapid and co-ordinated spread of excitation This network of specialised cardiac muscle cells is known as THE CONDUCTION SYSTEM This conduction consists of: the Sino-atrial node (known as the SA node or pacemaker) the Atrio-ventricular node or AV node the Bundle of His conduction fibres called Purkinje fibres As the stimulus for contraction of the heart originates from within cardiac muscle, the heartbeat is described as being MYOGENIC

The Conduction System of the Heart The origin of the heartbeat is from within a specialised patch of cardiac muscle tissue, located in the wall of the right atrium, and known as the sino-atrial node or SA node Another node of specialised tissue known as the AV node is located in the right portion of the septum between the atria and close to the AV valves AV node SA node in wall of right atrium Bundle of His with left and right bundle branches The left and right bundle branches divide into smaller branches called Purkinje fibres that spread throughout the ventricular muscle The AV node connects with a bundle of large fibres called the bundle of His, which divides into left and right bundle branches

The Conduction System of the Heart When the SA node emits spontaneous electrical impulses, they spread rapidly across both atria due to the inter-connecting nature of the cardiac muscle cells As the impulses spread across the atria, they stimulate a wave of contraction within the atrial walls and atrial systole is triggered When the electrical impulses reach the border between the atria and ventricles they are blocked by a band of non- conducting fibrous tissue Fibrous Tissue Impulses are conducted from AV node along the bundle of His AV Node In order to reach the ventricles, electrical impulses must pass through the AV node, which slows down the speed of electrical transmission The bundle fibres divide into numerous Purkinje fibres that permeate throughout the ventricular muscles This delay, called the AV delay, is extremely important as it allows the atria to complete their contraction before the ventricles begin to contract The spread of electrical impulses throughout the ventricles triggers ventricular systole

The Electrocardiogram (ECG) The contraction of muscles is associated with electrical changes called ‘depolarisation’, and these changes can be detected by electrodes attached to the surface of the body When the electrical changes associated with cardiac muscle contraction are inscribed on a ruled strip of paper, they provide an electrocardiogram that is a permanent record of cardiac activity In order to understand the ECG trace, it is necessary to consider the electrical properties of cardiac muscle

Electrical Properties of Cardiac Muscle During diastole, when cardiac muscle cells are at rest, they display an unequal distribution of charge across the membrane At rest, cardiac muscle cells are internally negative and externally positive Cardiac Muscle Cell Intercalated Disc This charge difference creates a small voltage across the membrane, which can be detected by electrodes across the chest wall When cardiac muscle cells are at rest and therefore internally negative and externally positive, they are described as being POLARISED

Electrical Properties of Cardiac Muscle When a stimulus from the SA node spreads along the cardiac muscle cell membranes, there is a reversal of the charge distribution Stimulus from SA node Cardiac Muscle Cell Intercalated Disc The muscle cell is now internally positive and externally negative and the cell is described as being DEPOLARISED Depolarisation stimulates the cardiac muscle to contract Depolarisation results in a voltage change across the membrane and this is detected by electrodes applied to the chest

Electrical Properties of Cardiac Muscle Cardiac muscle cells form an interconnected network The stimulus originating from the SA node spreads from cell to cell creating a wave of depolarisation throughout the muscle network Depolarised Cardiac Muscle Cell Depolarisation results in muscle contraction and thus a wave of contraction spreads throughout the network

Electrical Properties of Cardiac Muscle As the stimulus dies away, the muscle cells return to their POLARISED STATE and relax This event is termed REPOLARISATION The ECG electrodes detect the waves of depolarisation and repolarisation occurring during the cardiac cycle and the ECG trace is a record of these events

The ECG Trace R T wave P wave Q S The ECG trace for each heartbeat displays a P wave, a QRS wave or complex and a T wave

The ECG Trace R T wave P wave Q S The P wave is the result of depolarisation spreading across the atria from the SA node; it coincides with atrial contraction or systole The QRS wave or complex is the result of depolarisation of the ventricles and coincides with ventricular systole The T wave is the result of repolarisation of the ventricles as the ventricles begin to relax; repolarisation of the atria is not detected as the small voltage changes involved are masked by the QRS wave

The ECG Trace P – R interval R T wave P wave T – P interval Q S The P – R interval is the time, which elapses between the events of atrial systole and ventricular systole This period represents the time taken for the impulse to spread from the SA node through the atria, plus the delay in transmission to the AV node, together with the conduction time through the bundle of His and Purkinje fibres The T – P interval is the time spent by the heart in diastole before the next atrial systole begins

Control of the Heart Rate Although the origin and transmission of the heartbeat are properties of the heart itself, it is necessary for the heart rate to be modified to meet the different demands of the body The heart rate is regulated by both the nervous and hormonal systems of the body The autonomic nervous system is responsible for the regulation of the heart rate The autonomic nervous system has two divisions, i.e. the sympathetic and parasympathetic nervous systems

Control of the Heart Rate AUTONOMIC NERVOUS SYSTEM SYMPATHETIC NERVOUS SYSTEM PARASYMPATHETIC NERVOUS Sympathetic nerves release the neurotransmitter noradrenaline at their terminals Parasympathetic nerves release the neurotransmitter acetylcholine at their terminals The heart is supplied with both sympathetic and parasympathetic nerves and the chemicals that they secrete modify the heart rate

Control of the Heart Rate Two autonomic nerves link the cardiovascular centre in the brain with the SA node of the heart A sympathetic nerve, when stimulated, releases noradrenaline at its terminus with the SA node and this chemical speeds the heart rate This parasympathetic nerve is a branch of the vagus nerve A parasympathetic nerve, when stimulated, releases acetylcholine at its terminus with the SA node and this chemical slows the heart rate The heart rate is therefore determined by the balance between sympathetic and parasympathetic nerve activity Numerous sympathetic nerves also innervate (link to) the walls of the two ventricles where they increase the force of contraction of these chambers Sympathetic activity dominates during periods of exercise, stress and excitement Parasympathetic activity dominates during periods of rest and sleep Increased sympathetic activity also stimulates the release of the hormone adrenaline from the adrenal glands; adrenaline increases both the heart rate and its force of contraction

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