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Applied exercise physiology heart, called the right and left ventricles. In doing so, it passes the atrioventricular (AV) valves. The right AV valve is the tricuspid valve and the left AV valve is the bicuspid valve. The purpose of these valves is not merely to separate the atria from the ventricles but also to ensure that the blood can only flow in one direction through the heart. When the ventricles contract, blood on the right side of the heart is forced through the semi-lunar pulmonary valve into the pulmonary artery from where it travels towards the lungs. Meanwhile, blood from the left ventricle enters the aorta via the semi-lunar aortic valve. The aorta branches into many different arteries, which then transport the blood around the whole body. Once again, the semi-lunar valves ensure the unidirectional flow of blood, preventing backflow of blood into the heart. As the left side of the heart is responsible for pumping blood around the whole body, the wall of cardiac tissue (myocardium) surrounding the left ventricle is much thicker than that on the right side of the heart. HOT LINKS Visit a website such as www.zerobio.com/videos/dissect_ sheepheart1.html or heartlab.robarts.ca/dissect/dissection. html to see the dissection of a heart Task 4.01 Copy and complete Table 4.01, identifying the relevant function for each anatomical structure of the heart in the list. Structure of the heart Aorta Av bicuspid valve Right ventricle Pulmonary vein Septum 54 Function The cardiovascular system The conduction system of the heart yourself with the components of this system. Cardiac tissue is extremely specialised. To start with, it is ‘myogenic’, which means that it can generate its own electrical impulses and does not require stimulation by the brain. It also possesses an intricate network of nerves. Together, these two factors ensure an efficient flow of blood through the heart and around the body. The cardiac cycle Key terms Myocardium: cardiac muscle that makes up the heart Myogenic: the ability of the heart to produce its own impulses The cardiac impulse originates from the sinoatrial node (SA node). This is a specialised area of cardiac muscle fibres located in the muscular wall of the right atrium and acts as the heart’s intrinsic pacemaker. Once the SA node has emitted the electrical impulse, it rapidly spreads throughout both atria, creating a wave of excitation and causing them both to contract. The impulse then arrives at and activates another specialised area of cardiac tissue known as the atrioventricular node (AV node). The AV node initially delays the transmission of the cardiac impulse from spreading to the ventricles (for approx 0.1 seconds). This enables the atria to contract fully before ventricular contraction begins. After this short delay, the impulse is sent down the septum of the heart via the bundle of His and throughout the muscular walls of the ventricles via Purkinje fibres. Both ventricles now contract, forcing the blood out of the heart and around the body. Fig 4.03 traces the journey of an impulse through the heart’s conduction system. Take a few minutes to familiarise Left atrium Sinoatrial node (SAN) The cardiac cycle refers to the HIS bundle electrical and mechanical events Left bundle Right atrium branch(LBB) that take place in the heart during Left posterior one complete heartbeat. Typically, Atrioventricular node fascicle(LPS) (AVN) at rest, one complete heartbeat Right bundle Left ventricle will occur every 0.8 seconds and branch(RBB) occurs approximately 72 times Left anterior per minute. During this time, the fascicle (LPS) Right ventricle heart will at first relax and fill with Purkinje fibres blood – known as the diastolic (PF) phase – and then contract, Fig 4.03 Control of the heart rate is myogenic forcing blood from one part of the heart to another or forcing blood out of the heart altogether – this is referred to as the systolic phase. The control of the heart rate It is possible to summarise the cardiac cycle into It was stated earlier that the heart is myogenic – it four stages: generates its own impulses from its own intrinsic Stage 1 Atrial diastole pacemaker, the sinoatrial node (SA node). However, 0.5 seconds the rate at which cardiac impulses are fired can Stage 2 Ventricular diastole be altered and controlled by mechanisms external Stage 3 Atrial systole 0.3 seconds to the heart. During exercise, for example, the Stage 4 Ventricular systole heart rate must increase and the SA node must fire impulses more rapidly in order to meet the body’s demands for oxygen. It is able to do this through two REMEMBER! main regulatory mechanisms: The cardiac cycle and conduction system of the heart are inextricably linked. • neural control mechanism • hormonal control mechanism. Table 4.02 A summary of the events of the cardiac cycle Stage of cardiac cycle Description Action of valves Atrial diastole (relaxation) Atria fill with blood Atrioventricular valves are closed Semi-lunar valves are open Ventricular diastole (relaxation) Rising pressure in the atria causes the AV valves to open and the ventricles to fill with blood Atrioventricular valves open Semi-lunar valves are closed HOT LINKS Atrial systole (contraction) Atria contract, forcing blood into the ventricles Atrioventricular valves open Semi-lunar valves are closed View an animation of the conduction system of the heart at www.bostonscientific.com/templatedata/imports/ HTML/CRM/heart/interact_8.html Ventricular systole (contraction) Ventricles contract, increasing pressure in the Atrioventricular valves are forced to close ventricles, and forcing blood into the aorta and pulmonary artery 55