Physiology of the Heart

The cardiovascular system controls the movement of blood through thousands of miles of capillaries so that every tissue in every part of the body is perfused. Essential nutrients are supplied and waste products are removed. The blood has to be kept fluid and the regional circulation has to be adjusted in relation to the needs of a tissue or organ at that time. For example skeletal muscle will need an increased blood flow during exercise. Hence numerous control mechanisms are necessary to supply essential nutrients to each cell of the body.

The heart acts as a pump that propels arterial blood through the aorta and receives the venous blood through the superior and inferior vena cava. The left ventricle is thicker as it has to pump blood at a higher pressure into the aorta. The right ventricle works at a lower pressure and pumps into the pulmonary artery.

The venous blood from the vena cavae passes into the right atrium and through the tricuspid valve into the right ventricle. From here during ventricular systole blood is pumped through the pulmonary valve into the pulmonary artery and the lungs where oxygenation takes place. The oxygenated blood is then returned to the left atrium through the four pulmonary veins. From the left atrium the blood passes through the mitral valve to the left ventricle during diastole and during ventricular systole through the aortic valve into the aorta.

The electrical impulse that initiates the heart beat originates in the sino atrial (SA) node. From here the impulse passes across both the atria to the atrio-ventricular node (AV node).There is a physiological delay of the impulse in the AV node and there is also a limit to the number of impulses that can pass through. Thus if a large number of impulses come from the atria as in atrial fibrillation only a certain lesser number of impulses can go through to activate the ventricles. From the AV node the impulses pass through the Bundle of his to divide into the right and left bundle branches. The left bundle branch divides further into two fascicles–the anterior and posterior. The bundle branches end up as the Purkinje system. Conduction problems can occur at all levels. Thus you could have a sino atrial block, AV nodal block of varying degrees,right or left bundle branch block and fascicular block. These will be discussed in the sections on arrhythmias and ECG. The conduction system is shown in the diagram below:

Inrinsic conduction system of the heart

The function of the 4 heart valves is to prevent the blood from leaking back (regurgitation,incompetence) into the chamber just proximal to the valve during the corresponding part of the cardiac cycle. Thus the two semilunar valves aortic and pulmonary should prevent regurgitation into the left and right ventricle respectively during diastole. During systole the ventricle and corresponding great artery are in continuity as the valve is open. If these valves are stenosed(narrowed) the problem will be in systole when the ventricle has to get the blood through the valve. The two atrio-ventricular valves, the mitral and tricuspid have to prevent regurgitation into the left atrium and right atrium respectively during ventricular systole. If these valves are stenosed (narrowed) the problem will be in diastole when the atrium has to get the blood through the valve into the corresponding ventricle.