Cardiac Cycle

THE CARDIAC CYCLE The human heart is situated between the two lungs in the intra-thoracic cavity. The heart weighs approximately 125g in a healthy human and is considered to be the size of a clenched human fist. The apex of the heart is situated to the left of the midline and is described anatomically as being in the left mid-nipple line in the fourth inter-costal space in a man. The heart is divided into four chambers, the 2 receiving chambers being the atria (singular – atrium), while the two ventricles are strong and muscular and are responsible for ejecting blood out of the heart. The left side is completely separated from the right side by the inter- atrioventricular septum. The right atrium is separated from the right ventricle by the tricuspid valve, while the left atrium is separated from the left ventricle by the bicuspid (mitral) valve. The right side receives deoxygenated blood from the body via the superior and inferior venacavae, which open independently into the right atrium. In a similar sense, the left atrium receives blood returning after oxygenation from the lungs via two pairs of pulmonary veins. Blood received by the two atria flows directly into the two ventricles, but this ventricular filling is enhanced by the contraction of the atrial walls (atrial systole). During atrial systole the two atrio-ventricular valves remain open while the two semi-lunar valves (situated at the entrance of the pulmonary artery and aorta) remain closed. Following atrial systole the two ventricles prepare for their contraction. The onset of ventricular systole is marked by a brief isometric phase, during which time all four valves close and an enormous tension builds up in the ventricle walls. The tension developed in the left ventricular wall far exceeds that of the right ventricle, as the musculature on the left side is substantially thicker than that of the right. This is a functional adaptation brought about to help the left ventricle generate a pressure strong enough to propel the blood through the entire systemic circulation. During ventricular systole the pressure in the left ventricle reaches approximately 120 mm Hg. The ventricular wall then contracts powerfully, ejecting all the blood contained in it into the aorta. A similar process occurs on the right side except that pressures rarely exceed 25mm Hg. Since the pressure in the ventricles far exceed that of the great arteries, blood pulsates out along a pressure gradient, forcing the semi-lunar valves open. Towards the end of ventricular ejection, there comes a point when the blood has diminished so much in the left ventricle and increased so much in the aorta, that the pressure in the aorta begins to exceed that of the ventricle. At this point, due to reversing pressure gradients, reverse flow may occur, but the semi-lunar valve snaps shut so that backflow of blood is prevented. Following ventricular systole, the entire heart undergoes relaxation (diastole) during which time all four valves are open. As the heart muscle relaxes, recoils and expands there is the simultaneous filling of blood, which marks the onset of a new cardiac cycle. It is important to note that the two atria and the two ventricles although structurally separated, contract together as one, augmenting the effect of each other synergistically so that the beats are rhythmic and controlled as one fluid cycle. The human circulation comprises of a deoxygenated systemic (body) circulation and an oxygenated pulmonary (lung) circulation, which are connected and controlled by

the central pump, which is the heart. Humans therefore have a double closed circulation.