Sample Healthcare Essays on The Cardiac Cycle

The cardiac cycle refers to the sequence of events that occur within the heart during each heartbeat. The chambers of the heart are in repeated contraction and relaxation phases, known as systole and diastole respectively. During filling of the heart, both the tricuspid and the mitral valve are open. This permits the passive flow of blood from the veins and coronary sinus into the chambers. The aortic and pulmonary valves are however closed to prevent the backflow of the blood ejected during ventricular systole.

The cardiac cycle begins with atrial diastole. The atria are usually in a state of relaxation for about 0.7 seconds as blood flows from the veins and coronary sinus into them. Atrial diastole usually occurs simultaneously with ventricular diastole, as the mitral and tricuspid valves are open. This is then followed by atrial systole and this phase corresponds to the P wave visible on an ECG. Taking an approximate 0.11 seconds, it is only responsible for very little flow into the ventricle, as much of the ventricular filling occurs in the atrial diastole phase (Koeppen & Stanton 2017). After atrial systole, the ventricle begins to contract. The pressure build-up results in closure of the tricuspid and mitral valves. Increased pressure in the ventricles is eventually responsible for the opening of the aortic and pulmonary valves and blood is ejected from the chambers. This phase takes 0.27 seconds and is represented by the QRS complex of the ECG. Lastly, ventricular relaxation happens because of the repolarization of the myocardium. Generally, this is the period responsible for ventricular filling.

 

Physiology

The physiology underlying the cardiac cycle is a question of myocardial contractions, relaxation, and pressure changes within the chambers. More events can be recorded physiologically due to the pressure changes.         At the beginning of the cardiac cycle, atrial contraction occurs due to the depolarization of the atrial musculature. Atrial systole occurs to top up the direct venous return that flows past the atrioventricular valve into the ventricle. It is not an essential occurrence though. The wave of depolarization spreads past the atrioventricular node to induce isovolumetric ventricular contraction. The ventricular wave of depolarization induces contraction of the ventricular myocardium. Pressure builds up within the chamber resulting in the closure of the mitral/ tricuspid valves, which correlates with the first heart sound. The ventricular volume remains unchanged since the pressure is not yet sufficient to open the semilunar valves. After a very brief period, rapid ventricular ejection then occurs. Enough pressure is generated by the myocardium that causes the semilunar valves to give way. Blood from the ventricles is forced into the aorta/ pulmonary artery at very high pressure. As the ventricular pressure falls, some blood flows past the aortic valve due to kinetic energy though at a rate that is lower than that present during the rapid ventricular ejection phase. Reduced ventricular pressure below the aortic pressure then results in closure of the aortic valve, which produces the second heart sound. However, no apparent change in ventricular volume occurs hence the term isovolumetric ventricular relaxation. The heart musculature then relaxes causing ventricular diastole. The mitral valve opens, releasing atrial blood gained from venous return into the left ventricle. During this phase, a third heart sound, indicative of volume overload, can be heard especially in children (Koeppen & Stanton 2017). Lastly, reduced ventricular filling occurs which is the longest phase of the cardiac cycle. This phase occurs due to the slow venous return, which ultimately causes increased ventricular blood volume.

 

 

Reference

Koeppen, B. M., & Stanton, B. A. (2017). Berne and levy physiology e-book. Elsevier Health Sciences. Retrieved from https://books.google.co.ke/books?hl=en&lr=&id=rVSOC3q3QJ0C&oi=fnd&pg=PP1&dq=berne+and+levy+physiology&ots=-24nYMxU2B&sig=F2zT1nbf3iGBcqjyYMbsDIHlvbk&redir_esc=y#v=onepage&q=berne%20and%20levy%20physiology&f=false