Cardiac Cycle: Definition, Anatomy, Essential Phases And Its Tremendous and Crucial Functions In Circulatory System.

Cardiac Cycle Definition

The cardiac cycle involves a series of contractions of the heart, which pressurise the heart’s chambers and force blood to circulate in a single direction. The cardiac cycle consists of two parts. The ventricles relax and fill with blood during diastole. During systole, the muscles contract, forcing blood through the arteries.

Cardiac Cycle Explained

Due to differences in the anatomy of the heart, the cardiac cycle differs across species. A two-chambered heart with unidirectional flow is seen in fish. The sinus venous, atrium, and ventricle are the three chambers of the heart in several species. Reptiles and amphibians, for example, exhibit this behaviour.

Mammals and birds have evolved a more effective four-chambered heart that can provide higher blood pressure to creatures. The 4-chambered heart is essentially a 3-chambered heart with more chambers. The sinus venous is reduced to a node on the right atrium called the sinoatrial node. The four-chambered heart divides the atrium and ventricles, enabling a distinct conduit to the lungs to be constructed. Pulmonary circulation (to the lungs) and systemic circulation (to the rest of the body) are the two distinct circulation routes to the body.

The four-chambered heart of a mammal is represented by the cardiac cycle stages below. Animals with three-chambered hearts have a similar cardiac cycle, with the exception that the atria and ventricles are not entirely split, if at all. Hagfish and other species with a more primitive heart just use their hearts to slowly transport liquid through their bodies. Because the heart does not pump up a distinctive rhythm in species like these, the cardiac cycle is considerably less distinct.

The heartbeat is controlled by nerves in the sinoatrial node and carried out by nerves throughout the heart in most mammals. The cardiac muscle cells are generally interconnected laterally, allowing them to transmit the nerve signals they acquire towards each of their counterparts, culminating in rhythmic contractions.

Cardiac Cycle Phases

The two fundamental phases of the cardiac cycle, systole and diastole, happen sequentially. Each phase of the cardiac cycle includes essential processes that guarantee adequate blood flow. Contraction of heart chambers and appropriate opening and shutting of certain cardiac valves are among these stages.


The diastole phase begins with the relaxation of all heart muscles. At diastole, blood comes back to the heart and begins to refill the atria and ventricles. The mitral and tricuspid valves release when the ventricle is under pressure, enabling blood to pass through the atria into the left and right ventricles, respectively.

A massage delivered towards the sinoatrial node causes the contraction of each atrial muscle group. This simultaneously forces blood out of the atria and then into the ventricles. During this phase of the cardiac cycle, the bulk of blood is expelled from the atria. Action potential is transported from the atria and ventricles through the heart’s muscles and nerves.


When the ventricles enter ventricular systole and start to contract, a second wave of ventricular contraction commences. As the pressure in the ventricles increases, the mitral and tricuspid valves close. The pressure forces open the aortic and pulmonary valves. Here, the systole phase of the cardiac cycle starts.

The ventricles quickly contract, discharging the bulk of the blood they contain into the pulmonary and systemic circulations. The aorta, which is attached to the left ventricle, is the main artery that feeds the body with oxygenated blood. Containing unoxygenated blood, the pulmonary artery exits the right ventricle and proceeds to the lungs. This blood subsequently comes back to and fills the left atrium. The blood is subsequently transported to the remainder of the body via the left ventricle.

The body uses oxygen before delivering the blood to the right atrium to start the cycle once more. While it might require a prolonged duration for one blood pump to circulate the whole body, the heart’s cardiac cycle continues indefinitely to ensure the transportation of nutrients as well as oxygen around the body, as well as the elimination of undesirable metabolic waste products.

The ECG, which can be seen as the green line in the graph above, is an important instrument for measuring the heart cycle. The ECG is a signal that can be monitored by sensitive medical electronics and shows the heart cycle. The green line’s “QRS” highlights major spots in the signal, which correlate to ventricular contraction. Electrocardiogram analysis may provide valuable information about how the heart works. The existence of a signal, as well as its amplitude and the spacing between occurrences, are all important indicators of cardiac function.

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