Question

What is a cardiac cycle? How does it occur in a four-chambered heart?

Short answer

A cardiac cycle is a sequence of events in the heart that includes the contraction (systole) and relaxation (diastole) of heart muscles. In a four-chambered heart, it consists of atrial systole (contraction), ventricular systole (contraction), and diastole (relaxation). The blood flow is regulated by the tricuspid, pulmonary, mitral, and aortic valves. The cycle includes ventricular filling, closure of atrioventricular valves, ventricular contraction and ejection, closure of semilunar valves, and ventricular relaxation with atrial filling.

Step-by-step solution

Definition of Cardiac Cycle

A cardiac cycle is a sequence of events that takes place in the heart, from the start of one heartbeat to the initiation of the next one. It includes both the contraction (systole) and relaxation (diastole) of the heart muscle.

Overview of the Four-Chambered Heart

The four-chambered heart consists of two atria (right and left) and two ventricles (right and left). The right side of the heart pumps deoxygenated blood to the lungs, while the left side pumps oxygenated blood to the rest of the body. The blood flow through the heart is regulated by a series of valves: the tricuspid valve, the pulmonary valve, the mitral (bicuspid) valve, and the aortic valve.

Atrial Contraction & Ventricular Filling

At the beginning of the cardiac cycle, the atria contract (atrial systole) simultaneously. This pushes blood through the open tricuspid (right atrium to the right ventricle) and mitral valves (left atrium to the left ventricle). The ventricles are in a relaxed state (diastole) during this phase, allowing them to fill with blood.

Closure of Atrioventricular Valves

As the ventricles begin to contract (ventricular systole), the pressure inside the ventricles increases, causing the tricuspid and mitral valves to close. This prevents the backflow of blood to the atria.

Ventricular Contraction & Ejection

The ventricular contraction (systole) continues and sends the blood through the open pulmonary (right ventricle to lungs) and aortic (left ventricle to the body) valves. Oxygen-poor blood is pumped into the pulmonary artery, while oxygen-rich blood is pumped into the aorta.

Closure of Semilunar Valves

As the ventricles relax (ventricular diastole), the pressure within the ventricles drops, and the pulmonary and aortic valves close. This prevents the backflow of blood into the ventricles.

Ventricular Relaxation & Atrial Filling

As the ventricles continue to relax (diastole), the atria also relax and start filling with blood from the veins (superior and inferior vena cava for the right atrium, and pulmonary veins for the left atrium). This phase marks the end of one cardiac cycle and the beginning of the next. In summary, a cardiac cycle is the series of events that occur in the heart from one heartbeat to the next, consisting of both contraction and relaxation phases. In the four-chambered heart, it involves atrial systole, ventricular systole, and diastole, with blood flow and circulation governed by the opening and closing of the various valves within the heart.