Cardiac Cycle:

The cardiac cycle is a series of events that occur during one heartbeat, including the contraction (systole) and relaxation (diastole) phases. It can be divided into several phases:

Atrial Contraction (Atrial Systole): The cardiac cycle begins with the contraction of the atria, which forces blood into the ventricles. The atrioventricular (AV) valves (tricuspid and mitral) are open during this phase.

Ventricular Contraction (Ventricular Systole): Once the ventricles are filled with blood, they contract, causing the AV valves to close and the semilunar valves (pulmonary and aortic) to open. This action forces blood into the pulmonary artery and aorta, sending oxygenated blood to the lungs and the rest of the body.

Isovolumetric Relaxation: After ventricular contraction, the ventricles relax briefly. All heart valves are closed during this phase, preventing backflow of blood.

Atrial Filling (Atrial Diastole): As the ventricles relax, the atria begin to fill with blood from the superior and inferior vena cava and pulmonary veins. This phase completes the cardiac cycle.

The cardiac cycle repeats continuously, ensuring a continuous flow of blood through the heart and the entire circulatory system, providing oxygen and nutrients to body tissues while removing waste products.

This process is regulated by electrical signals generated by the heart’s conduction system, specifically the sinoatrial (SA) node, atrioventricular (AV) node, and Purkinje fibers, which coordinate the rhythmic contractions of the heart muscle. These electrical impulses initiate and control each heartbeat.