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The cardiac cycle is the sequence of events that takes place in the heart during one complete heartbeat, involving the contraction and relaxation of the heart chambers to pump blood throughout the body. It is divided into three main stages: atrial systole, ventricular systole, and diastole.

Atrial systole is the first stage of the cardiac cycle, in which the atria (the upper chambers of the heart) contract to push blood into the ventricles (the lower chambers). At the beginning of this stage, the sinoatrial (SA) node, also called the natural pacemaker of the heart, generates an electrical impulse that spreads across both atria, causing these to contract. As a result, blood flows from the atria into the ventricles through open atrioventricular (AV) valves, while the aortic and pulmonary valves remain closed to prevent backflow.

In the second stage of the cardiac cycle called ventricular systole, the ventricles contract in response to another electrical impulse generated by the atrioventricular (AV) node, which is located at the border between the atria and ventricles. This electrical signal spreads to the ventricles through the Bundle of His and Purkinje fibers, causing a strong and coordinated contraction. Consequently, the pressure inside the ventricles increases, closing the atrioventricular valves to prevent blood from flowing back into the atria. At the same time, the aortic and pulmonary valves open, and blood is forced out of the ventricles into the aorta and pulmonary arteries, respectively, which carry oxygenated and deoxygenated blood throughout the body.

The third and final stage of the cardiac cycle is diastole, which involves the relaxation of both the atria and ventricles, allowing them to passively fill with blood. The aortic and pulmonary valves close to prevent the backflow of blood into the ventricles, while the atrioventricular valves open to facilitate blood flow from the atria. During this phase, the heart is in a state of relaxation, and the pressure within the heart chambers is low. In summary, the cardiac cycle is crucial for the efficient functioning of the heart and the circulation of blood throughout the body. It consists of three main stages: atrial systole (contraction of the atria), ventricular systole (contraction of the ventricles), and diastole (relaxation of all heart chambers). The proper coordination of these stages ensures that blood is pumped effectively to maintain healthy blood flow and supply oxygen and nutrients to the body's tissues and organs.