Physiology of the Heart

Cardiac contraction is initiated in the excitable cells of the sinoatrial (SA) node by both spontaneous depolarization and sympathetic activity.

An electrocardiogram, or ECG, is a recording of the heart's electrical activity as a graph over a period of time.

The two major heart sounds are "lub" (from the closure of AV valves) and "dub: (from the closure of aortic and pulmonary valves).

The cardiac cycle describes the heart's phases of contraction and relaxation that drive blood flow throughout the body.

Cardiac output (Q or CO) is the volume of blood pumped by the heart, in particular by the left or right ventricle, in one minute.