Cardiac output is the volume of blood pumped from each ventricle in one minute, and it is an important measure of heart function. It is determined by two factors: stroke volume and heart rate. Stroke volume (SV) is the volume of blood pumped from each ventricle per heartbeat. Multiplying this volume by the heart rate (HR), heartbeats per minute, yields the cardiac output (CO).
CO = SV x HR
At normal resting values of a stroke volume of 70 ml/beat and a heart rate of 72 beats/min, the cardiac output is 5,040 ml/min. This means that the total volume of blood, 4 to 6 liters, passes through each ventricle of the heart each minute. Cardiac output increases with exercise because both stroke volume and heart rate increase.
Heart function is regulated by factors both internal and external to the heart. For example, venous return, the amount of blood returning to the heart during diastole, is an internal factor that affects stroke volume. If venous return increases, more blood enters and is pumped from the ventricles, increasing the stroke volume and cardiac output. Heart rate is primarily controlled externally by the autonomic nervous system, although hormones and certain ions also affect it.