Cardiac output (CO) is an integral aspect of human physiology, reflecting the heart's efficiency and responsiveness to the body's needs. It represents the volume of blood that the left or right ventricle ejects into the aorta or pulmonary trunk each minute. The CO is calculated by multiplying the heart rate (HR)—the number of heartbeats per minute—by the stroke volume (SV)—the amount of blood pumped out with each heartbeat.

In an average resting adult male, the typical cardiac output averages around 5.25 L/min, taking into account an average heart of 75 beats/min and a stroke volume of 70 L/beat.

A crucial concept related to cardiac output is cardiac reserve—the difference between maximum cardiac output and resting cardiac output. This measure is indicative of the heart's adaptability and resilience. An average individual possesses a cardiac reserve of about four to five times their resting value, while top endurance athletes may have a seven or eight-times greater cardiac reserve. Conversely, those with severe heart disease may exhibit little to no cardiac reserve, restricting their ability to perform even simple daily tasks. This underlines cardiac output and reserve's significant role in health and physical performance.