How much blood the heart pumps.

Cardiac output = stroke volume × heart rate; a resting person and an athlete reach 5 L/min and far more by different mixes.

Hit exactly 5 L/min with a 0.8 s cycle, then reach 20 L/min without the heart rate going above 180.

Cycle time does not depend on heart rate.

• Start: all four chambers are relaxed (joint diastole). The tricuspid and bicuspid valves are open, so blood from the pulmonary veins and vena cava flows through the left and right atria into the left and right ventricles. The semilunar valves are closed.
• The SAN fires an action potential that makes both atria contract together: atrial systole. Their squeeze pushes roughly 30 per cent more blood down into the ventricles.
• The impulse is carried to the ventricles by the AVN and AV bundle, and the bundle of His spreads it through the ventricular muscle. The ventricles contract (ventricular systole) while the atria relax (atrial diastole).
• Rising ventricular pressure closes the tricuspid and bicuspid valves as blood tries to flow back into the atria. As pressure rises further, the semilunar valves of the pulmonary artery (right) and aorta (left) are forced open and blood leaves the ventricles.
• The ventricles then relax (ventricular diastole); falling pressure closes the semilunar valves, preventing backflow. As pressure falls further, blood collected in the atria from the veins pushes the tricuspid and bicuspid valves open, blood flows freely into the ventricles again, and the heart is back in joint diastole until the SAN fires once more.
• This cyclically repeated sequence of systole and diastole of atria and ventricles is the cardiac cycle. With 72 beats per minute, one cycle lasts 0.8 seconds.
• Each ventricle pumps out about 70 mL of blood per cycle: the stroke volume. Stroke volume × heart rate = cardiac output, the volume pumped by each ventricle per minute, averaging 5000 mL (5 litres) in a healthy person.
• The body can change both stroke volume and heart rate and so the cardiac output; an athlete's cardiac output is much higher than an ordinary person's.
• Two prominent heart sounds are heard through a stethoscope each cycle: the first, lub, from closure of the tricuspid and bicuspid valves; the second, dub, from closure of the semilunar valves. They are of clinical diagnostic value.

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