Resting: about one beat a second, most of it spent filling.
Pick a heart rate · Slow to watch the valves
Heart rate
60 bpm
Per beat
70 mL
Per minute
4.2 L/min
ValvesOxygen-rich blood, left sideHeart muscleEach beat starts with a small squeeze of the two upper chambers, the atria. Then the two lower chambers, the ventricles, contract together: the valves between upper and lower chambers snap shut, pressure builds, and the outflow valves open. The right side sends oxygen-poor blood (blue) to the lungs; the thick-walled left side sends oxygen-rich blood (red) to the whole body. Cords inside the ventricles (chordae) tie the valve leaflets to small muscles in the walls, so the valves hold shut instead of blowing back into the atria. The monitor below tracks the ECG, the left ventricle's volume and when each set of valves is open.At rest, about 70 mL leaves each ventricle per beat, 4.2 litres a minute. Flat out, the heart beats about three times as often and empties more fully with each beat, so its output rises to roughly 20 litres a minute. That is the delivery side of VO₂max.Typical adult values (end of filling ≈ 120 mL, end of ejection ≈ 50 mL at rest); individual hearts differ. Blue and red follow the textbook convention: venous blood is dark red, not blue. As the ventricles squeeze, the valve plane is pulled toward the tip of the heart, which draws blood into the atria at the same time. The ECG is a drawing timed to the beat. The heart's 3-D shape, the chambers' depth, the muscle ridges and the swirl of blood as the left ventricle fills are drawn from textbook anatomy, not measured.