Stroke volume

Stroke volume is the amount of blood ejected by a ventricle in a single contraction. It is found by subtracting the end-systolic volume, the blood left in the ventricle after it contracts, from the end-diastolic volume, the blood it holds when filling is complete. In a resting adult the left ventricle fills to about 120 mL and keeps about 50 mL, so the stroke volume is roughly 70 mL; during hard exercise it can climb well above 100 mL.

Three factors set stroke volume. Preload is how far the ventricle wall is stretched by filling; by the Frank–Starling mechanism, a fuller heart contracts more forcefully and ejects the extra blood. Contractility, also called inotropy, is the strength of contraction at a given stretch, raised by sympathetic nerves and epinephrine. Afterload is the pressure the ventricle must overcome to open the aortic or pulmonary valve, so a higher afterload, as in high blood pressure, reduces the volume pumped out.

Multiplying stroke volume by heart rate gives cardiac output. Dividing it by end-diastolic volume gives the ejection fraction, a common echocardiogram measure of pumping strength that falls in many forms of heart failure.