During exercise, the amount of blood pumped by the heart increases in order to deliver more oxygen to the body’s muscles. In a healthy adult, cardiac output—a measure of the amount of blood pumped by the heart—can increase from 3 liters of blood per minute per square meter of body surface to 18 liters per minute per square meter of body surface.
To determine overall heart function, doctors measure cardiac output, the amount of blood pumped by each ventricle in one minute. Cardiac output is equal to the heart rate multiplied by the stroke volume, the amount of blood pumped by a ventricle with each beat. Stroke volume, in turn, depends on several factors: the rate at which blood returns to the heart through the veins; how vigorously the heart contracts; and the pressure of blood in the arteries, which affects how hard the heart must work to propel blood into them. Normal cardiac output in an adult is about 3 liters per minute per square meter of body surface.
An increase in either heart rate or stroke volume—or both—will increase cardiac output. During exercise, sympathetic nerve fibers increase heart rate. At the same time, stroke volume increases, primarily because venous blood returns to the heart more quickly and the heart contracts more vigorously. Many of the factors that increase heart rate also increase stroke volume. For example, impulses from sympathetic nerve fibers cause the heart to contract more vigorously as well as increasing the heart rate. The simultaneous increase in heart rate and stroke volume enables a larger and more efficient increase in cardiac output than if, say, heart rate alone increased during exercise. In a healthy adult during vigorous exercise, cardiac output can increase six-fold, to 18 liters per minute per square meter of body surface.