cardiac output (CO)

cardiac output (CO)

the volume of blood ejected per minute from each ventricle of the heart simultaneously, i.e. to the lungs via the pulmonary artery, and to the rest of the body via the aorta (although often defined solely as the output from the left ventricle). The product of stroke volume (SV) and heart rate (HR): at rest, e.g. 70 mL × 70 per minute = 4.9 L.min-1(4-6 L.min-1 varying with body size). Increase in CO in exercise involves increase in heart rate, accompanied by greater filling during diastole (by increased venous return to the heart), maintaining then increasing stroke volume, due to stretch of the ventricular muscle and to enhanced contractility from sympathetic stimulation; also the residual volume remaining in the ventricles after ejection decreases. Maximal CO in exercise increases with athletic training (by hypertrophy of ventricular muscle, with raised SV and lowered HR at rest, allowing a greater increase) exceptionally up to SV 200 mL × HR 200 = 40 L.min-1, but a more modest maximum is typical.