Afterload

thumb|Ventricular systole. Red arrow is path from left ventricle to aorta. Afterload is largely dependent upon aortic pressure. Afterload is the pressure that the heart must work against to eject blood during systole (ventricular contraction). Afterload is proportional to the average arterial pressure. As aortic and pulmonary pressures increase, the afterload increases on the left and right ventricles respectively. Afterload changes to adapt to the continually changing demands on an animal's cardiovascular system. Afterload is proportional to mean systolic blood pressure and is measured in millimeters of mercury (mm Hg).

The clinical concept of afterload was given its modern operational definitions through the work of American cardiologist Edmund Sonnenblick and colleagues at the National Institutes of Health in the early 1960s. Sonnenblick's force-velocity studies of isolated cardiac muscle demonstrated that afterload directly determines the velocity of myocardial fiber shortening, providing the theoretical basis for afterload reduction as a therapeutic strategy in heart failure.