afterload is the pressure 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).