Cardiovascular Function: Definition of Terms and Regulation
Cardiac output is the total blood volume pumped by ventricles per minute (heart rate × stroke volume). Stroke volume is the blood pumped by the left or right ventricle per beat; in a resting adult, it averages 60 to 80 mL of blood. Systole is the contraction phase of the cardiac cycle, when ventricles pump stroke volumes. Diastole is the resting phase of the cycle, which occurs between heartbeats. End-diastolic volume is the blood volume in each ventricle at the end of diastole: 120 mL at rest. End-systolic volume is the blood volume in each ventricle after contraction: 50 mL at rest. To maintain equal flow through pulmonary and systemic circuits, the left and right ventricles maintain the same cardiac output. The resting cardiac output is 4.8 to 6.4 L/min. Cardiac output increases (20-85%) during intense exercise to transport more oxygen to muscles. This greater blood flow is caused by higher blood pressure and arteriolar vasodilation in muscles, which is due to smooth muscle relaxation.