The volume of blood pumped by each ventricle per minute is called the cardiac output (CO), usually expressed in liters per minute. It is also the volume of blood flowing through either the systemic or the pulmonary circuit per minute. The cardiac output is determined by multiplying the heart rate (HR)—the number of beats per minute— and the stroke volume (SV)—the blood volume ejected by each ventricle with each beat: CO = HR x SV. Thus, if each ventricle has a rate of 72 beats/min and ejects 70 ml of blood with each beat, the cardiac output is: CO = 72 beats/min x 0.07 L/beat = 5.0 L/min
These values are within the normal range for a resting average-sized adult. Coincidentally, total blood volume is also approximately 5 L, and so essentially all the blood is pumped around the circuit once each minute. During periods of strenuous exercise in well-trained athletes, the cardiac output may reach 35 L/min; that is, the entire blood volume is pumped around the circuit seven times a minute! Even sedentary, untrained individuals can reach cardiac outputs of 20–25 L/min during exercise. The following description of the factors that alter the two determinants of cardiac output—heart rate and stroke volume—applies in all respects to both the right and left sides of the heart since stroke volume and heart rate are the same for both under steady-state conditions. It must also be emphasized that heart rate and stroke volume do not always change in the same direction. For example, stroke volume decreases following blood loss while heart rate increases. These changes produce opposing effects on cardiac output.