Your body's total volume of blood, equaling 4 to 6 L, passes through your heart every minute during rest. Changes in cardiac output during exercise increase blood cycling rate up to 21 L per minute in active individuals and 35 L per minute in elite athletes. Heart health and exercise performance are regulated by your cardiac output ability. Understanding cardiac output during activity enables informed exercise decisions.
According to the American Council on Exercise, cardiac output is the amount of blood flowing out of your heart per minute. Cardiac output is calculated by multiplying heart rate in beats per minute by stroke volume in mL. Direct measurement of cardiac output requires invasive measurements in a clinical setting. The book "Anatomy and Physiology" by Kenneth Saladin defines typical resting cardiac output as 5,250 mL per minute for adults.
Heart rate and stroke volume dictate cardiac output. Heart rate is the number of heart beats occurring per minute while stroke volume is the amount of blood pumped from each ventricle per minute. Your heart is comprised of four chambers called atria or ventricles that pump oxygenated blood to working tissue while sending deoxygenated blood back to your lungs for more oxygen.
Changes during Exercise
Active muscles require more oxygen than resting muscles. At the onset of exercise your muscles signal your heart to pump faster for increased blood flow. In addition, working muscles increase stroke volume by sending higher amounts of blood volume back towards the lungs for oxygen. Therefore, cardiac output rises during exercise due to increased stroke volume and heart rate. The deference between your resting and active cardiac output is called cardiac reserve.
Adaptations to Exercise
Cardiac output adapts throughout a training program. The "American Council on Exercise's Personal Trainer Manual" lists exercise adaptations as increased ventricle size, decreased exercise heart rate and increased stroke volume. Therefore, your heart can maintain a high cardiac output with less effort. Most improvement to cardiac output is contributed to increased stroke volume. Positive adaptations occur in as little as three months of aerobic training.
Enhancing cardiac output allows you to maintain lower heart rates during physical activity. For example, at the start of a program you may have a heart rate of 150 beats per minute while running at a 6 mph pace. After three or more months of training increased cardiac output enables you to sustain the same running intensity at a lower heart rate such as 125 beats per minute. Please consult a doctor before starting any new training program.