Cardiac output is the amount of blood pumped by a ventricle in 1 minute. A certain level of cardiac output is needed at all times to transport oxygen to tissues and to remove waste products. During exercise, cardiac output must increase to meet the body’s need for more oxygen. We will return to exercise after first consider-ing resting cardiac output.
To calculate cardiac output, we must know the pulse rate and how much blood is pumped per beat. Stroke volume is the term for the amount of blood pumped by a ventricle per beat; an average resting
stroke volume is 60 to 80 mL per beat. A simple for-mula then enables us to determine cardiac output:
Cardiac output = stroke volume x pulse (heart rate)
Let us put into this formula an average resting stroke volume, 70 mL, and an average resting pulse, 70 beats per minute (bpm):
Cardiac output = 70 mL x 70 bpm
Cardiac output = 4900 mL per minute (approximately 5 liters)
Naturally, cardiac output varies with the size of the person, but the average resting cardiac output is 5 to 6 liters per minute. Notice that this amount is just about the same as a person’s average volume of blood. At rest, the heart pumps all of the blood in the body within about a minute. Changes are possible, depend-ing on circumstances and extent of physical activity.
If we now reconsider the athlete, you will be able to see precisely why the athlete has a low resting pulse. In our formula, we will use an average resting cardiac output (5 liters) and an athlete’s pulse rate (50):
Cardiac output = stroke volume x pulse
5000 mL = stroke volume x 50 bpm
5000/50 = stroke volume
100 mL = stroke volume
Notice that the athlete’s resting stroke volume is significantly higher than the average. The athlete’s more efficient heart pumps more blood with each beat and so can maintain a normal resting cardiac output with fewer beats.
Now let us see how the heart responds to exercise. Heart rate (pulse) increases during exercise, and so does stroke volume. The increase in stroke volume is the result of Starling’s law of the heart, which states that the more the cardiac muscle fibers are stretched, the more forcefully they contract. During exercise, more blood returns to the heart; this is called venous return. Increased venous return stretches the myocar-dium of the ventricles, which contract more forcefully and pump more blood, thereby increasing stroke vol-ume. Therefore, during exercise, our formula might be the following:
Cardiac output = stroke volume x pulse
Cardiac output = 100 mL x 100 bpm
Cardiac output = 10,000 mL (10 liters)
This exercise cardiac output is twice the resting cardiac output we first calculated, which should not be considered unusual. The cardiac output of a healthy young person may increase up to four times the rest-ing level during strenuous exercise. This difference is the cardiac reserve, the extra volume the heart can pump when necessary. If resting cardiac output is 5 liters and exercise cardiac output is 20 liters, the car-diac reserve is 15 liters. The marathon runner’s cardiac output may increase six times or more compared to the resting level, and cardiac reserve is even greater than for the average young person; this is the result of the marathoner’s extremely efficient heart. Because of Starling’s law, it is almost impossible to overwork a healthy heart. No matter how much the volume of venous return increases, the ventricles simply pump more forcefully and increase the stroke volume and cardiac output.
Also related to cardiac output, and another measure of the health of the heart, is the ejection fraction. This is the percent of the blood in a ventricle that is pumped during systole. A ventricle does not empty completely when it contracts, but should pump out 60% to 70% of the blood within it. A lower percent-age would indicate that the ventricle is weakening. These aspects of physiology are summarized in Table 12–2.