## Introduction to cardiac output

Cardiac output, - is the total volume of blood pumped by the ventricle per minute, or simply the product of heart rate (HR) and stroke volume (SV). The stroke volume at rest in the standing position averages between 60 and 80 ml of blood in most adults. Thus at a resting heart rate of 80 beats per minute the resting cardiac output will vary between 4.8 and 6.4 L per min. However, the cardiac output of Olympic medal winners in cross country skiing increased 8 times above resting cardiac output to approximately 40 liters for one minute of maximal work with an accompanied stroke volume of 210 ml per beat.

 The average adult body contains about 5 L of blood, so this means all of our blood is pumped through our hearts about once every minute

## Cardiac output during exercise

Now that we have discussed both of the components of cardiac output (heart rate and stroke volume), we can put this information together to understand what happens to cardiac output during exercise. Changes in cardiac output, because it is the product of both heart rate and stroke volume, are predictable with increasing work levels.

Because:

cardiac output = heart rate x stroke volume (CO = HR x SV)

changes in either heart rate or stroke volume will have an impact on the other component.

For an untrained person who has a resting heart rate of 72 beats/min and a stroke volume of 70 ml, the resting cardiac output is calculated as follows:
• Cardiac output = Heart rate x Stroke
• Cardiac output = 72 x 70
• Cardiac output = 5040 ml/min
• Cardiac output = 5.04 L/min

(NOTE: 1000 ml = 1 L)

During the initial stages of exercise, increased cardiac output is due to an increase in both heart rate and stroke volume. When the level of exercise exceeds 40% to 60% of the individual's capacity, stroke volume has either plateaued or begun to increase at a much slower rate. Thus further increases in cardiac output are largely the result of increases in heart rate.

 Only 15% of the resting cardiac output goes to muscle, but during Olympic cross-country skiing the muscles receive 60% to70% of the cardiac output.

During Olympic cross country skiing, cardiac output increases primarily to match the need for increased oxygen supply to the working muscles. Compare the Olympic cross country skiers cardiac output with popular sports. You can see that cross country skiers cardiac output (men = 40 liters and women = 35 liters) are much higher than basketball players. Compare your favorite sport with cross country skiing.

While cross country skiing, blood is forced out of the heart and circulation speeds up. Blood is redirected, through the action of the sympathetic nervous system, away from areas where it is not essential, to those areas that are active. This causes constriction of vessels in those areas by reducing blood flow to the kidneys, liver, stomach, abd intestines. Thus, blood flow is concentrated to the skeletal muscles where it is needed. This ensures that adequate supplies of the needed materials oxygen and nutrients reach the tissues and that waste products, which build up much more rapidly during cross-country skiing, are quickly cleared away.

Look at the graph. During exercise, muscles receive 66% of the cardiac output, but the kidneys only receive 3%! At rest the liver has the highest percentage of cardiac output (27%), while the muscles only receive 15%!

 Only 15% of the resting cardiac output goes to muscle, but during Olympic cross-country skiing the muscles receive 60% to 70% of the cardiac output.

© April, 1998, Montana State University-Bozeman