How Is Cardiac Output Increased During Exercise?

The human heart is an amazing organ, pumping up to 2,000 gallons of blood each day through a network of about 60,000 miles of blood vessels. An adaptive feature of the heart is its ability to respond to changes in the environment or activity such as exercise. Blood flow increases during exercise to provide energy as well as remove wastes. Exercise intensity determines the degree to which this occurs, as measured by cardiac output.

What Is Cardiac Output?

Cardiac output measures the activity of your heart based on how much blood it pumps in one minute. It is measured as liters per minute (L/min). During rest, your cardiac output may hover around 5 L/min. The amount is determined by your heart rate multiplied by stroke volume or the amount of blood pumped per heartbeat in milliliters per beat (ml/beat). For example, an out-of-shape person may have a stroke volume of 50 ml/beat. If his pulse was 70, his cardiac output would be 70 times 50 or 3,500 milliliters, or 3.5 L/min.

Exercise Intensity

Your cardiac output increases during exercise simply because your heart rate is higher, resulting in faster blood flow. During exercise, your heart rate may increase upwards of 130 beats per minute or more during moderate exercise, depending upon your age and level of fitness. In addition, stroke volume will also increase to a similar figure. The change reflects the added demands exercise places on the body for more fuel and oxygenated blood. The increases in heart rate and stroke volume then impact cardiac output.

Adaptations to Exercise

Trained individuals will have a higher stroke volume at rest or during exercise due to adaptations to exercise. The heart, after all, is like any other muscle in which you train. The harder you work your heart, the larger the muscle will become. The increase in size translates into a higher stroke volume. This adaptation also means that the heart has become more efficient. The body can deliver more oxygenated blood per beat due to increases in red blood cells and oxygen-containing proteins.

Handling Increased Blood Flow

Your body will better be able to handle the higher cardiac output with regular exercise. The body adapts by increasing the flexibility of blood vessels, which in turn can manage the increase in blood flow. This effect explains why exercise can help you control your blood pressure. In addition, more capillaries will develop within the lungs and muscle tissue, providing more surface area for gas exchange. All of these adaptations reflect the body's heightened capacity to cope with increases in cardiac output during exercise.