Why must the heart beat faster during exercise?

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Jennifer H. Haythe, MD
Cardiology (Cardiovascular Disease)
The heart must beat faster during exercise because by increasing the heart rate, the body is able to increase cardiac output and deliver the necessary blood flow to the muscles. During exercise the body’s muscles increase their activity level and consume more oxygen. Increased blood delivery is necessary to allow for this increased demand.
When you exercise the muscles in your body must contract, in order to do that they need oxygen, glucose, a molecule called ATP, and amino acids. As your muscles use these compounds and contract themselves, they will create waste products like carbon dioxide, and lactic acid that must be carried away from the muscles. When exercising many muscles will all require nutrients and elimination of waste products constantly at the same time. To meet this demand the heart must rapidly increase the rate at which it beats and pushes blood through the body. This is why the heart beats significantly faster during exercise. 
Discovery Health
Administration

Your heart is a muscle, and gets a workout during exercise. Its job is to get more blood out to the body's muscles when they work hard.

When you are exercising, the heart's blood flow increases by around four or five times from when you are at rest. Your body does this for you by increasing the rate of your heartbeat, thus increasing the amount of blood that goes out to the rest of the body.

Cardiac output is the rate of blood pumped by the heart. You get that from multiplying the heart rate, the rate at which the heart beats, by the stroke volume, which is the volume of blood ejected by the heart with each beat. When you are at rest, the cardiac output is about 5 liters a minute. With exercise, sympathetic nerves stimulate the heart to beat faster and more forcefully; the heart rate can actually increase about threefold.

At the same time the veins constrict due to the sympathetic nerve stimulation. More blood being returned from the working muscles, along with the vein constriction, increases the venous return, or the amount of blood returned to the heart. This increased venous return increases the stroke volume by about 30 to 40 percent.

When the heart pumps at full force, the cardiac output can be about 20-25 liters per minute.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.