Your body works hard to keep all its systems running smoothly. When it senses something is out of the ordinary, stabilizing mechanisms help you return to a steady state. For example, when you eat a large meal, your stomach expands to accommodate the food. If you eat a box of candy and your blood sugar rises, your body secretes insulin to lower it back down. These stabilizing mechanisms are known as negative feedback.
Exercise acts as stress on the body, bringing you out of homeostasis, which is your body's preferred, constant state of existence. Your body tries to correct itself with negative feedback, including a risen heart rate, to achieve a consistent internal environment.
Read More: How to Immediately Lower Heart Rate
How Exercise Affects You
When you exercise, you put extra demands on your body, throwing it out of a balanced state. Working muscles are deprived of blood, your cells need more oxygen to support their efforts and greater cell respiration means you have more carbon dioxide to expel. Your body temperature increases and your body uses more stored energy to keep going.
All of these factors throw your body out of its normal rhythms, so the circulatory and respiratory systems, as well as your skin, kidneys and liver, step in to normalize you in the form of negative feedback.
Your heart rate rises during exercise to help meet the increased demands placed on your body. A higher heart rate reflects that blood is pumping at a far faster rate so your working tissues get oxygen faster and waste, in the form of carbon dioxide, is removed quicker from those taxed cells in your bloodstream.
It's your medulla, a portion of the brainstem, that instigates the negative feedback. It reacts to the rise in carbon dioxide and sends a signal to your adrenal glands to release epinephrine — or adrenaline — to raise your heart rate.
This heart rate response is coupled with increased respiration — or breath rate — which increases the pace and volume of oxygen brought into the body and quickens the rate at which carbon dioxide is removed.
If you've ever been exercising and just can't go any harder, that's the point where your body can no longer fight to maintain homeostasis. When you slow down or stop exercising, carbon dioxide levels in your blood decrease and your systems — including your heart rate — come back down to normal.
Read More: What Factors Can Influence Heart Rate?
Other Negative Feedback
Your skin has a negative feedback response to exercise, too. Sweat and increased blood flow to the skin are ways your body tries to regulate your body temperature as you heat up.
Your liver works to mobilize more stored glycogen, a fuel source, as you exercise to keep your energy supplies steady. As you sweat, you lose water and the concentration of salts in your blood changes. The kidneys react by raising your thirst levels and reducing the secretion of water (urine) in response.