Whether you are at rest or exercising, physiological functions must remain within a narrow range for you to survive. Negative feedback loops work to keep physiological parameters such as heart rate within this target range, or homeostatic set point. For example, the average resting heart rate should remain between 60 to 100 beats per minute, according to the National Institutes of Health. Without negative feedback loops, there would be little regulation of crucial physiological functions.
A negative feedback loop works by adjusting an output, such as heart rate, in response to a change in input, such as blood pressure. A basic loop consists of a receptor, a control center and an effector. If you are at rest and your blood pressure increases, pressure receptors in your carotid arteries detect this change in input and send nerve impulses to the medulla of your brain, or control center. This signals the brain to reduce nerve impulses that stimulate your heart muscle, an effector, to contract. Your heart contracts more slowly and your output, or heart rate decreases, causing your blood pressure to decrease to within target levels.
Exercise and Heart Rate
When you exercise, your muscle tissues convert nutrients such as glucose into chemical energy in a process called aerobic cellular respiration. Aerobic respiration requires oxygen, so your muscle tissues demand more oxygen to maintain higher energy output when exercising. Oxygen enters your lungs and binds to hemoglobin in your blood, which transports it to your muscle tissue. Your body increases your heart rate and blood pressure when exercising to increase blood flow to your muscle tissue and satisfy the increased demand for oxygen.
Exercise and Negative Feedback
The increase in your heart rate and blood pressure when exercising is not physiologically abnormal; your body increases blood flow to muscle tissue in response to the increased demand for oxygen. The homeostatic set points of heart rate and blood pressure are therefore "reset" higher. For example, when exercising vigorously your heart rate may increase to as fast as 200 beats per minute depending on your age and fitness level, according to the Cleveland Clinic. Negative feedback loops then act to maintain heart rate and blood pressure within these new higher target ranges. After you exercise your muscle tissues no longer demand as much oxygen and your homeostatic set points are reset back to their original target ranges.
When exercising, negative feedback loops work to maintain homeostatic set points other than heart rate and blood pressure. For example, your blood pH must remain within a narrow range of 7.35 to 7.45 for crucial physiological processes to occur. When you exercise, your muscle tissues consume oxygen and produce carbon dioxide as a waste product. Carbon dioxide is acidic, and when it accumulates in your body your blood pH decreases. Chemoreceptors in your blood vessels detect this change in pH and signal your brain to increase the respiration rate of your lungs, so that you breathe out excess carbon dioxide more rapidly and increase your blood pH back to within the target range.