How Negative Feedback Loops During Exercise Affect Heart Rate and Blood Pressure

A negative feedback loop may sound like something you'd encounter in a toxic workplace. But it's actually a totally normal process your body uses to make sure all your systems are stable and operating smoothly.

Here's the negative feedback loop definition: "A negative feedback loop is a way of controlling deviations from a set point in the body," says Mike T. Nelson, PhD, professor of human performance at the Carrick Institute for Functional Neurology. "It helps you maintain homeostasis." Homeostasis is a stable base state in which things are level.

Let's say your thermostat is set to 70 degrees in the winter. The thermostat has a sensor that monitors room temperature. Whenever it dips below 70, the sensor triggers the furnace to kick on. Once the room reaches the programmed temp, the furnace turns off. This process of upholding a constant temperature is a negative feedback loop.

The same concept can be applied to blood sugar regulation in your body: Insulin lowers your blood glucose when levels in your body are high and glucagon raises your blood glucose when levels in your body are low.

Yep, those exist too. What is a positive feedback loop? Unlike negative loops, which help keep your body in a steady state, positive feedback loops have a snowball effect. They amplify changes in your system, pushing you farther from equilibrium.

"A positive feedback loop is like a chain reaction, intensifying and building on the initial response," says R. Kannan Mutharasan, MD, co-program director of sports cardiology at the Northwestern Medicine Bluhm Cardiovascular Institute.

A good example of a positive feedback mechanism would be training a puppy. If your dog fetches a ball and you give them positive feedback, like a pat and a treat, they'll be encouraged to repeat the behavior and fetch more balls. In contrast, if your puppy chews up your running shoes and you give them negative feedback by scolding them, they will be less likely to destroy your footwear in the future.

Every feedback loop — positive or negative — has the following five steps.

1. Receptors in your body detect what's happening in a certain system and how different biological states change. (This includes your heart rate, temperature and glucose levels.)
2. This information goes to your brain.
3. Your brain integrates the info and figures out how to respond.
4. Your brain sends instructions throughout your body to respond.
5. Your body follows your brain's instructions.

What is a negative feedback loop when exercising? When you're working up a sweat, your body adjusts to the uptick in intensity in a number of ways. It uses negative loops to help you perform at your desired level of intensity, while also keeping you healthy and thriving.

To understand the role of negative feedback loops in your body during exercise, let's take the example of temperature again — but this time, instead of exploring how home heating is regulated, we'll look at body temperature.

The average body temperature is 98.6 degrees Fahrenheit, so whenever your temp rises above this, your body attempts to cool down. "You generate heat when you're working out, your body senses your temperature rising, and it responds by shunting heat," Dr. Mutharasan says.

The capillaries and blood vessels that go to your skin dilate, bringing heat to the surface of your skin where it can be released. In addition, your sweat glands release perspiration, which cools you as it evaporates off your skin.

Extreme fatigue after exercise is another negative feedback loop example.

"If you are overtraining and pushing too hard, you may notice fatigue," Dr. Mutharasan says. "This is a negative feedback loop mechanism to tell your body to slow down so you don't continue to overload yourself."

Negative feedback also happens if you get hurt during physical activity.

"When your body detects an injury, it sends a pain signal to the brain," he says. "In turn, the brain sends a message to the body to take it easy."

When you're working out, your body requires extra blood flow to power the muscles you're using. This influx of blood flow brings oxygen to your muscles, while also clearing out waste products like carbon dioxide.

As you begin exerting yourself, your brain signals the vessels in your muscles to dilate so a greater amount of blood can stream through them, allowing you to increase your cardiac output.

To accommodate the heightened demand for blood flow, your heart rate goes up so it can pump blood more quickly through your system. As your heart rate rises, it drives up your blood pressure.

This is a positive feedback loop in your body. As you ramp up the intensity of exercise, your body in turn raises your heart rate and blood pressure to help you fulfill the physical demands of exertion.

Then, once you reach "cruising altitude" — say, if you're on a tempo run — negative feedback loops keep your heart rate and blood pressure in homeostasis.

"If your heart rate or blood pressure shoot too high, your body will signal you to slow down to keep your vitals in check," Nelson says. If they dip too low to keep you moving at a steady pace, your body will make the necessary adjustments to get you back up to speed.

At the end of your workout, negative feedback loops for blood pressure and heart rate also help you decelerate.

"You don't need as much blood flow, so your blood vessels constrict, causing your blood pressure and heart rate to drop," Dr. Mutharasan says.