If you step out of a cool building on a warm and sunny day, you will probably notice that your skin starts to blush. Beads of sweat form as the heat settles on your body. These reactions, as uncomfortable as they are, are meant to help you survive. Both responses are part of the body’s automatic way of restoring internal temperature to the typical 98.6 degrees Fahrenheit.
That process is one of many self-regulatory mechanisms in the body, all of which work to maintain a constant internal environment in all conditions — not just temperature, but blood pressure and pH as well. Your body makes these physiological adjustments without having to think. This superpower is called homeostasis.
The stakes are high. “The further your body moves away from the status quo…the more dangerous it can be,” says Etain Tanseya reader at Queen’s University Belfast, which runs the Center for Biomedical Sciences Education. “If homeostasis wasn’t there, if the mechanisms that are normally there are overwhelmed, then that can lead to illness and disease.”
What is Homeostasis?
The term “homeostasis” comes from two Greek words, “homio,” meaning “similar to” and “stasis,” which translates to “standing still.” All organisms on Earth, with the exception of viruses, use forms of self-regulation to maintain a constant internal environment. For certain human functions, equilibrium is reached in a relatively narrow range called a “set point”. For example, around 98.6 degrees is usually the optimal body temperature. But for other functions, the setpoint is more dynamic.
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All of these adjustments happen through a signaling system in your body. Three things – receptors, an integrating center and effectors – maintain homeostasis. Throughout the body, receptors, which could be a special class of proteins or specialized nerve cells, for example, monitor conditions such as temperature or blood pressure. When a receptor detects a change, it sends a signal to the integrating center, also called the control center. In humans, this is usually an area of the brain called the hypothalamus, which regulates temperature, heart rate, hunger, water balance, circadian rhythms, and other essential conditions. “The integration center is where the change is compared to the set point,” explains Tansey.
The last step is the reaction, which often uses the endocrine system or nervous system. Signals sent by those systems arrive at effectors, the parts of the body that trigger the necessary physiological responses. For example, to regulate temperature, effectors include blood vessels and sweat glands in the skin. Or to raise blood pressure, the kidneys retain more water. To increase oxygen, the diaphragm and respiratory muscles will breathe more or deeper. Other effectors change behavior. If you are thirsty or have an appetite for salty foods, it could be a homeostatic response.
Once the function in question has returned to its normal range, the receptors send a new message, called negative feedback, to the integrating center. This tells the other parts of the body to stop overcompensating, shutting down the process.
Biology’s Air Conditioning
Humans lose heat to the environment under most conditions, says Tansey. But in a particularly hot climate, the body heats up through solar radiation and conduction from warmer things around it.
While there are temperature receptors in the skin, “core body temperature is most important,” she says. Therefore, receptors also exist in the hypothalamus itself, around the veins, viscera, and spinal cord.
When it burns out, all of these thermoreceptors send signals to the hypothalamus. Researchers think this brain region determines how far body heat is from 98.6 degrees by sensing how the nerve cells that respond to temperature changes throughout the body fire differently, Tansey says. Then the hypothalamus sends signals that alert the blood vessels in the skin. There, the smooth muscle around the vessels relaxes, causing increased blood flow to the surface of the skin, dissipating heat from the core of the body. This process, called vasodilation, also makes the skin look pinker and redder. Millions of sweat glands are also activated, and those droplets finish the job by transferring heat from the skin’s surface to the air through evaporation.
When the body’s AC stops working
As the body begins to struggle to get rid of enough heat, it can become overwhelmed, he says Dawn Emerson, clinical assistant professor of exercise science at the University of South Carolina. When a body cannot sweat, it stops transferring heat away from the body. That, she says, is classic heat stroke. (Exertional heat stroke can look a little different, she says, because patients who overheat from exercise can still sweat.)
Things start to go wrong when the central nervous system stops functioning properly. People with heat stroke “show signs of abnormal behavior, they get very aggressive, they kind of forget what’s going on because their brains aren’t functioning the way they should,” says Emerson. Other organs will begin to shut down in the body’s attempt to regain control.
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This type of full system failure is not unique to overheating. “If the body isn’t in homeostasis, it’s going to keep trying to stay there, and that causes a lot of stress,” says Emerson. A stressed body will compromise other functions in an attempt to achieve homeostasis.
However, some forms of overheating do not cause this reaction. For example, when someone has a fever as an immune response to an infection or illness, the body sets a new set point, resetting the thermostat to run hotter. That’s because a toasty inflammatory response is a useful part of fighting a pathogen, Emerson explains. “Heat kills pathogens,” she says, and an increase in blood flow creates more white blood cells to fight the infection.
Hack homeostasis
Some people may be able to override their homeostatic set points. To take “The Iceman” Wim Hof. He has run a half marathon on snow and ice barefoot and can stay submerged in ice cubes for more than 112 minutes. A pair of neuroscientists at Wayne State University studied the techniques “The Iceman” uses, such as controlled breath retention, hyperventilation, and meditation. That behavior, the scientists say, can have primed Hof’s brain to withstand the stress submerge himself in icy water.
Studies like these open up the possibility that we might be able to tweak our setpoints a bit, but forcing an override could be dangerous.
“You can override homeostasis, for example, to drink a lot of water when you don’t need it,” says Tansey. But if you drink too much, your cells’ sodium levels can drop as they swell with water, which is especially dangerous if it happens to brain cells. “That can be deadly,” says Tansey.
A more common hack occurs during fitness routines. While exercising, it’s possible to get your body used to new conditions so it doesn’t have to work as hard to maintain homeostasis, says Emerson. The body of someone who starts exercising will handle the heat of that activity better. Over time, she says, the rate of sweat will increase to cool you faster, the blood flow of the skin will also change, and your cardiovascular load will decrease. It takes about 10 to 14 days, Emerson says, to get used to and adjust to that exercise — setting a new baseline for your body’s balancing superpower.