If you step out of a cool building into a hot and sunny day, you’ll probably notice that your skin starts to flush. Beads of sweat form as the heat settles on your body. These reactions, as uncomfortable as they are, are designed to help you survive. Both responses are part of the body’s automatic way of restoring its internal temperature to the typical 98.6 degrees Fahrenheit.
That process is one of many self-regulating mechanisms in the body, all working to maintain a constant internal environment in any condition—not just temperature but also blood pressure and pH. Your body makes these physiological adjustments without the need 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 Tansey, a reader at Queen’s University Belfast who directs the Center for Biomedical Sciences Education. “If homeostasis wasn’t there, if the mechanisms that are normally there are overwhelmed, that’s what can lead to illness and to disease.”
What is homeostasis?
The term “homeostasis” originates from two Greek words, “homio,” which means “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 constant internal environments. For certain human functions, balance is achieved in a relatively narrow range called a “setpoint.” Around 98.6 degrees, for example, is usually the body’s optimal temperature. But for other functions, the setpoint is more dynamic.
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All of this adjustment happens through a signaling system within your body. Three things—receptors, an integrating center, and effectors—maintain homeostasis. Throughout the body, receptors, which can be a special class of protein 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, sometimes called the control center. In humans, this is usually a region in the brain called the hypothalamus, which regulates temperature, heart rate, hunger, water balance, circadian rhythms, and other essential conditions. “The integrating center is the place where the change is compared to the setpoint,” explains Tansey.
The final step is the response, which often uses the endocrine system or the nervous system. Signals sent through those systems arrive at effectors, which are whatever parts of the body that cause the necessary physiological responses. To regulate temperature, for example, effectors include blood vessels and sweat glands in the skin. Or to increase blood pressure, kidneys will retain more water. To increase oxygen, the diaphragm and respiratory muscles will take more or deeper breaths. Other effectors change behavior. When you feel thirsty or crave salty foods, that might be a homeostatic response.
Once the function in question has returned to its normal range, the receptors will send a new message, called negative feedback, to the integrating center. This tells the other parts of the body to stop overcompensating, turning the process off.
Biology’s air conditioning
Humans lose heat to the environment under most conditions, Tansey says. But in a particularly hot climate, the body warms up from solar radiation and conduction from warmer things around it.
Although there are temperature receptors in skin, “it’s the core body temperature that is most important,” she says, which is why receptors also exist in the hypothalamus itself, around the veins, the viscera, and the spinal cord.
When it’s scorching out, all of these thermoreceptors send signals to the hypothalamus. Researchers think this brain region determines how far off the body’s warmth 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, notifying the blood vessels in the skin. There, smooth muscle surrounding the vessels relaxes, which causes increased blood flow to the surface of the skin, carrying heat away from the body’s core. This process, called vasodilation, also makes the skin appear pinker and more flushed. Millions of sweat glands also activate, 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
If the body starts to struggle to get rid of enough heat, it can become overwhelmed, says Dawn Emerson, clinical assistant professor of exercise science at the University of South Carolina. When a body can’t sweat, it stops being able to transport heat away from the body. That, she says, is classic heat stroke. (Exertional heat stroke can look a little different, she says, because patients overheating from exercise could still be sweating.)
Things start to go awry as the central nervous system stops functioning properly. People with heat stroke “exhibit signs of abnormal behavior, they become really aggressive, they kind of forget what’s going on, because their brain is not functioning the way that it should,” Emerson says. Other organs will start to shut down in the body’s attempt to regain control.
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This kind of full-system failure isn’t unique to overheating. “When the body is not in homeostasis, it’s going to keep trying to fight to be in that, and that causes a lot of stress,” Emerson says. A stressed body will compromise other functions in an attempt to achieve homeostasis.
Some kinds of overheating, though, don’t trigger this response. When someone has a fever as an immune response to an infection or illness, for example, the body establishes a new setpoint, resetting the thermostat to be hotter. That’s because a toasty inflammatory response is a helpful part of the battle against a pathogen, Emerson explains. “Heat kills off pathogens,” she says, and an increase in blood flow brings more white blood cells to fight off the infection.
Some people might be able to override their homeostatic setpoints. Take “The Iceman” Wim Hof. He’s run a half-marathon on snow and ice while barefoot, and can remain 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. Those behaviors, the scientists say, may have primed Hof’s brain to withstand the stress of submerging himself in ice-cold water.
Such studies open up the possibility that we might be able to adjust our setpoints a little bit, but forcing an override can be dangerous.
“You can override homeostasis to, for example, drink lots of water when you don’t need it,” Tansey says. But if you drink too much, your blood’s sodium levels will become too diluted, and the extra water can enter your cells. This causes your cells to swell, which is particularly dangerous when it happens in the brain. “That can be fatal,” Tansey says.
A more common hack occurs during fitness routines. When training, it is possible to acclimate your body to new conditions so it doesn’t have to work so hard to maintain homeostasis, Emerson says. The body of someone who starts exercising will become better at handling the heat of that activity. Over time, she says, sweat rates will increase to cool off more quickly, skin blood flow will also change, and your cardiovascular strain will decrease. It takes about 10 to 14 days, Emerson says, to get used to that exercise and adapt—setting a new baseline for your body’s balancing superpower.