Keeping Things Stable

If you've ever shivered in cold weather, you've experienced one of the many ways your body retains a stable environment, also known as homeostasis. Homeostasis is the body's "tendency to resist change in order to maintain a stable, relatively constant internal environment." It is crucial to humans' survival; humans can die when homeostasis becomes unsuccessful.

Our external environments (such as a bedroom or car) are usually not the ideal temperature for our body: 98.6 degrees Fahrenheit (37 degrees Celsius). Your thermostat is probably set somewhere between 60 and 80 degrees Fahrenheit, which isn't that close to the body's necessary 98.6! That's why the body self-regulates its internal temperature, pH level, blood glucose (sugar the body produces from food consumption) level, and more. If any of these values stray too far away from their respective balance points, the body is in danger of functioning improperly.

For example, body temperature must be regulated so that enzymes can work properly. Enzymes are "proteins that speed up the rate of a chemical reaction in a living organism." They need very specific optimum temperatures in order to function because if the temperature is too far from this optimal value, the enzyme will denature (its physical structure will be damaged) or the rate of chemical reactions in the body will not be ideal.

Within the body, homeostasis can regulate internal environments in many locations, from the stomach to individual cells. This is because each part of our body is highly specialized. The stomach's acidic environment is ideal for food digestion, but its acid content would consume surrounding body tissues if not for the stomach's protective lining. To give you a sense of how acidic the stomach's environment is, consider the pH scale. It ranges from 0 to 14 with 0 being the most acidic, 14 the most basic, and 7 completely neutral. Stomach acid is usually between 1-3!

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Homeostatic mechanisms work by using negative feedback loops; this means that when the variable is too far above or below the set point, homeostatic circuits are activated to restore necessary conditions.

For temperature regulation, sensors in the skin and brain alert the hypothalamus (a part of the brain) of high/low external temperatures, and the body takes action. If the body is hot, blood vessels dilate (this is called vasodilation) to increase the rate at which heat leaves the body (this is why your cheeks may look pink/red after exercise!). The body also starts sweating because sweat causes evaporative cooling (this means that the body temperature decreases as the sweat droplets evaporate), and heavy breathing occurs to accelerate heat loss.

Contrarily, if the body is cold, you might start shivering and see goosebumps; blood flow to the skin will also decrease. The movement of shivering causes muscles to generate heat, and reducing blood flow to the skin reduces heat loss (which was desirable when the body was overheated). Goosebumps can trap air under the skin, but they used to be more helpful when humans had a thicker coat of hair; at that time, activating goosebumps provided humans extra insulation.

While negative feedback loops are common in biological body systems, a key example of a positive homeostatic feedback loop occurs during pregnancy. Keep in mind that for positive feedback loops, signals enhance a condition instead of seeking to restore it to its balance point. The pressure of a baby's head on the cervix activates neurons in the brain; these neurons signal to the pituitary gland that oxytocin must be released. Oxytocin is then released, increasing uterine contractions and pressure on the cervix. This example demonstrates how the stimulus amplifies the initial effects, instead of inhibiting them as would happen in a negative feedback loop.

As you can see, homeostasis is crucial to maintain proper internal conditions for our bodies. Without homeostasis, all of the body functions we normally rely on would be affected, or even ceased altogether!