Homeostasis

What is homeostasis?

Homeostasis is how your body regulates your internal systems so they function correctly. Your body works best when its internal environment — including things like temperature or oxygen levels — is just right.

The term homeostasis comes from two ancient Greek words meaning “equal” and “holding still.” In other words, homeostasis means reaching and maintaining a state of balance. In this context, balance isn’t about keeping you from toppling over. It’s about having the right balance of everything you need.

Balance is key because too much of even the most essential things can be harmful. For example, everyone knows that water and oxygen are good things, and you need them to survive. But drinking too much water too quickly can cause hyponatremia. And breathing pure oxygen for too long can cause oxygen toxicity. Both of those conditions can be deadly.

How does homeostasis work?

Homeostatic processes are automatic, and a specific part of your brain, the hypothalamus, runs them. There are two main ways that homeostasis happens:

• Negative feedback
• Positive feedback

Negative feedback homeostasis

Negative in this context doesn’t mean bad. It means your body senses a change and tries to counteract or reverse the unwanted change. Most homeostatic processes in your body rely on negative feedback.

Examples of negative feedback homeostasis include:

• Blood pressure. Your blood pressure keeps blood flowing up toward your head against the pull of gravity.
• Body temperature. If you’re too hot, your body makes you sweat to cool you down. If you’re cold, your body makes you shiver to generate heat. These are examples of thermal (temperature) homeostasis.
• Fluid balance. If you drink too much water, you’ll soon feel the urge to pee to get rid of the extra fluid.
• Oxygen levels. Your heart and breathing rates increase and decrease depending on how active you are. That’s an example of respiratory and circulatory homeostasis.

Positive feedback homeostasis

Positive feedback homeostasis means the processes add to themselves and get stronger. That will keep on happening until the initial cause is gone.

Examples of positive feedback homeostasis include:

• Labor and delivery. When labor starts, your cervix starts to stretch, which triggers nerve signals that go to your brain. Your brain releases the hormone oxytocin, which causes uterus contractions. The more your cervix stretches, the more oxytocin your brain releases and the stronger the contractions get. That stops once delivery is complete and your cervix returns to its normal size.
• Hemostasis (blood clotting). When you have a wound that bleeds, clotting factors in your blood activate to seal the wound. They also call for reinforcements, summoning even more clotting factors. They’ll keep on calling for more clotting factors until the bleeding stops.
• Immune responses and inflammation. When you have an infection or injury, your immune system causes inflammation at that site. Inflammation is your body’s way of shining a spotlight on a specific place so immune cells know exactly where to go. And immune cells at the scene will keep inflammation going as long as they need more help.

Why is homeostasis important?

You can’t survive without homeostasis and the processes that drive it. And every disease or medical condition involves a disruption of homeostasis.

When you get hurt or sick, your body adjusts its homeostatic processes so you can recover. That’s why you get a fever with infections or your blood clots when you have a bleeding wound.

Chronic conditions happen when something goes wrong and your homeostatic processes can’t adjust enough to make up for it. For example, high blood pressure (hypertension) happens when your body can’t keep the pressure within a healthy range. Osteoporosis happens when your bones lose calcium faster than your body can replace it.

Some diseases happen because homeostatic processes malfunction and make things worse. Examples include:

• Allergic reactions. Allergies happen when your immune system reacts to something that it thinks is a threat. Mild allergies may just be unpleasant. But if the reaction is too strong, that homeostatic process can be dangerous. For example, a bee sting causes brief pain and mild swelling for most people. But a sting can cause deadly anaphylaxis for someone with an allergy.
• Autoimmune diseases. These happen when your immune system malfunctions and mistakenly attacks part of your own body. The reaction itself would likely be normal and healthy if the target were an infection or injury instead of your own healthy tissues.
• Cytokine release syndrome (CRS) or sepsis. Immune system reactions naturally strengthen with time and will stop once the initial cause is gone. But those processes can rapidly become dangerous if your immune system reacts too strongly early on. That’s why CRS and sepsis can become deadly so quickly.

How does the nervous system maintain homeostasis?

Your nervous system balances its own activity using two subsystems that do opposite jobs. Those subsystems, which are part of your autonomic nervous system, are your:

• Sympathetic nervous system. This activates to protect you when you’re in danger. It’s responsible for your “fight-or-flight” response.
• Parasympathetic nervous system. This is what relaxes your body. It takes the lead when you feel safe and comfortable.

How does the skeletal system maintain homeostasis?

Your skeletal system constantly goes through a maintenance cycle of breaking down and replacing old bone tissue. You need vitamin D and calcium for that to work properly. But that maintenance process slows down naturally as you age. And if you lose bone tissue faster than your body can replace it, you can develop conditions like osteopenia or osteoporosis.

How does the respiratory system maintain homeostasis?

Your respiratory system helps maintain multiple types of homeostasis. Two key examples include:

• Oxygen levels throughout your body. That’s why physical activity makes you breathe faster and harder. Your body needs more oxygen to meet the demand.
• Blood pH level. Your body uses carbon dioxide to manage your blood pH (acid-base balance). Your blood is slightly basic (alkali), the opposite of an acid. When blood pH strays too far from normal, respiratory acidosis or respiratory alkalosis can happen.

How does the endocrine system maintain homeostasis?

Your endocrine system supports homeostasis for multiple systems. That’s partly because your endocrine system creates many key hormones, which are like chemical messengers. They can tell your body systems when to start or stop certain processes. Examples include your digestive system, metabolism, and the male and female reproductive systems.

How does the muscular system maintain homeostasis?

Your muscular system relies on and supports homeostasis in multiple ways. One example is how your muscles constantly break down and rebuild themselves. Your muscles are also important for temperature homeostasis, especially when they generate heat to keep you warm.