Staying in balance

Homeostasis is the body’s ability to keep its internal conditions steady — body temperature, water levels, blood sugar, the acidity of the blood, and dozens of other variables — even as the outside world changes constantly around it. It is one of the most important ideas in all of physiology, because nearly every system you will study in this course exists, at least in part, to help maintain it.

Systems rarely work alone to hold a variable steady — they cooperate. When you exercise, your muscles burn fuel faster and produce heat and carbon dioxide; in response, your heart beats faster to deliver more oxygen and clear waste, you breathe faster and deeper to bring in more oxygen and expel more carbon dioxide, and blood vessels near your skin widen to release excess heat. That is homeostasis in action — several systems adjusting together, automatically, to keep your internal environment stable while your activity level changes around it.

Keep the idea of homeostasis in mind as you move through the rest of this course. Nearly every organ system you build in the Body Lab — the lungs bringing in oxygen, the kidneys filtering the blood, the endocrine system releasing hormones — is, from one angle, a tool the body uses to keep some internal condition inside a healthy, stable range.