Homeostasis

Passages on this topic usually hand you a graph or a short description of a loop and ask what kind of control it shows. A negative feedback loop works like a thermostat: a sensor detects that some value has drifted from its set point, and the body's response pushes that value back the other way. Stand in a snowstorm and your core temperature starts to drop, so your hypothalamus triggers shivering and narrows skin blood vessels to cut heat loss, driving temperature back up. Stand in a sauna and the same sensor triggers sweating and widened blood vessels to dump heat, driving temperature back down. Same set point, opposite corrections, because the response always opposes the direction of drift.

Blood sugar regulation runs the same way. Eating a meal raises blood glucose above its resting level. The pancreas detects that rise and releases insulin, which drives cells to pull glucose out of the blood, bringing the level back down toward the fasting baseline. The loop only needs to know which direction the value moved to know which direction to push.

Positive feedback gets mislabeled as a malfunction, but it isn't. It amplifies a change instead of opposing it, and the test expects you to recognize it as normal when the process has a built-in stopping point. Labor contractions are the classic case: cervical stretch triggers oxytocin, oxytocin strengthens contractions, stronger contractions stretch the cervix more, and the loop escalates until delivery ends it. Blood clotting works the same way, escalating until the wound is sealed. The question to ask is whether the loop corrects a drift or drives toward a finish line.