Your body's core stays near 37 °C, on a cold winter morning or a hot summer afternoon. It is the clearest example of the negative feedback you just learned.
Key idea
The hypothalamus in the brain is the control center. When the body is too warm, it triggers sweating and widens blood vessels in the skin. When the body is too cold, it triggers shivering and narrows blood vessels in the skin. Each response pushes temperature back toward the set point.
Why it works
Two pieces of physics make it work. Evaporation: when sweat turns from liquid to vapor, it takes heat with it, cooling the skin. Blood flow: wide skin vessels bring warm blood near the surface, where its heat escapes to the air; narrow vessels keep warm blood deep in the body. Shivering is fast, small muscle contractions, and working muscles release heat from cellular respiration.
Worked example: A hot day
A runner's temperature rises during a race on a hot day. Describe the feedback loop.
- Stimulus: temperature rises above the set point.
- Sensors in the skin and brain detect it; the hypothalamus responds.
- Effectors: sweat glands release sweat and skin vessels widen.
- Sweat evaporates and warm blood near the skin loses heat.
Answer: Temperature falls back toward 37 °C: negative feedback.
Worked example: Why humid days feel worse
On a very humid day, sweat drips off instead of drying. Why does the body cool less?
- Cooling comes from evaporation.
- Humid air already holds a lot of water vapor, so sweat evaporates slowly.
Answer: Less evaporation carries away less heat.
Other animals use the same ideas in different ways. Dogs pant, which evaporates water from the mouth and tongue. Elephants flap their large ears, which are full of blood vessels. Lizards and snakes, which get most of their heat from their surroundings, control their temperature mainly by behavior: moving into the sun or into shade.
Common mistake
It seems natural to think sweat cools you because it is cold. Sweat comes out at about body temperature. It cools the skin as it evaporates, because turning liquid water into vapor takes heat, and that heat comes from your skin.