Homeostasis and feedback.

Step 1 of 6

Thirty-seven degrees

The body’s cells work within narrow conditions: temperature, water, sugar and salts in the blood. Enzymes, for example, work best around 37 °C. Keeping those conditions steady whatever happens outside is called homeostasis.

In the Scene a person stands for an hour in a room. The thermometer shows the core temperature, of the brain and the inner organs, and the chart shows how it changes through the hour. The room is now cool, 10 °C, like a winter night in an unheated house.

Step 2 of 6

A freezing night

Temperature is measured in the hypothalamus, a small region at the base of the brain, and by sensors in the skin. When the body cools, the hypothalamus narrows the vessels in the skin, so less warm blood reaches the surface, and starts shivering: quick contractions of muscles that make heat. In the heat it does the opposite: it widens the skin’s vessels and switches on the sweat glands.

Step 3 of 6

The thermostat

The hypothalamus works like a thermostat in a house: it compares the temperature with a set point and switches heating or cooling on. What happens when it does not work? In the Scene you can switch it off: skin blood flow, sweating and shivering stay as they were in a pleasant room, whatever happens outside.

Step 4 of 6

Hotter than the skin

Zoom to the skin is at Skin up close: the layers of the skin, the blood vessels, the sweat glands and the hair muscles. Heat always flows from hot to cold. As long as the air is cooler than the skin, the skin gives it heat by radiation and convection. But on a 40 °C day the air is hotter than the skin.

Step 5 of 6

The comfort zone

Between the cold that makes you shiver and the heat that makes you sweat there is a band where the body manages the cheapest way: it only widens and narrows the skin’s vessels, setting how much heat reaches the surface. Exercise and Thermostat are fixed, and Air temperature is yours.

Step 6 of 6

Negative feedback

Everything you have seen is built from the same loop: a sensor measures, a control centre compares with the set point, and effectors change the state in the direction opposite to the drift. The loop is called negative feedback, and it also holds the body’s blood sugar, its water and its blood pressure.