Why is constant body temperature important?

Humans can only survive within a very narrow range of temperatures - like other mammals and birds they have to keep their body temperature constant at all times, around 37°C. Just five degrees too high or low would be a dangerous condition.

How does the body achieve a constant temperature?

To achieve this, the human body keeps a careful balance between heat production and heat loss. Heat production is easy - most of the energy we take in with our food is transformed into heat by our muscles and internal organs. When we get too cold, our muscles start to shiver, thus producing more heat, and our capillaries contract, keeping the heat inside.
 
Why do we sweat?

You think that getting flushed and sweaty in summer is a nuisance? Actually it keeps you from collapsing in the heat! The most important organ for heat loss is the skin. It sweats, benefiting from the cooling effect of evaporation. Another strategy is to radiate heat, especially when the capillary vessels under the skin are dilated and a lot of blood is transported through the skin. The lungs also take part in the business of heat loss: we virtually breathe out the heat.

What else can be done in extreme temperatures?

The organ that controls body temperature is called the hypothalamus, a tiny spot deep in the brain. It regulates the sweating and shivering, but it also sends signals to the brain suggesting action: putting on a sweater, stepping into the shade or taking off a coat might help our body to keep cool or hot.