Why do people sweat?
Sweating removes excess salt and wastes through the skin. It also helps to keep you cool, because sweat takes heat away from the body as it evaporates on the skin. If you have been exercising and are very sweaty, and then stand in a draught, you will notice this cooling effect very quickly. Sweat is a watery liquid, but other skin glands release an oily substance called sebum that helps to lubricate the skin and keep it supple.
Why do I need to sleep?
You spend about one-third of your life asleep, but no one is sure why this is necessary. During deep sleep, the body produces large amounts of growth hormone that help to repair or replace damaged cells and tissue. Sleep also speeds healing. While you are sleeping, the brain remains very active. The body goes through stages of being very limp and relaxed and other stages when the eyes move about beneath closed eyelids. Dreaming takes place during this rapid eye movement period, called REM sleep. Dreaming is very important. If you prevent someone from dreaming by waking them every time they enter REM sleep, that person can become very disturbed and ill after several dreamless nights.
How does the body repair damaged tissues and cells?
Cells are able to divide very quickly to replace those that are old or have died. Nerve cells are the only ones that cannot be replaced. However, even nerve cells can sometimes grow new connections if the long fibres along which messages are passed become damaged. Dead and dying cells are removed by white blood cells in the blood stream, which actually eat them. The liver is also able to break down red blood cells, which are only able to survive for a short time.
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