Naked Science Forum

Life Sciences => Physiology & Medicine => Topic started by: jeg29 on 12/10/2006 20:50:41

Title: Hot and Sweaty!
Post by: jeg29 on 12/10/2006 20:50:41

I listen to the podcast while walking the dog here in Hawaii and I've started wondering how much a person can sweat before they can't sweat anymore. With no further fluid intake, How many buckets full before a person would "dry up"?

Aloha from Waikoloa

Title: Re: Hot and Sweaty!
Post by: chris on 12/10/2006 22:34:54

Sweating is the body's means of cooling down. Liquid excreted onto the skin surface absorbs heat and either runs or drips off, or evaporates. As it vapourises the water molecules pick up more energy (referred to as the latent heat of vapourisation), helping to cool the skin.

Sweat is produced by sweat glands, which are tiny tubular structures sited in the dermis and opening onto the skin surface. The body is covered by about two and a half million of them. They are supplied by so called "sudo-motor" branches of the sympathetic nervous system which activates them during the "fight or flight" reaction, the aim being to keep the body cool as you run away. This is why a stressful situation makes your palms sweat.

Around the base of each sweat gland is a nest of capillaries (tiny blood vessels), which have small holes in their walls. These minute perforations sieve out water and dissolved salts but are too small to allow cells and other components of the blood to escape. The filtered water collects in the lumen (tube) of the sweat gland and runs out to the skin surface. Sweat is therefore produced by filtering blood, like a coffee percolator.

In cold countries the average person can pump out about 1 litre of sweat per hour, but a heat-adapted individual living somewhere hot can easily lose 2-3 litres of water every hour. That's equivalent to more than the volume of a can of Coke or Pepsi every 6 minutes.

As a result, without adequate volume replacement, it's possible to become water depleted very rapidly, and even fatally. Water makes up about two thirds of our body weight, and as water is lost from blood to make sweat, more water moves out of tissues and cells to replace the lost volume. But this process can only compensate for so long, and more than about 20-25% water loss can provoke irreversible injury.

Chris



"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx