Working up a sweat

How do we sweat, how do our exocrine glands work? Christof Schwiening is here to give us the details...

Human beings make a lot of strange liquids: saliva, both smelly and non-smelly sweat, digestive enzymes in your gut, milk, acid, mucus, earwax, and more. They may be a bit unpleasant, but they all serve a purpose. But where do these things come from, and how do we make them?

Epithelial cells have two sides, one facing the inside of the body and one facing the outside. Epithelial cells come in many shapes and sizes - some are rather boring, looking like a sheet of bricks bonded together with tight junctions. But, dotted around the body, in numerous places, these cells form little structures that can secrete an array of different chemicals: we call these exocrine glands.

The ability to secrete fluid comes from the fact that different proteins occur in each side of the cell allowing the cells to effectively pull chemicals into the cell from inside the body and then push them out across the other side into the outside world.
The processes that do this pushing and pulling are diverse. Sometimes they are proteins, rather like special revolving doors in the cell membrane that can allow only certain chemicals out - we call these transporters. Sometimes they are special small holes like 'cat-flaps', letter-boxes, or vents that can also only allow certain chemicals out - we call these channels. And, sometimes stuff literally gets thrown out of the cell in small package. And, sometimes, specific cells just shed bits of themselves into the outside world - a kind of micro-dandruff, just in case these things weren’t icky enough...

The exocrine glands are exquisitely arranged with a mix of different cell types in specific structures. The eccrine sweat glands are a nice example. They consist of a coiled duct made from cells at the base which produce the sweat and then a straighter duct that reabsorbs some of the salt before the sweat reaches the surface of the skin.

Exocrine glands are very sensitive to the world around us. When sweat glands are working properly, they start producing sweat when the brain detects a sufficiently large rise in brain temperature.

The exact temperature at which that occurs depends on many variables and can be trained by exposure to prolonged heat. Equally, the sweat glands themselves can be trained. If you sweat a lot, the sweat glands can become larger and be able to produce more sweat

There are many other stimuli that can activate exocrine glands. Saliva is produced even if you think hard enough about food. And the 'polygraph' or lie-detector test is based upon the idea that stress activates sweat glands - but stress can be caused by many things, one of which may, or may not, be lying!

Exocrine glands are critically important for life - they take the simple watery bit of the blood and produce from it fluids that can propel our sperm and keep it swimming, allow us to digest and swallow our food, stay cool, feed babies, and less usefully allow our glasses to gradually slide down our nose.