But exercise, you’ll sweat
The hottest place I’ve ever lived was beautiful Perth. I was even offered a job there after I finished my postdoctoral fellowship, but a whole month of cloudless, blue sky and a top temperature of 45℃ in the shade were too much for me. And yet, despite that high temperature I sweated less than I did at 29℃ in S’pore. How come? Sweating is, of course, a response of the body’s sweat glands to cool the body, but the cooling effect depends on how quickly the sweat can evaporate and that depends not just on the ambient temperature but the amount of moisture (the humidity) in the air. It is the vaporization of the sweat that cools the body and the high air humidity in Singapore slowed down the evaporation process.
Humans and their monkey relatives possess so-called eccrine sweat glands that produce a colourless and odourless watery liquid all over their body. A second class of sweat glands, in humans restricted to certain places like the armpits, also secrete an odourless but very slightly oily liquid that supports the growth of bacteria, which render this kind of sweat smelly. All mammals other than humans, monkeys and horses have mostly apocrine sweat glands, which are less effective ‘coolers’ then eccrine glands and are more sparsely distributed on the body than they are, for example, on the feet, ears, eyelids and anal/urogenital area of dogs and cats. Their function seems to be that of a pheromone, involving recognition and communication between different individuals and males and females. An evolutionary derivative of these apocrine glands are the milk-secreting mammary glands of all mammals.
In order to secrete sweat, the sweat glands (irrespective of whether eccrine to cool the body or apocrine to emit an odour) need to receive a command. And that is initiated by the part of the brain known as the hypothalamus. It monitors not just the blood temperature but also the hormonal content of the blood, which may be affected by emotions like fear, excitement, and other feelings. From the hypothalamus nerve fibres (called “sympathetic”, because they are not under our conscious control: we can’t switch sweat on and off at will) lead to the sweat glands and release the neurotransmitter acetylcholine (= Ach). The Ach binds to receptor molecules on the sweat gland and causes an increase of calcium ions in the latter, which changes the permeability of the cells’ membranes and leads to a build up of sweat. In humans a 2-5 mm long duct runs from the subcutaneous bulbous basal part of the sweat gland to the surface of the skin. In this duct sodium may be reabsorbed and saved for the body, but sweat rate and sodium reabsorption depend on the speed at which Ach is hydrolysed. If too much sweat in a very short time is produced, sodium reabsorption may not be able to cope and the sweat could become slightly salty. The 2-4 million sweat glands in the human body can actually produce 10 litres of sweat in a day.
Animals without those cooling sweat glands have to seek the shade when it’s hot and they may enter the water (like our cat did when we had the hottest day in Perth), or they are panting to let the liquid on the tongue evaporate. However, not all animals that exude liquid through their skins “sweat” to cool down their bodies. Some frogs have wet skins, because they need to get rid of excess water and blood-sucking leeches “sweat” for that same reason: they want the solid components of the ingested blood and not the aqueous part. With humans it’s different: we sweat and need to replace the water – sometimes with something a little tastier than just water! Cheers.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2020.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.