Sweat, perspiration, hidrosis. Its all very relevant to fluid balance. They say once your febrile patient has began to sweat, the fever has "broken" and you don't need to prescribe antipyretics. Of course, they will proceed to wipe themselves with towels and whatnot, thereby abolishing all chances of losing heat by evaporation. Nevermind this tangent. In fact the effects of water loss (by sweat and other means) is discussed in the page dedicated to the physiological response to dehydration. This chapter addresses the physiological properties of sweat, and the physiology of sweating.
The 2023 CICM Primary Syllabus does not contain sweat, which is ironic, because it contains a lot of blood, and is powered entirely by tears. In terms of exam value, this topic has never been interrogated in the CICM primary exams, as it is very specific and unlikely to become important in the ICU environment. The issue of evaporative water loss has some importance in terms of affecting daily fluid balance and sweat has appeared as one of the answers in questions on thermoregulation, but the specific properties and contents of this fluid have never been of any interest to the college examiners.
Sweat has several physiological roles, which can be said to be common to most species of animals. In general, it can be safely said that sweat glands have quite different roles across different species, and within the same species between body regions.
Sweat volume and content depends on who you are and on your degree of acclimatization, but the average value for osmolality is about 120mosm, with a broad range (62 to 192 mosm/Kg).
If you want to know the human sweat electrolytes in grim detail, this article is for you. There is even a picture of a horrific box draped with garbage bags inside which the principle investigator is peddling on an ergometric cycle in his underwear. There is a gentlemanly discourse on this topic is available from 1932; it was written by a H.H Mosher, from the Climax Rubber Company, and it compares the electrolyte composition of sweat to that of urine. Nothing is measured in osmoles, but there is a savage joy in reading about somebody being put into a heating chamber swaddled in sheets of rubber. A more sober account of sweat osmolality can be found here.
The electrolyte concentrations offered here are obviously not definitive. There is significant variation between people, particularly people who are acclimatised to different ambient heat levels. As the level of heat in your environment increases, you adapt by decreasing the electrolyte concentration of your sweat, such that the more you sweat and the longer you spend in a hot country, the more aldosterone will try to preserve the sodium. There will be a reduction in sodium loss. They say it could be as little as 5mmol/day. The process of adaptation to hot climates, where you learn to produce greater volumes of sweat and lose less sodium, is called "acclimatisation". Allan and Wilson (1971) found acclimatised subjectes excreted about half as much sodium in their arm-sweat than unacclimatised subjects. Moreover, there is significant regional variation in sweat volume and composition within any given person. Interestingly, the forehead appears to be the sweatiest region by volume (Patterson, 2000).