The properties and contents of sweat

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.

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.  

Physiology of sweat secretion

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. 

• Thermoregulation: Sweat being essentially water, and with every gram of evaporating water representing a 0.58 kcal of heat. loss, one is able to use sweat to get rid of waste heat by convection. Thus, with 800ml of water per day evaporating away, 464 kcal of heat are lost, which ends up being  25% of basal heat production.
• Lubrication / friction: Through sweat secretion, some mammals (for instance, dogs, cats, rodents) increase the "grippyness" of their footpads during periods of fight-or-flight reaction. Clearly there is some evolutionary advantage to being able to scuttle away rapidly from a predator. In modern human society this effect is  somewhat squandered, as skin friction offers minimal advantage in being able to scuttle away from a hostile audience or an awkward date. This is not the effect of adrenaline; for example Chalmers and Keele (1951) were not able to produce a sweating response to the local injection of adrenaline. Instead, postganglionic sympathetic cholinergic fibres control this process.
• Excretory: Sweat glands can act as organs of excretion for water and electrolytes, although this is neither an efficient nor reliable way of clearing either. In man, these functions are bugs rather than features. Electrolyte losses and water losses which cannot be measured by straining through tubes into bags are difficult to quantify (therefore, "insensible") and patients can become quite dehydrated and electrolyte-depleted by this process. According to Torii (1994), the maximum sweat volume is 1500-2000 ml/hr. Perspiration water loss of about 11-12 litres per day is thought to be maximal (eg. by Robinson & Robinson, 1954) because this is the daily water loss observed in highly acclimatised soldiers in the California desert as they performed "severe muscular work". This information probably has little use in the ICU, where we prefer to remove fluid by means of an extracorporeal circuit.
• Scent: For the purpose of chemical signalling between members of a species, sweat glands can be modified to carry informative scents. This, in herding artiodactyls, guards "against the extinction of the species" (Jenkinson, 1976) and among humans sends strong chemical messages regarding one's attention to hygiene. 
• Antimicrobial: Apart from helping the flow of sebum across the skin, sweat contains some endogenous antibacterial molecules, for example dermcidin (Schittek et al, 2001). Skin already has a bunch of antimicrobial peptides (eg.  cathelicidins and β-defensins ) which are expressed by keratinocytes in response to injury; dermcidin is something unique to sweat, and has broad-spectrum antibacterial and antifungal activity.  

Electrolyte content of sweat

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).