The Properties and Contents of Sweat

Created on Mon, 06/29/2015 - 19:05
Last updated on Sun, 04/30/2017 - 16:44

Anyway: 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.

Evaporative, “insensible” water loss

The daily loss is about 800ml of "insensible water loss";

Half is lost from the skin and half from the respiratory tract.

Insensible Fluid Losses

This is FREE WATER LOSS; solutes are concentrated as a result.

Consider: if initially you have 19 L of ECF and 280mOsm/L of solute, you must have 5320mOsm in total.

That's your total ECF solute.

Obviously if 800ml of fresh water is removed, you will have some concentration.

You still have 5320 mOsm of solute, but it is now distributed in 18.2 L of ECF.

The resulting ECF concentration is thus 292 mOsm/L.


Evaporative heat loss through insensible water loss

Every gram of water evaporating loses 0.58 kcal of heat.

Thus, with 800ml of water per day evaporating away, 464 kcal of heat are lost.

That's 25% of basal heat production.


Blood flow to the skin

Normally, the skin receives 300mls of blood per minute.

Under heat stress, it could receive up to 8000 mls of blood.


Electrolyte content of sweat

This again depends on who you are and on your degree of acclimatization.

There are average values, which I will use here.

The total osmolality is on average about 120mosm.

The range is broad: 62 to 192 mosm/Kg

Sweat electrolyte composition

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.

Fluid loss through sweat

Sweat is a dilute solution of electrolytes. Its pretty much just water.

It is produced by ECCRINE glands (they cover 99% of the body)

In contrast, the smelly groin and armpits contain APOCRINE glands.

The sweat glands have a muscarinic sympathetic innervation.

The central control is from the core-temperature-sensing homeostatic neurons of the hypothalamus.

The maximum sweat volume is 1500-2000 ml/hr, or a maximum of 12 litres per day.


Sodium loss through sweat

At first, there is 65mmol/L of sodium in sweat. This also means you lose 65 mmol of chloride.

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 "acclimatization".


Heat loss through sweat

That depends on how efficient the air is in sucking the water off your skin.

One litre of sweat removes 580 kcal of heat as it evaporates.

Humid, supersaturated air from the West of Sydney is useless at exchanging heat with your skin, because the rate of evaporation will be very low.


The importance of isotonic rehydration

Why is this relevant to the casual drug user at the hippy outdoor dance festival?

sweat fluid loss and consequences of hypotonic rehydration

  • Say, you are acting the fool at a dance party, and you produce 2 litres of sweat.
  • That means you lose 2 of the 19 litres of your ECF, and 260mosm out of a total 5320mosm of solute. Thus, the osmolality of the remaining 17 litres of ECF rises to 297mosm/L.
  • Now, you replace the fluid deficit with very expensive bottled water, or some sort of sugary rave-juice... Lets say 2 litres of it. Thus, there is a net even water balance, but the extracellular fluid loses sodium.
  • The osmolality of the ECF is now 266mosm.
  • The consequence is a redistribution of free water into the intracellular fluid, and the cells swell.

This is bad for the brain. Cerebral oedema ensues, with embarrassing seizures and a potentially party-ending involvement of the police and ambulance services.



Most of this information derives from easily accessible physiology textbooks, such as Ganongs Review of Medical Physiology 23rd edition.