This document was created by Alex Yartsev ([email protected]); if I have used your data or images and forgot to reference you, please email me.
The contents and properties of sweat
-
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.
0.58 kcal of heat per ml of
evaporating water
25% basal heat production
280mosm/L
280mosm/L
292mosm/L
280mosm/L
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. Thats 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 received 300mls of blood per minute.
Under heat stress, it could receive up to 3000 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
Magnesium 1mmol/L
Calcium 1mmol/L
Potassium 4mmol/L
Sodium: 50mmol/L
CATIONS
ANIONS
16 or so mmol of other anions:
bicarbonate, ammonia, sulfate, lactate,
amino acids. Electronegativity is
preserved, but how? It is hard to find a
source that estimates the anionic
concentration in sweat.
Chloride 40 mmol/L
From Ganongs Review of Medical Physiology 23rd edition, Vanders Renal Physiology 7th edition, Wests Respiratory Physiology: the Essentials, as well as public works by the eminent Dr Kerry Brandis to whom I owe
much of the inspiration for my shambolic efforts at self-education. 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. A sober account of osmolality of sweat can be found here.
This document was created by Alex Yartsev ([email protected]); if I have used your data or images and forgot to reference you, please email me.
Fluid loss through sweat
Sweat is a dilute solution of eectrolytes. 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 parasympathetic 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.
From Ganongs Review of Medical Physiology 23rd edition, Vanders Renal Physiology 7th edition, Wests Respiratory Physiology: the Essentials, as well as public works by the eminent Dr Kerry Brandis to whom I owe
much of the inspiration for my shambolic efforts at self-education. 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. A sober account of osmolality of sweat can be found here.
This document was created by Alex Yartsev ([email protected]); if I have used your data or images and forgot to reference you, please email me.
The importance of isotonic rehydration
Why is this relevant to the casual drug user at the hippy outdoor dance festival?
Initial conditions:
- 19L of ECF water
- 5320 mOsm of ECF solute
(280mOsm/L of solutes)
Ravenous consumption of
hypotonic lolly-water
2L H2O
260mOsm of solutes
(mainly NaCl)
297mosm/L
280mosm/L
266mosm/L
Dehydrated conditions:
-
17L of ECF water
5060 mOsm of ECF solute
(297mOsm/L of solutes)
280mosm/L
Rehydrated conditions:
-
19L of ECF water
5060 mOsm of ECF solute
(266mOsm/L of solutes)
270mosm/L
270mosm/L
Redistribution:
Water distributes into the intracellular
fluid; cellular volume increases, and
cerebral oedema is the result.
 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, 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.
From Ganongs Review of Medical Physiology 23rd edition, Vanders Renal Physiology 7th edition, Wests Respiratory Physiology: the Essentials, as well as public works by the eminent Dr Kerry Brandis to whom I owe
much of the inspiration for my shambolic efforts at self-education. 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. A sober account of osmolality of sweat can be found here.