3.3: Sweat

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Sweating is important for body temperature regulation but can also be a major source of water and solute loss. The heat loss can be quite significant because there is a loss of 0.58 kcals for every ml of water evaporated.

Maximum rate of sweating is up to 50 mls/min or 2,000 mls/hr in the acclimatised adult. This rate cannot be sustained but losses up to 25% of total body water are possible under severe stress: this could be fatal.

Losses due to Sweating

Fluid loss - Can be large in a hot environment, or if physically active
Solute loss - Decreases with 'acclimatisation'
Heat loss - Can be large due to high latent heat of evaporisation of water; hence important role in body temperature regulation

There are several different types of sweating, but from the fluid perspective, only the sweating from eccrine sweat glands is important. The volume of fluid from apocrine sweating is very low.

The eccrine glands are specialised skin appendages which are present in over 99% of the skin surface. They are innervated by sympathetic cholinergic neurones. The muscarinic receptors can be blocked by atropine and this will prevent sweating.

Control

Sweating is controlled from a center in the preoptic & anterior regions of the hypothalamus where thermosensitive neurones are located. The heat regulatory function of the hypothalamus is also affected by inputs from temperature receptors in the skin. High skin temperature reduces the hypothalamic set point for sweating and increases the gain of the hypothalamic feedback system in response to variations in core temperature. Overall though, the sweating response to a rise in hypothalamic temperature (core temp) is much larger than the response to the same increase in average skin temperature.

Sweat is not pure water; it always contains a small amount (0.2 - 1%) of solute . When a person moves from a cold climate to a hot climate, adaptive changes occur in their sweating mechanism. These are referred to as acclimatisation: the maximum rate of sweating increases and its solute composition decreases. The daily water loss in sweat is very variable: from 100 to 8,000 mls/day. The solute loss can be as much as 350 mmols/day (or 90 mmols/day acclimatised) of sodium under the most extreme conditions. In a cool climate & in the absence of exercise, sodium loss can be very low (less than 5 mmols/day). [Na⁺] in sweat is 30-65 mmol/l depending on degree of acclimatisation.

Main Differences between Sweat and Insensible Water Loss
	Sweat	Insensible Fluid
Source	From specialised skin appendages called sweat glands	From skin (trans-epithelial) and respiratory tract
Solute loss	Yes, variable	None
Role	Body temperature regulation	* Cannot be prevented. * Evaporation of insensible fluid is a major source of heat loss from the body each day but is not under regulatory control