Sweating and Evaporative Cooling

When dry routes of heat loss become insufficient, the human body relies on the evaporation of sweat to dissipate heat. Eccrine sweat glands, activated by the thermoregulatory system, secrete a dilute fluid onto the skin; as that fluid evaporates it draws the latent heat of vaporization from the body, providing the principal avenue of cooling during exercise and heat exposure.

Definition

Sweating is the secretion of fluid by eccrine sweat glands under thermoregulatory (sympathetic cholinergic) control, and evaporative cooling is the heat removed from the body when that fluid evaporates from the skin, absorbing the latent heat of vaporization.

Scope

This topic covers the eccrine sweating response, its neural activation, the composition of sweat and how it is modified, the physics of evaporative cooling and what limits it, and how training and heat acclimatization alter the response. It treats sweating as thermoregulatory physiology and not as a basis for hydration or electrolyte recommendations.

Core questions

• How are eccrine sweat glands activated, and what governs the sweating rate?
• What determines the volume and composition (especially sodium) of sweat?
• How does evaporation of sweat remove heat, and what limits evaporative cooling?
• How do heat acclimatization and training modify the sweating response?

Key concepts

• Eccrine sweat glands
• Sympathetic cholinergic activation of sweating
• Sweat rate and its determinants
• Sweat composition and sodium reabsorption in the duct
• Latent heat of vaporization and evaporative cooling
• Skin wettedness and ambient humidity as limits
• Heat acclimatization (enhanced, more dilute sweating)

Mechanisms

Eccrine sweat glands are innervated by sympathetic cholinergic fibres; as thermal drive rises, more glands are recruited and each secretes faster, raising whole-body sweat rate. The glandular secretory coil produces a precursor fluid resembling plasma, and as it passes along the duct sodium and chloride are reabsorbed, so the sweat that reaches the skin is hypotonic - the more so at lower flow rates and after acclimatization. The cooling itself is physical: each gram of sweat that evaporates removes a fixed quantity of heat (the latent heat of vaporization), so the evaporative cooling actually achieved depends on how much sweat evaporates rather than merely on how much is secreted. Evaporation is limited by skin wettedness and by the water-vapour gradient between skin and air, so in humid conditions sweat may drip without contributing to cooling. With repeated heat exposure, the sweating response acclimatizes - beginning at a lower core temperature, reaching higher rates, and becoming more dilute - improving evaporative heat loss while conserving sodium.

Clinical relevance

The sweating response sets both the body's main cooling capacity and the magnitude of fluid and sodium loss during exercise, which is relevant to understanding heat tolerance and exercise-associated disturbances of body water and electrolytes. This entry describes the physiology for reference and does not provide hydration, electrolyte, or treatment guidance.

Evidence & guidelines

The physiology of sweat-gland function, sweat composition, and the limits of evaporative cooling is reviewed comprehensively by Baker (2019); its integration with skin blood flow and with hyperthermia and performance draws on Charkoudian (2003) and Nybo et al. (2014), and the consequences of sweat-driven fluid loss on Cheuvront and Kenefick (2014). These are descriptive reviews rather than guidelines.

History

Study of human eccrine sweating advanced through twentieth-century physiology, which established its cholinergic sympathetic control, the ductal reabsorption that renders sweat hypotonic, and the way repeated heat exposure enhances and dilutes the response. Contemporary reviews have consolidated this understanding of sweat-gland function and its role in evaporative cooling.

Key figures

• Lindsay B. Baker
• Nina Charkoudian
• Michael N. Sawka
• Lars Nybo

Seminal works

• baker-2019
• charkoudian-2003

Frequently asked questions

Does sweating itself cool the body?

Sweat cools the body only when it evaporates; the evaporation absorbs the latent heat of vaporization from the skin. Sweat that drips off without evaporating contributes little to cooling, which is why high humidity reduces the effectiveness of sweating.

Why is sweat saltier in some situations than others?

Sweat begins as a plasma-like fluid and loses sodium as it passes along the gland's duct; at high sweat rates there is less time for reabsorption, so sweat is saltier, whereas heat acclimatization improves reabsorption and makes sweat more dilute.