How Sweating Works: The Physiology of Thermoregulation

Sweating (perspiration) is the primary mechanism for dissipating heat in humans.

Mechanism

1. Thermoreceptors (hypothalamus, skin) detect ↑ core/skin temperature.
2. Hypothalamus (preoptic area) activates sympathetic cholinergic fibers to eccrine glands.
3. Eccrine glands (2–4 million) secrete primary sweat (isotonic, NaCl-rich) from coiled secretory portion.
4. Ductal reabsorption: Na⁺, Cl⁻ reabsorbed → hypotonic sweat on skin (conserves electrolytes).
5. Evaporation: Phase change (liquid → vapor) absorbs 2.43 kJ/g (580 kcal/L) latent heat → cools skin → cools blood → cools core.

Sweat Composition

• 99% water.
• Electrolytes: Na⁺ (20–80 mmol/L), Cl⁻, K⁺, Ca²⁺, Mg²⁺.
• Trace: lactate, urea, ammonia, cytokines, antimicrobial peptides.

Regulation

• Acclimatization: ↑ sweat rate, ↓ electrolyte concentration, earlier onset.
• Hydration: Hypohydration ↓ sweat rate, ↑ core temp.
• Medications: Anticholinergics ↓ sweating; some antidepressants ↑.

Disorders

• Hyperhidrosis: Excessive (primary focal or secondary).
• Anhidrosis/Hypohidrosis: Risk of heat stroke.