Sweat

Sweat is a clear, watery secretion produced by sweat glands in the skin of mammals. In humans, two main gland types contribute to sweating: eccrine glands, which are widespread across the body and produce a mostly odorless, watery secretion, and apocrine glands, concentrated in the armpits and groin and releasing a thicker fluid that can develop odor after bacterial action. Both gland types are innervated by the sympathetic nervous system, with eccrine sweating driven mainly by heat and physical activity and apocrine sweating linked to stress and hormonal signals.

During production, eccrine glands secrete a fluid that is initially similar in osmolarity to plasma, but as it travels through the duct, sodium and chloride are reabsorbed, making sweat usually hypotonic with relatively low electrolyte content. Sweat production increases with ambient temperature, exercise, fever, and emotional stimuli. The amount and composition of sweat can vary with hydration status and acclimation to heat.

Because sweating results in evaporative cooling, body temperature is regulated as heat is lost to the environment. Evaporation is promoted by low ambient humidity and adequate airflow; clothing and wet surfaces can impede cooling. Typical sweat contains mostly water with small amounts of electrolytes (sodium, chloride, potassium), urea, lactate, ammonia, and trace metabolites, as well as antimicrobial peptides produced by glands. The pH is slightly acidic, often around 4.5 to 6.

Clinical notes: Hyperhidrosis is excessive sweating; anhidrosis is a lack of sweating. Bromhidrosis refers to unpleasant odor from bacterial breakdown of apocrine secretions. Sweat testing of chloride is used in diagnosing cystic fibrosis.