Perspiration

Perspiration, commonly known as sweat, is a watery fluid secreted by specialised sweat glands in the skin of mammals. In humans it plays a central role in thermoregulation, enabling the body to maintain a stable core temperature through the evaporation of moisture from the skin surface. Sweating also has social, diagnostic and physiological relevance, as variations in sweat quantity or character may reflect emotional states, environmental conditions or underlying medical disorders.

Types of Sweat Glands

Humans possess two principal types of sweat glands, each differing in structure, distribution and function:

• Eccrine glands, found across most of the body, produce a dilute, watery secretion. They are primarily responsible for cooling the body and are activated mainly by rises in core temperature.
• Apocrine glands, located in the armpits and certain other areas, secrete a thicker, oily substance that is initially odourless. Characteristic body odour arises when skin bacteria metabolise this secretion.

In adults, sweat production can reach up to several litres per hour in hot environments or during strenuous activity, whereas prepubertal children typically show lower rates. Animals with few sweat glands, such as dogs, rely on panting rather than sweating to regulate temperature.
Only a limited range of mammals—including humans, horses, some primates and certain bovidae—use sweating extensively for thermoregulation. Horses, for instance, produce a protein-enriched sweat containing BPIFA4P that helps spread moisture across the coat.

Terminology

The medical vocabulary related to sweating includes several specialised terms:

• Diaphoresis / hidrosis – generally denote sweating, though both can also refer to excessive sweating depending on clinical context.
• Hypohidrosis – reduced or insufficient sweating.
• Hyperhidrosis – excessive sweating, either localised (such as in the armpits, palms, soles or face) or generalised across the body.
• Secondary hyperhidrosis – excessive sweating arising from an underlying condition.
• Hidromeiosis – diminished sweating caused by sweat gland obstruction, often in humid conditions.
• Sudorific – a substance that induces sweating.

Signs and Symptoms

Sweat contributes to body odour when its components are broken down by skin bacteria. Diet, medication and certain metabolic disorders—including diabetic ketoacidosis and renal failure—may alter its smell.
Excessive sweating can occur in response to physical exertion, fever, menopause, emotional stimuli such as fear or anxiety, or high environmental temperature. Because most sweat glands are innervated by sympathetic cholinergic neurons, they respond rapidly to autonomic nervous system signals.
Diaphoresis may accompany numerous pathological conditions, such as hyperthyroidism, infections producing high fever, shock and myocardial infarction. It is also a feature of serotonin syndrome, pneumothorax, neuroleptic malignant syndrome and haematological malignancies. Hypoglycaemia in diabetics is a frequent cause, and several drugs—including stimulants, opioids, alcohol, antidepressants and antipsychotics—can provoke sweating. Withdrawal from alcohol, narcotics or benzodiazepines may also trigger it.
Mercury compounds were historically used as diaphoretics, resulting in toxic consequences. Childhood mercury poisoning (acrodynia) is characterised by excessive perspiration. A minority of individuals develop sweat allergy, a reaction to proteins produced by skin bacteria; washing and agents like tannic acid may reduce symptoms.

Hyperhidrosis

Hyperhidrosis affects millions of individuals yet often remains untreated due to stigma or lack of awareness. Though not life-threatening, it may significantly impair quality of life and cause psychological distress. Localised forms most frequently involve the armpits, hands, feet or face.
Management options include:

• antiperspirants containing aluminium salts,
• iontophoresis,
• local procedures to remove or destroy sweat glands,
• botulinum toxin injections to inhibit neural stimulation,
• endoscopic thoracic sympathectomy for severe cases.

Night Sweats

Night sweats, or nocturnal hyperhidrosis, refer to heavy sweating during sleep unrelated to excessive bedding or high room temperature. Common non-pathological causes include hormonal transitions during perimenopause and menopause. Severe nocturnal sweating may also signal significant underlying disease, such as chronic infection. Medical evaluation is necessary when sweating saturates sleepwear or bedding, or when it occurs with systemic symptoms.

Mechanism of Thermoregulation

Sweating serves a critical homeostatic function. The preoptic and anterior hypothalamus houses thermosensitive neurons that integrate signals from both core and skin temperature receptors. Rising core temperature strongly stimulates sweat production, while higher skin temperature lowers the hypothalamic threshold for sweating.
Cooling occurs through evaporative heat loss: as high-energy water molecules escape from the skin into the air, they remove heat from the body surface, decreasing overall core temperature. This system enables humans to maintain thermal balance in hot climates and during physical exertion, provided adequate hydration is maintained.