Sweat glands are small, coiled tubular structures embedded within the skin, functioning as part of the body’s exocrine system. They produce sweat, a fluid secreted onto the skin surface to facilitate two fundamental processes. The most recognized function is thermoregulation, where sweat evaporation helps cool the body and prevent overheating. Sweat glands also play a secondary role in excretion, eliminating small amounts of water and electrolytes.
The Two Primary Sweat Gland Types
The human body contains approximately three to four million sweat glands, categorized into two main types: eccrine and apocrine glands. These groups differ significantly in structure, location, and secretory product. Eccrine glands are simple, coiled, tubular structures that develop early in gestation, becoming functional shortly after birth.
Apocrine glands are larger and more complex, originating from the hair follicle. Unlike eccrine glands, they remain dormant until the hormonal changes of puberty trigger their activation. The eccrine gland duct opens directly onto the skin’s surface through a sweat pore.
In contrast, the apocrine gland duct secretes its product into the upper part of the hair follicle, traveling to the skin surface along the hair shaft. Eccrine glands release a watery, hypotonic secretion, while apocrine glands produce a thicker, viscous fluid rich in lipids and proteins. A third, less prevalent type, the apoeccrine gland, shares characteristics of both and is mainly found in the armpits.
Mapping the Body’s Cooling System (Eccrine Distribution)
Eccrine sweat glands are the most numerous and widely distributed across the skin surface, making them the primary mechanism for evaporative cooling. They are found almost everywhere, though concentration varies widely. The highest density is on the palms and soles, reaching up to 700 glands per square centimeter.
The secretion produced by eccrine glands is a clear, odorless fluid, composed of approximately 98% to 99% water. It is hypotonic, containing a lower concentration of dissolved solutes than plasma. The main solutes are sodium chloride, which gives sweat its salty taste, along with trace amounts of urea and other metabolites.
The eccrine gland’s structure facilitates this composition: the coiled secretory portion produces the initial fluid, and the duct reabsorbs a majority of the sodium and chloride ions. During intense activity, an individual can produce up to four liters of sweat per hour to prevent overheating. The secretion also helps maintain the skin’s slightly acidic protective layer, known as the acid mantle.
Localized Odor Production (Apocrine Distribution)
Apocrine glands follow a localized distribution pattern, restricted primarily to specific, hair-bearing regions. These areas include the axillae (armpits), the perianal and genital regions, and the skin surrounding the nipples (areola).
The function of apocrine glands is related to chemical communication, not generalized body cooling. Their secretion is initially odorless and is a viscous fluid rich in organic compounds, including lipids, proteins, and steroids.
Body odor, medically termed bromhidrosis, results when bacteria naturally present on the skin metabolize these organic molecules. This bacterial breakdown creates volatile, odorous compounds. Because apocrine ducts empty into the hair follicle, the hair serves as a pathway for the thick secretion to reach the surface and interact with bacteria.
How the Body Controls Sweating
Sweat production is managed by the autonomic nervous system, which controls involuntary bodily functions. Specifically, the sympathetic nervous system, known for the “fight or flight” response, triggers sweat gland activity. The body’s thermoregulatory control center, located in the hypothalamus, constantly monitors core body temperature.
When temperature exceeds a set point, the hypothalamus sends signals through sympathetic nerve fibers to activate eccrine glands across the body. The primary chemical messenger used to stimulate eccrine secretion is acetylcholine, which acts on receptors within the gland. This cholinergic pathway is the main driver of thermal sweating.
Sweating can also be triggered by emotional stimuli, regulated by the cerebral cortex and the limbic system. Emotional sweating is most noticeable on the palms, soles, and axillae, even when the body is not overheated.
Overactivity of the sympathetic nervous system can lead to hyperhidrosis, characterized by excessive sweating beyond temperature regulation needs. In hyperhidrosis, the eccrine glands are typically normal, but the nerves are overstimulated, releasing excessive acetylcholine.
This demonstrates that sweat production control is a function of nervous system signaling rather than a structural issue with the glands. The sympathetic system also influences apocrine glands, which are primarily associated with emotional or stress-related sweating.