Physiology of sweat gland function: The roles of sweating and sweat composition in human health

Abstract

The purpose of this comprehensive review is to: 1) review the physiology of sweat gland function and mechanisms determining the amount and composition of sweat excreted onto the skin surface; 2) provide an overview of the well-established thermoregulatory functions and adaptive responses of the sweat gland; and 3) discuss the state of evidence for potential non-thermoregulatory roles of sweat in the maintenance and/or perturbation of human health. The role of sweating to eliminate waste products and toxicants seems to be minor compared with other avenues of excretion via the kidneys and gastrointestinal tract; as eccrine glands do not adapt to increase excretion rates either via concentrating sweat or increasing overall sweating rate. Studies suggesting a larger role of sweat glands in clearing waste products or toxicants from the body may be an artifact of methodological issues rather than evidence for selective transport. Furthermore, unlike the renal system, it seems that sweat glands do not conserve water loss or concentrate sweat fluid through vasopressin-mediated water reabsorption. Individuals with high NaCl concentrations in sweat (e.g. cystic fibrosis) have an increased risk of NaCl imbalances during prolonged periods of heavy sweating; however, sweat-induced deficiencies appear to be of minimal risk for trace minerals and vitamins. Additional research is needed to elucidate the potential role of eccrine sweating in skin hydration and microbial defense. Finally, the utility of sweat composition as a biomarker for human physiology is currently limited; as more research is needed to determine potential relations between sweat and blood solute concentrations.

Keywords: Chloride; potassium; sauna; sodium; sweat biomarkers; thermoregulation.

Figure 1.

Comparison of the apocrine, eccrine, and apoeccrine glands in the axilla.

Figure 2.

Structure of the eccrine sweat gland (panels A-B) and mechanisms of sweat secretion in the secretory coil (panel C) and Na and Cl reabsorption in the proximal duct (panel D). ACh; acetylcholine; AQP-5, aquaporin-5; CFTR, cystic fibrosis membrane channel; ENaC, epithelial Na channel; NaCl, sodium chloride.

Figure 3.

An illustration of central and peripheral control of sweating and the factors that modify the sweating response to hyperthermia. Shifts in the onset (threshold) and sensitivity (slope) of the sweating response to hyperthermia are depicted by the dashed lines. Other potential factors that may directly or indirectly modify sweating (altitude/hypoxia, microgravity, menstrual cycle, maturation, aging) are discussed in the text.

Figure 4.

Top row (panels A-C): Variation in the size of human eccrine sweat glands taken from the backs of three different men who were described as poor (A), moderate (B), and heavy sweaters (C). Bottom row: Correlation between size of sweat gland and sweat rate_max per gland (panel D). Dose-response curves (expressed per unit length of tubule) of sweat rates of 7 men to methacholine. Closed symbols show moderate to heavy sweaters. Open symbols show poor sweaters. Reprinted from Sato and Sato 1983 [131]with permission.

Figure 5.

Frequency histograms of forearm sweat sodium concentration (Panel A) and predicted whole-body sweat sodium concentration (Panel B) in 506 skill-sport and endurance athletes during training/competition in a wide range of environmental conditions. The vertical line represents the mean value. Reprinted from Baker et al. 2016 [156] with permission.

Figure 6.

Relation between regional sweating rate and regional sweat [Na]. Values are means ± SE for 10 subjects’ regional (forearm) sweating rate and sweat [Na] while exercising at 50%, 60%, 70%, 80%, and 90% of maximal heart rate. The mean r for the group was 0.73 (P < 0.05). y = 59.7(x)+6.7. Reprinted from Buono et al. 2008 [39] with permission.

Figure 7.

Whole-body sweating rate and whole-body sweat [Na] and [Cl] comparison between low (45% maximal oxygen uptake) and moderate (65% maximal oxygen uptake) intensity cycling exercise in a warm (30°C and 44% relative humidity) environment (n = 11 men and women). Solid circles show individual data. Open circles show mean data (p < 0.05 between low and moderate intensity for sweating rate, sweat [Na] and sweat [Cl]). Redrawn from Baker et al. 2019 [159].

Figure 8.

Regional sweating rate vs. regional sweat [Na]. Data points represent the group (26 subjects) mean ± SEM at each regional site (DFA, dorsal forearm; VFA, ventral forearm). Regional sweating rate and sweat [Na] measured with the absorbent patch technique during cycling exercise in the heat (30°C, 42% relative humidity). Redrawn from Baker et al. 2018 [149].

Figure 9.

Regression of regional sweating rate vs. regional sweat [Na] within site for the dorsal forearm (A), and the 9-site aggregate (weighted for body surface area and regional sweating rate), as well as regression of whole-body sweating rate vs. whole-body sweat [Na]. Correlations between sweating rate and sweat [Na] were not significant (p > 0.05). Reprinted from Baker et al. 2018 [149] with permission.