Sweating and Shivering
Latent Heat of Vaporization, Hypothalamic Setpoints, and Brown Adipose Thermogenesis
Liquid sweat dripping off the chin or soaking into cotton clothing provides zero conductive cooling to the human body. Thermal energy leaves the skin strictly during the phase change from liquid water to water vapor. Breaking hydrogen bonds absorbs 2,426 kilojoules of latent heat for every liter of sweat that evaporates into the air.
Humans possess two to four million eccrine sweat glands distributed across the skin at densities up to 600 glands per square centimeter on the palms and forehead—ten times the density of chimpanzees. Uniquely within the sympathetic fight-or-flight nervous system, postganglionic sudomotor nerve fibers trigger thermal sweating by releasing acetylcholine rather than norepinephrine.
Unacclimatized travelers exercising in desert heat secrete sweat containing 60 millimoles per liter of sodium, risking hyponatremia and muscle cramps. Across ten days of heat exposure, rising adrenal aldosterone inserts additional epithelial sodium channels (ENaC) into the coiled eccrine duct walls. Reclaiming salt before the drop exits cuts sweat sodium to 15 millimoles.
Eccrine thermal sweat is 99 percent water and completely odorless when it emerges on the epidermis. Body odor originates from apocrine glands in the axillae, which secrete lipid- and protein-rich droplets that Corynebacterium microbes cleave into volatile thioalcohols. An East Asian ABCC11 single-nucleotide polymorphism abolishes this apocrine transport in two billion people.
Core body temperature is regulated by warm-sensitive neurons in the preoptic area of the anterior hypothalamus (POAH), which continuously measure the temperature of carotid arterial blood to within 0.1 degrees Celsius. By integrating central blood temperature with peripheral TRPM8 and TRPV3 skin signals, the POAH initiates sweating or vasoconstriction before core temperature shifts.
Before burning a single calorie shivering or losing a drop of sweat, the body regulates heat across a neutral thermal zone solely by shifting cutaneous blood flow from 0.2 to 8 liters per minute. Hairless glabrous skin on the palms, soles, and ears contains thick-walled arteriovenous anastomoses that shunt hot arterial blood straight into superficial veins.
When cutaneous vasoconstriction cannot stop core temperature from falling below 36.5 degrees Celsius, the posterior hypothalamus activates the primary motor center for shivering. Descending tracts drive opposing flexor and extensor muscles in asynchronous 10-Hertz micro-contractions that perform zero external mechanical work. Converting 100 percent of ATP hydrolysis into heat multiplies metabolic rate fivefold.
Human infants lack mature shivering circuitry and rely instead on non-shivering thermogenesis inside interscapular brown adipose tissue (BAT). Packed with iron-rich mitochondria expressing uncoupling protein 1 (UCP1 or thermogenin), brown fat opens a proton leak channel across the inner mitochondrial membrane. Short-circuiting the electron transport chain releases lipid oxidation energy directly as pure heat.
Shivering violently under heavy blankets while running a 39-degree-Celsius fever reflects a recalibrated thermostat rather than a broken one. Bacterial lipopolysaccharides stimulate immune macrophages to release interleukin-1, which induces endothelial prostaglandin $\text{E}_2$ (PGE2) inside the hypothalamus. PGE2 raises the target set-point to $39^\circ\text{C}$, making $37^\circ\text{C}$ blood feel freezing cold.
Swallowing aspirin or ibuprofen blocks cyclooxygenase (COX-1 and COX-2) enzymes, halting hypothalamic PGE2 synthesis and resetting the thermal target back down to 37 degrees Celsius. Instantly, the hypothalamus reads its own 39-degree arterial blood as dangerously overheated, triggering massive cutaneous vasodilation and drenching diaphoresis as the fever breaks.
Immersion in ice water initially triggers intense sympathetic vasoconstriction in the fingers and toes to preserve warm core blood. Sir Thomas Lewis discovered in 1930 that every five to ten minutes, cold-paralyzed smooth muscle in digital vessels suddenly dilates. This protective hunting reaction flushes warm blood through the fingertips to prevent frostbite necrosis.
Even when a subject rests motionless for twenty-four hours in a constant-temperature chamber, human core temperature oscillates by 0.8 degrees Celsius on an endogenous circadian sine wave. Core temperature hits its nadir near 4:30 a.m. and peaks near 6:00 p.m., when elevated muscle enzyme kinetics and nerve conduction velocity produce the majority of world athletic records.