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from Guyton and Hall Textbook of Medical Physiology (4th South Asia Edition) write a proper 5 mark short note on heat loss- enumerate methods, mechanism, and mechanism of sweating

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Heat Loss (Guyton & Hall, 4th South Asia Edition) — Short Note

Concept: Body temperature is a balance between heat production and heat loss. Heat generated deep in the body (liver, brain, heart, contracting muscle) must be conducted to the skin and then transferred to the surroundings. The rate of heat loss depends on: (1) how fast heat is conducted from the core to the skin, and (2) how fast it is then transferred from skin to environment - Guyton and Hall Textbook of Medical Physiology, p. 892.
Insulator system: Skin, subcutaneous tissue, and especially subcutaneous fat act as heat insulators (fat conducts heat only one-third as well as other tissues), keeping the core warm while allowing skin temperature to approach that of the surroundings - Guyton and Hall Textbook of Medical Physiology, p. 892.
Vasomotor control: Blood vessels beneath the skin (with an extensive venous plexus and arteriovenous anastomoses) carry heat from the core to the skin at a variable rate controlled by sympathetic vasoconstrictor tone. Heat conductance from core to skin can vary about 8-fold depending on the degree of vasoconstriction/vasodilation - Guyton and Hall Textbook of Medical Physiology, p. 893.

Methods (mechanisms) of Heat Loss from the Skin

In a nude person at rest in a normal room, heat is lost by four physical methods:
  1. Radiation (~60%) - loss of heat as infrared rays; the body radiates heat rays in all directions, and net loss occurs when body temperature exceeds surrounding temperature.
  2. Conduction to objects (~3%) - direct transfer of heat by molecular contact to solid objects such as a chair.
  3. Conduction to air, followed by convection (~15%) - heat is conducted to the thin layer of air touching the skin, which is then carried away as air currents (convection); wind increases this loss (cooling effect proportional to the square root of wind velocity). Conduction to water is far greater than to air because of water's higher specific heat and conductivity.
  4. Evaporation - 0.58 kcal of heat is lost per gram of water evaporated. Even without sweating, insensible evaporation from skin and lungs (~600-700 mL/day) causes continual loss of 16-19 kcal/hour. Evaporation becomes the only available mechanism when the environmental temperature exceeds skin temperature, since radiation and conduction then work in reverse (body gains heat).
Clothing reduces conductive/convective loss by trapping air next to the skin (a suit halves heat loss; arctic clothing reduces it to about one-sixth), but this insulating effect is largely lost when clothing becomes wet.

Mechanism of Sweating

  • Control: The anterior hypothalamic-preoptic area, when stimulated by heat (electrically or by a rise in core/blood temperature), sends impulses via autonomic pathways down the spinal cord and out through sympathetic fibers to the skin.
  • Innervation: Sweat glands are supplied by cholinergic sympathetic fibers (secreting acetylcholine, though they travel with sympathetic/adrenergic nerves). Circulating epinephrine/norepinephrine can also stimulate sweating to a lesser degree, which is important during exercise.
  • Gland structure: Each sweat gland is a tubular structure with (1) a deep subdermal coiled secretory portion and (2) a duct that carries fluid outward through the dermis and epidermis.
  • Secretion process:
    • The coiled portion actively secretes a primary (precursor) fluid resembling plasma but without plasma proteins (Na+ ~142 mEq/L, Cl- ~104 mEq/L).
    • As this fluid passes along the duct, Na+ and Cl- are reabsorbed, and the degree of reabsorption depends on flow rate.
    • At low sweat rates, the fluid moves slowly, allowing almost complete reabsorption of Na+/Cl- (down to ~5 mEq/L), which lowers osmotic pressure so water is also reabsorbed - concentrating solutes like urea, lactic acid, and potassium.
    • At high sweat rates, the fluid moves quickly through the duct, so far less reabsorption occurs and the sweat remains more dilute (closer to plasma electrolyte concentration), though still hypotonic overall.
  • Physiological significance: Because evaporation of sweat is the only heat-loss route available once ambient temperature exceeds skin temperature, sweating is essential for surviving hot environments; people born without sweat glands tolerate cold well but are at high risk of heatstroke in the tropics.
Source: Guyton and Hall Textbook of Medical Physiology, Chapter 74 "Body Temperature Regulation," pp. 892-895.
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