from Guyton and Hall Textbook of Medical Physiology (4th South Asia Edition) write a proper 5 mark short note on Response of the body when exposed to extreme cold,Hypothermia, and the use of induced hypothermia
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Response of the Body to Extreme Cold, Hypothermia, and Induced Hypothermia
(Guyton and Hall, Textbook of Medical Physiology)
1. Body's Response to Cold Exposure
When the body is exposed to cold, the hypothalamic thermoregulatory center activates heat-conserving and heat-producing mechanisms:
- Cutaneous vasoconstriction to reduce heat loss from the skin.
- Piloerection (less significant in humans).
- Shivering thermogenesis, initiated by the posterior hypothalamus, which can increase heat production 4-5 times normal.
- Increased secretion of norepinephrine, epinephrine, and thyroxine, producing chemical (non-shivering) thermogenesis, especially important in infants with brown fat.
- Behavioral responses (seeking warmth, curling up) driven by the sensation of cold.
2. Frostbite and Cold-Induced Vasodilation
With extreme, prolonged cold exposure, surface tissues (ear lobes, digits) may freeze - frostbite - causing ice crystal formation, permanent circulatory impairment, tissue damage, and gangrene requiring surgical removal. As a protective mechanism, when tissue temperature approaches freezing, the vascular smooth muscle becomes paralyzed by cold, producing sudden cold-induced vasodilation (skin flush) that delivers warm blood to the periphery, partly protecting against frostbite. This mechanism is less developed in humans than in cold-adapted animals.
3. Hypothermia
- If the body is exposed to severe cold (e.g., ice water) for 20-30 minutes without treatment, death usually occurs from cardiac standstill or ventricular fibrillation, by which time core temperature has fallen to about 77°F (25°C).
- Loss of temperature regulation: Once core body temperature falls below about 85°F, the hypothalamus loses its ability to regulate temperature; this control is already markedly impaired below about 94°F.
- Mechanisms: chemical (metabolic) heat production in each cell is depressed nearly 2-fold for every 10°F fall in body temperature, creating a vicious cycle of progressive cooling.
- Central nervous system depression - sleepiness progressing to coma - further suppresses the hypothalamic heat-control center and abolishes shivering, accelerating the fall in temperature.
- If rewarmed rapidly with external heat, the person can often be revived even from profound hypothermia.
4. Induced (Artificial) Hypothermia and Its Clinical Use
Artificial hypothermia is produced by:
- Administering a strong sedative to depress hypothalamic thermoregulatory reactivity.
- Cooling the body with ice or cooling blankets until temperature falls, then maintaining it below 90°F for days to a week using continual sprinkling of cool water or alcohol.
Clinical significance: This controlled cooling is used during cardiac (heart) surgery, allowing the heart to be stopped artificially for extended periods. Because cooling greatly slows the heart rate and depresses cellular metabolism, the body's tissues can tolerate 30 minutes to more than an hour without blood flow during the procedure - without causing tissue damage - since reduced metabolic demand matches the reduced oxygen delivery.
Reference: Guyton and Hall Textbook of Medical Physiology, Chapter on Body Temperature Regulation - "Exposure of the Body to Extreme Cold," p. 902.
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