Body Temperature Regulation (Thermoregulation) - 20082020 Notes

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Body temperature regulation (thermoregulation)

NOTES NOTES BODY TEMPERATURE REGULATION BODY TEMPERATURE REGULATION (THERMOREGULATION) NORMAL BODY TEMPERATURE ▪ 37 ± 0.5 °C (98.6 ± 0.9 °F) ▪ Hypothalamic thermoregulatory center acts as a thermostat ▫ Sets temperature set-point ▪ Thermoreceptors ▫ Peripheral (in skin) → sense surface temperature ▫ Central (in the body core—e.g. hypothalamus itself) → sense core temperature ▪ Temperature variations activate thermoreceptors → thermoreceptors inform hypothalamus → hypothalamus activates heat regulation mechanisms → temperature returns to baseline ▪ Body region variations ▫ Core: higher temperature, more stable ▫ Skin: lower temperature, more variable ▪ Core temperature varies with throughout the day ▫ Lower during sleep ▫ Higher when awake BODY TEMPERATURE MAINTENANCE ▪ Body temperature maintained by balancing heat-generation, heat loss Heat generation ▪ Activation of sympathetic nervous system ▫ Vasoconstriction of skin arterioles → blood bypasses skin → ↓ heat loss ▫ Adrenal glands release catecholamines (epinephrine, norepinephrine) → increased metabolic rate → ↑ heat production ▫ Piloerection (goosebumps) → heat trapping ▪ Thyroid hormones released from hypothalamus → ↑ metabolic rate → ↑ heat production ▪ Non-shivering thermogenesis using brown adipose tissue ▫ Activation of primary motor center for shivering in the posterior hypothalamus → skeletal muscle contraction → shivering → ↑ heat production ▫ Behavioral changes (adding garments, tightening the arms across the chest, moving around) Heat dissipation ▪ Inhibition of sympathetic activity in skin blood vessels → blood goes to skin → ↑ heat loss ▪ Activation of sympathetic cholinergic fibers innervating sweat glands → ↑ sweating → ↑ heat loss ▪ Behavioral changes (removing garments, reducing movements, fanning air over body) Fever ▪ Body temperature elevation due to change in hypothalamic set-point ▪ Pyrogens act on hypothalamus → hypothalamus releases prostaglandins → hypothalamic set-point temperature increases → heat-generating mechanisms kicks in → body temperature rises and reaches new baseline temperature ▫ Aspirin reduces fever by inhibiting prostaglandins production OSMOSIS.ORG 1
▪ Benefits of fever ▫ Inhibit bacterial growth by making growing conditions less favorable ▫ Increase efficiency of immune cells HYPERTHERMIA ▪ Elevation of body temperature without change in hypothalamic set-point ▪ Normal mechanisms of thermoregulation are overwhelmed by various factors ▫ Excessive environmental temperature ▫ Impaired ability to dissipate heat ▫ Excessive heat production Heat exhaustion ▪ Excessive sweating → significant water and electrolyte loss → ↓ blood volume → ↓ arterial pressure Heat stroke ▪ Hyperthermia > 40°C/105.1°F ▪ Potentially fatal ▪ Causes ▫ High environmental temperature ▫ Periods of intense physical activity ▪ Risk factors ▫ Susceptible individuals: infants, children (higher metabolic rate; ineffective sweating; physical, psychological limitations); elderly (pre-existing conditions; physical, psychological limitations) ▫ Medications: ones that inhibit heatdissipating mechanisms (beta blockers, diuretics) Malignant hyperthermia ▪ Genetic alteration of ryanodine receptor 1 (RYR1) in the muscle cells ▪ Normally: cell depolarization → RYR1 activation → calcium release from sarcoplasmic reticulum into cytoplasm → muscle contraction ▪ In malignant hyperthermia: cell depolarization → RYR1 hyperactivation → excessive calcium release → inappropriate muscle contraction, ↑↑ metabolic rate → excessive heat production ▪ Triggered by drugs ▫ Anesthetic gas: Alothane, Sevoflurane, Desflurane 2 OSMOSIS.ORG ▫ Depolarizing muscle relaxants: Succinylcholine, Decamethonium ▪ Potentially fatal ▪ Treatment ▫ Dantrolene (skeletal muscle relaxant) HYPOTHERMIA ▪ Abnormally low temperature ▫ Diagnosis: core temperature < 35°C/95°F ▪ Compensatory mechanisms responding to cold stress are overwhelmed ▪ ↓ core body temperature → ↓↓ metabolic rate → myocardial irritability, cold diuresis (↓ renal blood flow, water resorption) ▫ Progressive oliguria as ↓ core temperature → ↓ intravascular volume, ↑ hematocrit, central nervous system depression Risk factors ▪ Prolonged cold exposure ▫ E.g. inadequate clothing/shelter, cold water immersion ▪ Impaired thermoregulation ▫ E.g. hypothalamic dysfunction, metabolic derangement ▪ ↑ heat loss ▫ Multisystem trauma, shock, spinal cord transection ▪ Iatrogenic ▫ Cold IV fluid administration, inadequate operating room warming ▪ ↑ risk populations ▫ Older adults (↓ physiologic reserve, ↓ sensory perception, chronic medical conditions) ▫ Children (↑ body surface area to body mass ratio, ↓ glycogen stores, young infants unable to use shivering thermogenesis) Complications ▪ Cardiac arrhythmias, myocardial infarction, pulmonary edema, pulmonary embolism, lactic acidosis, disseminated intravascular coagulation (DIC), coma, death Signs & symptoms ▪ Mild hypothermia ▫ Core temperature 32–35°C/90–95°F
Chapter 22 Body Temperature Regulation ▫ Shivering, tachypnea, tachycardia, confusion ▪ Moderate hypothermia ▫ Core temperature 28–32°C/82–90°F ▫ ↓ shivering and muscle rigidity, hypoventilation, bradycardia, ↓ cardiac output, lethargy, arrhythmias, loss of pupillary reflexes ▪ Severe hypothermia ▫ Core temperature < 28ºC/82°F ▫ Apnea, ↓ cardiac activity → ventricular arrhythmias → asystole, coma, loss of ocular reflexes, ↓↓ metabolic rate Rewarming treatment ▪ Warmed blankets/forced warm-air system; heated, humidified oxygen; warmed crystalloid IV fluid; pleural, peritoneal lavage using warm saline solution; vasopressors ▪ Extracorporeal blood rewarming ▫ Venovenous rewarming, hemodialysis, continuous arteriovenous rewarming (CAVR), cardiopulmonary bypass (CPB), extracorporeal membrane oxygenation (ECMO) OSMOSIS.ORG 3

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