Chapter 14 - Body Temperature & Regulation

Ch. 14 - Body Temperature & Regulation What is the normal body core temperature & the variations in certain conditions? => Normal core body temperature: 36.5 - 37.5 °C => Diurnal variation: => Lowest in the early morning (4-6 AM) (36.0 °C) => Highest in the late afternoon (4-6 PM) (~37.7 °C) => Variation in Certain Conditions: => Exercise : Can increase to 38-40 °C => Menstrual cycle : Increases by ~0.5 °C during the luteal phase due to progesterone! => Fever : ≥ 38 °C due to infection or inflammation. => Hypothermia : <35 °C due to prolonged cold exposure. What Produces Heat in Our Body? => Heat is a byproduct of metabolism. • Basal Metabolic Rate (BMR) (largest contribution) • Muscle activity (exercise, shivering) • Sympathetic stimulation (epinephrine, norepinephrine) • Thyroxine secretion (long-term metabolic regulation) • Non-shivering thermogenesis (brown fat metabolism, esp in infants) Heat Insulators of the Body: => Subcutaneous fat layer (main insulator, prevents heat loss). => Skin & cutaneous vasculature (control heat dissipation). => Hair & clothing (external insulators). Methods of Heat Loss from the Skin Method Description Example Radiation Heat loss as infrared waves (accounts for 60% of total heat loss) Standing in a cool room, heat radiated from skin. Conduction Heat transfer by direct contact Sitting on a cold metal chair, standing in a cool swimming pool (heat radiates from the limb). Convection Heat loss by air or water movement Wind blowing over the skin, water currents during swimming. Evaporation Conversion of sweat into vapor (primary heat loss in hot conditions). Sweating on a hot day, breathing out moist air. => Evaporation occurs as: • Insensible water loss (~450 mL/day from lungs & skin). • Sensible sweating (active process for heat dissipation). What is Brown Fat? => Specialized adipose tissue found in infants & hibernating animals. => Contains many mitochondria with uncoupling protein-1 (UCP-1), allowing non-shivering thermogenesis. => Oxidizes fatty acids to generate heat instead of ATP. Constituents of Sweat. => Mainly water (~99%). => Electrolytes: Na+, Cl-, K+ => Urea, lactate, ammonia => pH: 4.5-7.0 Role of Aldosterone in Sweating Regulation => Prolonged exposure to heat -> Aldosterone secretion increases. => Enhances Na+ reabsorption in sweat glands -> reduces sodium loss in sweat. => Acclimatization to hot climates results in more dilute sweat. Role of the Hypothalamus in Temperature Regulation. => Preoptic area of the hypothalamus = primary temperature control center. => Receives input from: => Peripheral thermoreceptors (skin) => Central thermoreceptors (hypothalamus, spinal cord, abdominal organs) => Initiates responses to maintain core temperature via: - => Heat loss mechanisms (vasodilation, sweating). => Heat gain mechanisms (shivering, vasoconstriction, hormonal changes). Mechanisms of Temperature Regulation => When the body is too hot => Vasodilation : Increased blood flow to the skin -> heat loss via radiation & conduction. => Sweating : Evaporative heat loss. => Decreased heat production : Reduced muscle activity, inhibition of thyroxine secretion. => When the body is too cold => Vasoconstriction : Reduces blood flow to skin -> conserves heat. => Shivering : Increases heat production. => Piloerection : Hair stands up (minimal effect in humans). => Increased metabolic rate : via thyroxine, epinephrine, norepinephrine. Role of Cytokines in Fever. => Infection/Inflammation -> Immune cells release cytokines (IL-1, IL-6, TNFα, IFN-γ). => These stimulate the hypothalamus to increase the temperature set point. => Prostaglandin E2 (PGE2) is key in raising set point -> Fever. Sweat Mechanisms Mechanism of Sweat Secretion => Stimulated by SNS (cholinergic effect of fibers). => Sweat glands (eccrine glands) secrete a primary isotonic secretion, later modified to a hypotonic fluid by reabsorbing Na+ & Cl-. What stimulates sweating? => High body temperature. => Exercise. => Emotional stress (palms, soles, armpits - adrenergic response). Why do we sweat during exercise? => Increased metabolism -> heat production -> hypothalamus triggers sweating to dissipate heat. Heat Loss in Specific Conditions. Panting => Rapid shallow breathing enhances evaporation -> cooling from respiratory tract. During exercise => Heat loss via radiation, convection & sweating. => Skin blood flow increases -> enhanced heat dissipation. Pathophysiology Behind Temperature Disorders Fever => Cytokines stimulate hypothalamus -> PGE2 increases set point -> fever. Chills => Occurs as body temperature rises to meet the new set point -> vasoconstriction & shivering. Heatstroke => Extreme hyperthermia (>41°C) due to failure of thermoregulation. => Symptoms : Hot dry skin, confusion, collapse. => Complication : Multi-organ failure due to excessive heat load. Hot flushing => Sudden vasodilation episodes causing warmth & sweating (Menopause). Malignant Hyperthermia => Genetic disorder triggered by anesthetics. => Defective ryanodine receptors -> excessive Ca2+ release in muscles -> uncontrolled heat production. Frostbite => Vasoconstriction -> ischemia -> tissue freezing -> necrosis. Q: How to Decrease Body Temperature? Non-Pharmacologic Measures : => Cold Environment => Cooling blankets, ice packs (axilla, groin, neck). => Cold IV fluids. Pharmacologic Measures: => Antipyretics (Acetaminophen, NSAIDs): Inhibit PGE2 production. => Muscle relaxants (Dantrolene) for malignant hyperthermia.