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BAHAN KULIAH-Termoregulasi
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Thermoregulation Chapter 24 - Nutrition, Metabolism and Thermoregulation Objectives * Describe the thermoregulatory center and its functions. * Explain the mechanisms of heat transfer. * Explain the pathophysiological mechanisms involved in temperature disorders. Body Temperature * Shell temperature: * Temperature closer to skin * Oral temperature * 36.6o-37.0oC (97.9o-98.6oF) * Core temperature: * Most important temperature * Temperature of "core" (organs in cranial, thoracic and abdominal cavities) * Rectal temperature * 37.2o-37.6oC (99.0o-99.7oF) * Thermoregulation is a balance between heat input/production and heat loss * Heat input = environmental * Heat production = metabolic processes * Heat loss = environmental and/or metabolic Physiology * Body temperature is maintained within a narrow range by balancing heat load with heat dissipation. * Body's heat load results from both metabolic processes and absorption of heat from the environment * As core temperature rises, the preoptic nucleus of the anterior hypothalamus stimulates efferent fibers of the ANS to produce sweating and cutaneous vasodilation. Heat Production * Exergonic reactions: * Oxidation and ATP use. * Most heat generated by brain, heart, liver and glands at rest. * Skeletal muscles 20-30% at rest. Can increase 30-40 times during exercise. Thermoregulatory Center * Hypothalamus: * Preoptic area neurons: hypothalamic thermostat: * Heat-losing center * Heat-promoting center * Monitors temperature of blood and receives signals from peripheral thermoreceptors. * Negative feedback loops Thermoregulatory Center * Heat-losing center: * Activates heat losing mechanisms: * Dilation of dermal arterioles: increase blood flow to skin. * Sweating. * Increased respiration through mouth. * Behavioral: remove clothing. * Inhibits heat-promoting center. Thermoregulatory Center * Heat-promoting center: * Activates heat generating mechanisms: * SNS: * Vasoconstriction of dermal arterioles: decrease blood flow to skin * Stimulates arrector pili muscles: hair stands on end * Shivering thermogenesis: spinal reflex of alternating contractions in antagonistic muscles * Nonshivering thermogenesis: * Long-term mechanism stimulating thyroid hormone release T3 and T4. * Inhibits heat-loss center. Thermoregulatory Center * Hypothalamus: * Peripheral thermoreceptors: * Temperature of skin. * Central thermoreceptors: * Temperature of core. * Most important located in hypothalamus. Mechanisms of Heat Transfer * Radiation: * Infrared radiation. * Conduction: * Direct transfer of energy through physical contact. * Convection: * Heat loss to air around the human body. * Evaporation: * Energy change in water molecule from liquid to vapor. Heat Exhaustion * Acute heat injury due to dehydration. * Heat-loss center stimulated. * Sweat production increases. * Decrease in BP because heat loss center stimulates peripheral vasodilation. * Blood flow to brain decreases. * Compensatory mechanisms: * SNS * Aldosterone * ADH Heat Exhaustion * Symptoms: * Headache * Nausea * Actively sweating * Skin cool and pale * Fainting * Treatment: * Increase fluids * Shade or AC room * Cold wet towels * Can progress to heat stroke. Heat Stroke * Core rectal temperature approaches 41oC * 2 types: * Individuals with compromised homeostatic mechanisms: * Young children * Elderly * Exertional: * Healthy individual under strenuous exercise Heat Stroke * Renal and splanchnic vasoconstriction. * Peripheral blood flow decreases. * Impaired thermoregulation. * CNS dysfunction: * Cerebral edema and increased intracranial pressure. * Tissue damage due to uncoupling during oxidative phosphorylation. * Proteins denature. Heat Stroke Complications * Rhabomyolysis * Multiple organ failure: * Cardiac * Hepatic * Renal * DIC: fibrinolysis * CNS hallucinations, coma * Anhydrosis more common in classic than exertional heat stroke. Heat Stroke Treatment * Drop core temperature by 0.2oC/min * Institute evaporative cooling methods: * Remove clothing and spray body with water while cooling with fans. * Correct water deficit * Oxygen * Give chlorpromazine: * Depresses shivering during treatment Exercise * Maximum exercise, heat production can be 10-20 times resting. * Highest temperatures occur in the exercising muscles. * Body temperature increases during exercise and levels off a few degrees above normal (except at extreme temperatures). * Regulated response with heat loss = heat production at a stabilized core temperature. Exercise Acclimatization * Exercise in the heat for two weeks at a safe intensity. * Plasma volume increases 12%. * Sweating occurs at lower temperature. * Sweat rate increases as much as 3 times. * Sweat osmolality decreases. Hypothermia * Core temperature below 95oF. * Gradual loss of mental and physical activity. * Changes in motor coordination. * Shivering. * Slurred speech. * Abnormally slow rate of breathing. * Cold, pale skin. * Fatigue, lethargy. Hypothermia * If core temperature remains above 90oF, recovery is good. * If core temperature falls below 80o F, prognosis poor. * Treatment: * Move person out of cold. * Insulate the person's body. * Warm beverages. * Give warm IV fluids, slowly otherwise vasodilation occurs, bringing too large volume of chilled blood; cardiac arrhythmias. * Frostbite: * Vasoconstriction to cold, causes irreparable tissue damage. * Cells decrease ATP production, hypoxia. * Do not rub tissue. Classifying Thermal Relationships Homeothermy in Ectotherms Hyles lineata (Lepidoptera: Sphingidae) * Occurs in Mojave desert of SW USA * Polyphagous on desert annuals * Abundant in April and May, dormant rest of year * Population size varies drastically yr to yr * Caterpillars regulate Tb through position and postural changes Endothermy versus ectothermy Warming up by Basking Optimal temperature for enzyme functioning Behavioural adaptations for thermoregulation * animals often bathe in water to cool off or bask in the sun to heat up Shivering, sweating, and panting * honeybees survive harsh winters by clustering together and shivering, which generates metabolic heat * Inefficient - 75% of energy is lost in mechanical movement Surface area to volume ratios * Infants have: * Therefore, cannot tolerate a heat gradient (dt/dx). * To maintain body temperature during a minor cold stress: * Infant must produce 5x heat per kilo as an adult faced with the same stress Where do ectotherms thrive? * Where food items are: * scarce * small * In environments low in O2 Thermoregulation and food chains * Endotherms are often the top predator in food chains * Food chains with lots of ectotherms are often longer in length Thermoregulation * Homeotherms use the: * Environment * Vascular and metabolic adaptations * Shivering and sweating Involuntary Response to Cold Stress * Shivering in adults * Nonshivering thermogenesis in infants until 3-6 months of age Nonshivering Thermogenesis * Temperature sensors are in the skin (in newborns particularly the face), the spinal cord and the hypothalamus. * Temperature information is processed in the hypothalamus. Norepinephrine (NE) is released in response to cold stress. * Result: Vasoconstriction and increased metabolic activity. Nonshivering Thermogenesis * Vasoconstriction also occurs in infants but the primary response is increasing heat production from brown fat metabolism. * NE stimulates receptors on brown fat cells; activates lipase which releases intracellular fatty acids. * Conversion of T4 to T3 inside brown fat cells. T3 cellular metabolic rate Lecture #19 Date________ * Chapter 44 ~ Regulating the Internal Environment Homeostasis: regulation of internal environment * Thermoregulation internal temperature * Osmoregulation solute and water balance * Excretion nitrogen containing waste Regulation of body temperature * Thermoregulation * 4 physical processes: * Conduction~transfer of heat between molecules of body and environment * Convection~transfer of heat as water/air move across body surface * Radiation~transfer of heat produced by organisms * Evaporation~loss of heat from liquid to gas * Sources of body heat: * Ectothermic: determined by environment * Endothermic: high metabolic rate generates high body heat Energy Balance: About 50% used for Body Heat Body Temperature Balance: Homeothermic * Metabolic heat production usually required to maintain balance * Balance is very narrow range, usually higher than environment Body Temperature Balance: Homeothermic Thermoregulation: Homeostatic Balancing of Body Temperature * Peripheral and body core receptors - senses change * Hypothalamic thermoregulatory center - integrates & initiates: * Shivering, non-shivering thermogenesis, vasoconstriction Thermoregulation: Homeostatic Balancing of Body Temperature Thermoregulation: Prevention of Overheating * Sweat: evaporates from skin - cooling * Vasodilation of cutaneous vessels transports heat from core * Behavior: ? activity, exposure to heat Thermoregulation: Prevention of Overheating Thermoregulation: Pathologies * Hyperthermia: body temperature too high * Fever: pyrogens fight pathogens * Heat exhaustion (1020F) * Heat stroke (?1060F) ? death * Malignant hyperthermia - defective Ca++ release * Hypothermia: body temperature too low * Metabolism slows ? loss of consciousness, death * Surgical applications: heart surgery Summary * Eating provides carbohydrates, proteins, & fats for metabolism * Reversible reactions allow interconversion of nutrients * Energy is used for body heat and work: transport, synthesis, storage * Metabolic rate changes with age, sex, body fat, activity & diet * Insulin regulates anabolic cell activities & glucose uptake in cells Summary * Glucagon regulates catabolic reactions & prevents hypoglycemia * Diabetes is a major disease associated with insulin lack or tolerance * Maintaining homeothermy takes ?50% of our energy * Hypothalamic thermoregulatory center controls heat homeostasis Linear Heat Transfer Equation Within the Thermal Neutral Zone, heat loss is controlled by changing conductance (C). * Thermoneutral zone * Environmental temperature at which body temp can be maintained primarily by changes in vaso/pilomotor mechanisms * No additional energy expenditure (shivering, sweating, etc) to raise or lower body temp Control of Conductance Mechanisms of Heat Production Behavioral Thermoregulation Thermoregulation * Environmental gain/loss * Convection (+ or -) * Transfer of heat between tissue and fluid (usually moving fluid) * Tissue = body skin surface or within blood vessels * Fluid = liquid or gas * Wind movement, contact with fluid/water, respiratory heat losses (ie, panting) * Convection * Natural convection - heat changes arising from thermal gradient alone * Forced convection * Increased heat changes due to forced movement of fluid/air across tissues * Wind chill, incl/ fans, running speed * Water current * Blood circulation * Convection losses/gains dependent on thermal gradient * Will you get colder faster in 80 air/water or 40 air/water? * Fluid movement increases thermal gradients * Rapid air/water exchange continuously removes heat from tissue surface, maintains greater thermal gradients * The thermal gradient in convection loss/gain can be altered * Skin surface blood flow * Increased circulation = increased heat loss * Decreased circulation = decreased heat loss * Insulation (hair, piloerection, SQ fat) * Behavioral * Curling up * Huddling in groups * Increased respiratory rates, (panting) * Conduction (+ or -) * Direct transfer of heat through liquid, solid or gas * Example - lying on cold (or hot) concrete, contact with cold (or warmed) surgical table * Relatively minor source of heat loss in horses * Radiation (+ or -) * Movement of heat between two objects without direct contact * Solar radiation - up to 15% of heat gain * Heat reflected from hot concrete, sand, pavement, etc * Grass, water absorbs radiation rather than reflects, so feels cooler * Can also lose body heat to cooler surroundings * Lose body heat to cool walls even when air in between is warm * Evaporation * Heat loss through vaporization of sweat, saliva, respiratory fluids * Extremely important in horses * 85% sweating, 15% respiratory * May be only form of heat loss available when air temp exceeds body temp * Evaporation is the only pathway by which the body only loses heat, never gains. * Efficiency of evap varies based on: * Magnitude of thermal gradient * Hot horse/cool air or hot horse/hot air? * Velocity of air movement (convection) * Maintains thermal gradient * Ambient air humidity * High humidity decreases vapor pressure gradient, less efficient * Hs = Hm - [ Hcd Hc Hr + He] * Hs = heat stored in tissues * Hm = metabolic heat production * Directly proportional to oxygen consumption (VO2) * 20% efficient = 80% heat production * Hcd = conduction heat loss/gain * Hc = convection heat loss/gain * Hr = radiation heat loss/gain * He = evaporative heat loss * How important is cooling? * Heat generated during submaximal exercise ~ .56F/min => 33F/hr * Normal to potentially lethal in < 15 min of exercise if heat not removed * Enough heat generated during 50-mile endurance ride to melt and boil 150 lbs of ice * Heat generated during 1 1/2 mile race = 1.8 - 2.7F/min * Total heat production in 2 min race 3.6 - 5.4F - high but not dangerous due to short duration of exercise * Most of heat stored, little time to dissipate large amounts of heat during race, but decreases quickly after cessation of exercise * If continued to gallop at maximal speeds, and heat were not dissipated, could reach lethal temps in < 5 min * Environmental factors that increase thermal stress (Hs) * High ambient air temps * Sunny conditions * Lack of wind/air movement * High humidity * Physiologic factors that increase thermal stress * Unfit condition * Higher cardiovascular load, less capillary density, less developed sweating ability * Insulation * Hair, body fat, heavy muscling * Dehydration * Increased blood viscosity, decreased convection * Physiologic factors that increase thermal stress * Intensity of exercise * Thermic effect of feed * Protein > structural carbs > simple carbs > fats * Body mass * Surface area to body mass ratio * As body mass increases, surface area decreases * Most heat exchange occurs at skin surface => lower BSA:BM = less heat loss per m2 of skin surface * Desert species * Small body mass * Larger SA:BM ratio * Dissipate heat better * Harder for small animals to retain body heat in cold environments * Polar species * Larger body mass * Smaller SA:BM * Retain heat better * Few small species in cold environments * Compare humans to horses * Human body mass 80 kg * Skin surface 2 m2 * SA:BM = 1:40 * Horses 500 kg * Skin surface 5 m2 * SA:BM = 1:100 * Horses body mass 625% larger than humans, but only 250% increase in skin surface * Must develop more efficient methods of heat dissipation * Human max sweat rate 17 ml/m2min * Horses max sweat rate 50 ml/m2min or up to 15 L/hour (faster than any other species) * Amount of heat loss via evaporation of sweat is dependent on temperature and vapor pressure (humidity) of surrounding air * 1 liter evaporated water removes * 598 cal of heat @ 32F * 580 cal of heat @ 104F * Equiv to 1-2 min max exercise or 5-6 min of submaximal exercise * At maximum efficiency, evaporative sweat = up to 65% total heat loss * Respiratory losses = up to 25% of heat loss * Conduction, radiation, etc = remainder * If insufficient heat loss, the result is increasing core body temp, heat exhaustion * Evaporative efficiency * Maximized in cool, dry conditions * Less efficient hot, dry conditions * LEAST efficient hot, humid conditions * Potentially INEFFECTIVE * Rate of Hs (heat storage in tissue) in hot, humid conditions may be twice as rapid as under cool, dry conditions * Core temp can reach dangerous levels twice as quickly * Rate of evaporation is a function of the gradient of vapor pressure between ambient air and at skin surface * Sweat rate exceeds evaporation rate = sweat drips * Drips only 10% as effective at removing heat as evaporation * Most likely to occur when ambient conditions are hot and humid * Why quantify ambient conditions * Increasing precautions against heat exhaustion (FEI) * Decrease length, difficulty of course * Provide rest/cooling stops * Additional veterinary monitoring * Limit/cancel competition if conditions are extreme