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Diurnal temperature
Variations in temperature related to time of day
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Endothermy (homeotherms)
Physiological and behavioral maintenance of a relatively constant internal body temperature (typically show circadian fluctuations in temperature).
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Ectothermy (poikilotherms)
Fluctation of internal body temperature closely related to environmental temperature. Poikilotherms use only behavioral means to regulate temperature.
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van't Hoff rule
For every 10oC increase, the rate of biochemical reactions doubles (ie. for every increase 1oC -> increase 10% BMR)
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Heat stress
Early phase of hyperthermia: dehydration, hypovolemia, decreased peripheral vascular resistance, cutaneous vasodilation
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Heat Stroke
Body temp 41-43oC -> increased brain temp -> loss of neural control of thermoregulation
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Adaptation
Innate characteristics that allow an animal to survive in adverse environments (eg. Bos indicus adaptations to hot environment)
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Acclimation
Physiological change that result from prolonged exposure to a single component (eg. heat) and allow the animal to respond more effectively to the stressor. Eg. increased coat length and BMR in winter
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Acclimatization
Physiological changes with fluctuations in several parameters rather than just one component. Eg acclimization to high altitude -> hypoxia & cold
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Daily Torpor
Decreased body temp (<32oC) <-> decrease BMY <-> decreased energy expenditure. Eg small birds and bats.
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Hibernation and estivation
Hibernation is characterised by reduced BMR and body temp to near ambient temperature over prolonged times in the winter. Estivation is a similar form of dormancy in a dry and hot environment during summer
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Heat transfer in the body
- Newton's law of cooling
- Delta H/delta t = M*C*(T2-T1)
Temp gradientDelta H/delta t = heat flux from cooling body in watts - M = mass of animal
- C = coefficient of thermal conductivity
- (T2-T1) = temperature difference between body and environment
- Tissues
- 1. Basal metabolic rate (BMR) or heat production (kcal/day): rat > chicken > dog > sheep > human > cow
- 2. Most tissues have poor thermal conductivity - conduction not effective
- 3. Blood - heat redistribution by 'circulatory convection'
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Heat transfer in the body
- Mechanisms
- 1. Peripheral vasodilation (hot environment) and vasoconstriction (cold environment)
- 2. Arterio-venous anastomoses open (hot) or closed (cold) in peripheral tissues
- 3. Counter-current exchange mechanisms: heat loss from superficial veins (hot) or heat transfer from arteries to veins to maintain core temperature (cold)
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Counter-current exchange
- Warm conditions: heat lost from blood perfusing superficial capillaries
- Cold conditions: peripheral vasoconstriction with blood flow redirected to deep vascular beds - conserves heat
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Heat balance
- Heat gain:
- 1. Voluntary: exercise (~70-80% EE converted to heat)
- 2. Involuntary - thermogenesis: shivering, non-shivering
- Heat Loss:
- 1. Conduction
- 2. Convection
- 3. Radiation
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Heat Gain: Physiological adjustments
- Shivering thermogenesis
- 1. Involuntary contractions of antagonistic groups of muscle-no work
- 2. Chemical energy converted to heat (~4-5x heat)
- 3. Increase in BMR (~5-10x) is proportional to non-evaporative heat loss
- Non-shivering thermogenesis1. Increase in BMR without shivering
- 2. Increase in thyroid hormones and/or catecholamines
- 3. Oxidation of fatty acids in "brown fat" to produce heat (beta1 adrenergic receptors in brown fat, ~5-6% of BW in new born rabbits is brown fat)
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Heat Loss
- 1. Conduction
- Occurs when body is in direct contact with surfaces
- Eg. cold -stainless steel surgery tables, new-born piglets on cold floors
- 2. ConvectionOccurs with movement of air or water in contact with skin
- Natural convection: warmer air/H2O moves away from surface of animal
- Forced convection: cooler air/water is moved over skin -> thermal gradient maintained -> greater heat loss than natural convection
- Insulation eg. thick hair coat, piloerection, blubber in sea animals
- Radiation
- Heat transfer within both the visible (determined by color of absorbing surface) and infrared spectrum (independent of color) or electromagnetic waves
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Heat Loss: Physiological adjustments
- 1. Non-evaporative heat loss
- Zone of minimal thermoregulatory effort: heat exchange occurs by behavioral changes (eg. piloerection, thick hair coat, postural changes) and skin blood flow (eg. vasoconstriction of cutaneous vessels)
- 2. Evaporative heat loss
- 1. Panting: mostly closed mouth - except dogs
- Increased respiratory frequency + decreased tidal volume -> constant alveolar ventilation (no change in PCO2)
- Inspired air -> nasal turbinate bones -> vaporization from venous blood -> air expired through mouth + protrusion of moist tongue
- Rete mirbile at base of brain is a countercurrent exchanger = cool venous blood from nasal sinus + warm arterial blood from carotid arteries = regional heterothermy
- Gular flutter:
air rapidly passed through mouth and gular sac of birds - -convective heat loss from blood vessels + evaporative heat loss
- 3. Wallowing: pigs and water buffaloes
- 4. Sweating: sweat composition (~plasma) is altered by reabsorption of ions. At high secretion rate decreased reabsorption -> increased water & salt secretion
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Regulation of body temperature
Normal body temp -> Increase in body temp -> CNS signals skin blood vessels dilate & sweat glands to secrete -> Increase body heat loss -> Decrease body temp -> Normal body temp (~37 oC) Hypothalamic set-point -> Decrease body temp -> CNS signals skin blood vessels to constrict -> (two options) -> CNS body heat conserved or ->CNS signals muscles to contract (shivering) -> Increase body heat (which both feed into) -> Increase body temp -> Normal body temp
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Disorders of temp regulation
- Decreased temperature
- Hypothermia: Heat output> heat production; impaired hypothalamic thermoregulation at < 29oC (ex. Neonates - lambs, piglets, puppies), Anasthesia
- Frostbite: Extreme cold -> peripheral vasoconstriction -> tissue cooling below H2O freexing poin -> ice crystals formation in tissues
- Increased Temp
- Malignant hyperthermia: inherited skeletal muscle disorder triggered by exposure to volatile anesthetics and muscle relaxants
- Heat strok: Heat output > Heat production (Body temp -41.5 - 42.5 oC)
- Fever
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