Physiology: Thermoregulation

Variations in temperature related to time of day

Physiological and behavioral maintenance of a relatively constant internal body temperature (typically show circadian fluctuations in temperature).

Fluctation of internal body temperature closely related to environmental temperature.  Poikilotherms use only behavioral means to regulate temperature.

For every 10^oC increase, the rate of biochemical reactions doubles (ie. for every increase 1^oC -> increase 10% BMR)

Early phase of hyperthermia: dehydration, hypovolemia, decreased peripheral vascular resistance, cutaneous vasodilation

Body temp 41-43^oC -> increased brain temp -> loss of neural control of thermoregulation

Innate characteristics that allow an animal to survive in adverse environments (eg. Bos indicus adaptations to hot environment)

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

Physiological changes with fluctuations in several parameters rather than just one component. Eg acclimization to high altitude -> hypoxia & cold

Decreased body temp (<32^oC) <-> decrease BMY <-> decreased energy expenditure. Eg small birds and bats.

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

• Newton's law of cooling
• Delta H/delta t = M*C*(T₂-T₁) Temp gradient
• Delta 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'

• 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)

• Warm conditions: heat lost from blood perfusing superficial capillaries
• Cold conditions: peripheral vasoconstriction with blood flow redirected to deep vascular beds - conserves heat

• 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

• 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 thermogenesis
• 1. 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)

• 1. Conduction
• Occurs when body is in direct contact with surfaces
• Eg. cold -stainless steel surgery tables, new-born piglets on cold floors
• 2. Convection
• Occurs 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

• 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

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

• Decreased temperature
• Hypothermia: Heat output> heat production; impaired hypothalamic thermoregulation at < 29^oC (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