THERMOREGULATIONBody temperature is determined by balance between heat production and heat loss. Normal function depends on a relatively constant body temperature.
Why should I care about understanding thermoregulation? • Improve patient safety – Particularly during/after anesthesia • Improve patient comfort • Improve patient willingness to participate in procedures • Facilitate understanding of hypothermia & hyperthermia causes & treatments (clinical)
Metabolic rate and ThermoregualtionMetabolic Rate: – Rate that an animal consumes chemical energy. – heat energy released, O2 consumed, or CO2 produced.Basal Metabolic Rate (BMR): – Metabolic rate at rest with no temperature stress or digestion occurring. – Basic functioning of vital organs
Energy• An animal’s body is an energy transformer used for: – Growth – Muscular Movement – Maintenance, including enzyme synthesis of enzymes to digest food – Reproduction – Product production (such as milk, eggs)
Energy (& thus heat) comes from:• Organic compounds (food) glucose ATP production via glycolysis, Krebs (TCA, citric acid) Cycle, Electron Transport Phosphorylation, and anaerobic (lactic acid) metabolism• Brown adipose tissue • Most prominent in neonates • Important for small mammals in cold environment, hibernating animals • Inverse correlation between amount of brown fat & BMI
Body Temperature• The Body Temperature is the temperature recorded on a thermometer inserted into the rectum deep enough to record the core (or internal) temperature• Temperature of skin surface may be higher or lower than core• Body temperature may be ↑ or ↓ depending on surface the animal is placed on• Effects of both internal (adipose – brown & white) and external insulating layers (hair, feathers) is important during restraint
Metabolic rate and Thermoregualtion• Thermoneutral zone: range of ambient temperatures in which basal metabolic rate maintained.
Heat Storage in Camels• In times of water deprivation, camels can store heat during the day to dissipate overnight in cooler temperatures (instead of using evaporative mechanisms which would use more water).• Temp can vary from 34°C (93°F) up to 41°C (106°F); if hydrated only varies from 36°C (97°F) to 38°C (100°F) (Schmidt-Nielsen K. Osmotic Regulation in higher vertebrates. The Harvey Lectures, 1962-63. Series 58. London Academic Press, Inc., 1963:53-93)
Body temperature is affected by:– Ambient temperature– Level of activity– Digestion, food & water intake– Time of day (higher when typically active) • Diurnal animals – lower temp in am, then warms up • Nocturnal animals – higher temp in am, cools down during day– Time of year (hibernation)
Endotherms in Cold EnvironmentsTorpor: adaptive hypothermia. – The dormant, inactive state of a hibernating or estivating animal. – Birds, daily in cold months – Example chickadees • Fat stores accumulated during day supply energy for cold nights. • Stored fat is not enough to survive the night without torpor.
Endotherms in Hot EnvironmentsEstivation: period of torpor during hot periods (e.g., pocketmouse) – A state of dormancy or torpor during the summer. • Estivate during periods of food scarcity. • Duration of torpor proportional to severity of food deprivation. • Frogs, toads, snails, worms, lungfish
• Hibernation – An inactive state resembling deep sleep in which certain animals living in cold climates pass the winter. – Bats, ground squirrels, some other rodents• Brumation – A state of cold-period dormancy or reduced activity in cold-blooded animals. Unlike hibernation, the animal may remain partially active during brumation.
Homeotherm (endotherm)• Capable of regulating temperature internally• Also called “warm-blooded”• Primarily controlled unconsciously within the body: Core temperature rises heat energy is transported to skin surface via blood released into environment by conduction, convection, radiation or the evaporation of sweat• Birds and mammals
Homeotherm (endotherm)• When body heat production is stable, blood flow into dermis (layer of the skin / integument) depends on changes in ambient temperature
Homeotherm (endotherm)• Vertebrates also regulate temperature via hypothalamus – Autonomic nervous system, involuntary response – Receptors in hypothalamus monitor blood temp in brain – Skin receptors monitor temp as well and send info to hypothalamus
Poikilotherm (ectotherm)• Rely on external sources for heat and coolness to regulate body temperature• Also called “cold-blooded”• Primarily controlled through behavior adaptions• Reptiles, fish, amphibians
Heat Production• Heat gained/temperature increased by: – increased production or – by absorption from the environment• Production is through: 1. ↑ muscle tone - shivering 2. Exercise 3. Eating 4. Fever (pyrexia) 5. Brown adipose
Endotherms in Cold Environments• Thermogenesis: a. Shivering: brief contractions of antagonistic muscle groups produces heat without motion. b. Non-shivering thermogenesis: fat metabolized to produce heat; regulated by sympathetic nervous system. • White adipose tissue: fats reduced to fatty acids for metabolism elsewhere in body. • Brown fat: fat metabolized within fat cells for rapid release of heat (thermogenin uncouples electron transport from ATP production) Fig. 17-2
Physics of Heat Transfer• Heat flows from areas of high temperature to areas of low temperature.• Forms of heat transfer: 1. Conduction: between bodies in contact. 2. Convection: movement of air or water over a surface; continuous replacement of fluid maximizes heat transfer. 3. Radiation: emission of electromagnetic energy from an object. Animals radiate primarily in the infrared. 4. Evaporation: conversion of liquid to gas; causes cooling.
Heat from environment is via: 1. Radiation 2. Conduction 3. Convection
Radiation• Electromagnetic waves transfer energy without heating the intervening air. Sun most important source All warm objects (including animals) give off radiant energy
Conduction• Direct transfer of heat between an animal and an object.• The direction is from higher to lower temperature
Convection• Transmission of heat by movement of a medium surrounding or within an object: – Air – Water – Blood
Ectotherms in hot environments• Localized vasodilation: shunt heat from cool regions to warmed regions. Example: marine iguana – Chilled at night and basks in morning sun. – Warmed blood causes vasodilation on both dorsal and ventral sides. – Air is cooled on ventral side by convection. – Cool belly acts as heat shunt.
Heat Conservation• Heat is conserved through vascular responses: 1. Blood vessels near surface constrict to allow skin temperature to drop without jeopardizing core temperature 2. Velocity of blood flow increases which decreases exposure time to cold
Endotherms in Cold EnvironmentsInsulation: a. Hair, feathers; • Pilomotor control. • Shedding/molting allows seasonal changes. b. Blubber • Metabolically inactive. • Vasoconstriction reduces heat loss.
Metabolic rate and ThermoregualtionNegative feedback: change in temperature triggers physiological response to counteract change. Skin blood vessels dilate Hypothalumus Body temperature activates cooling drops Sweat glands activate Heating Body temperature Cooling Skin blood Vessels constrict Body temperature Hypothalamus increases activates warming Skeletal muscles shiver
Behavioral adaptations allow animals to exist in different climates:
Ectotherms in hot environments Change orientation: reduce amount of surface exposed to sun (lizard). – Orient vertically to minimize impact of sun’s rays. – Compress ribs to reduce surface area.
Ectotherms in hot environmentsAvoidance: find refuge during hot periods – Example: Desert tortoise constructs burrow as daily retreat and for hibernation – Shallow burrow collects water causing evaporative cooling. – Water balance fluctuates with availability of free water and vegetation.
Ectotherms in hot environmentsWater exchange through skin – Example: spadefoot toad lives in deserts • Dig burrow during rainy season: absorb water from soil. • During dry season: retain urea → increased internal osmolarity → continued water absorption from soil.
HypothermiaBody temperature decreases when heat loss exceeds heat production or gain• Below 93.2 degrees in homeotherms – impaired – below 86 completely eliminated• Newborns have undeveloped thermoregulatory ability
Causes of Hypothermia• Exposure to wind • Surgery• Soiled or moistened – Placement hair coat – shaving & wetting• Restraint on a cold – cleaning solutions surface – open incision – vasodilatory drugs• Prolonged immobility• Chemical restraint drugs & anesthesia
Hypothermia TreatmentWarming of whole body is necessary• Circulating warm air (Baer Hugger)• Warmed fluid bags, water bottles, rice socks, towels…• Warm water bath• Warm water enema• Warm IV fluids• Circulating water pad• Incubator• Heat lampsMonitor until you are sure they are thermoregulating appropriately on their own
Hyperthermia• Excessive elevation of core temperature (not necessarily fever)• Enzymes (proteins), which control metabolic reactions in the body, work best in a narrow range of temperature (and pH)• Proteins tend to denature above 45°C (113°F)
Hyperthermia Treatment• IV fluids with line iced• May need oxygen, intubation• Cool water: bath, spray, moistened towels• Ice packs wrapped in moist towels• Enema (cool water)• Alcohol bath, alcohol on foot pads• Circulate air (fan, open window)• DO NOT OVER-COOL! – (I generally stop active cooling at ~104°F)
Hyperthermia Treatment• Provide diuresis, supportive care• Hyperthermia may cause delayed organ dysfunction• Watch P for several days, repeat CBC/Chemistry