Lec 09 Thermoregulation
description
Transcript of Lec 09 Thermoregulation
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
http://www.elmhurst.edu/~chm/vchembook/612citricsum.html
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
• Thermoneutral zone: range of ambient temperatures in which basal metabolic rate maintained.
Metabolic rate and Thermoregualtion
Rectal TemperaturesSpecies °F +/- 1° °C +/- 0.5°
Cattle
Beef cow 101 38.3
Dairy cow 101.5 38.6
Cat 101.5 38.6
Dog 102 38.9
Goat 102.3 39.1
Horse
Mare 100 37.8
Stallion 99.7 37.6
Pig 102.5 39.2
Rabbit 103.1 39.5
Sheep 102.3 39.1
www.merckvetmanual.com *Adapted from Andersson B.E. and Jónasson H., Temperature Regulation and Environmental Physiology, in Dukes’ Physiology of Domestic Animals, 11th ed., Swenson M.J. and Reece W.O., Eds. Copyright©1993 by Cornell University.
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 Environments
Torpor: 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., pocket mouse)
– 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. 17-22
• 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.
Physics of Heat Transfer
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.
Heat Reduction (cooling)
• Takes place through:
1. Conduction
2. Convection
3. Radiation
4. Evaporation
5. Excretion
Negative feedback: change in temperature triggers physiological response to counteract change.
Bodytemperature
Heating
Cooling
Hypothalumusactivates cooling
Skin bloodvessels dilate
Sweat glandsactivate
Body temperaturedrops
Body temperatureincreases
Hypothalamusactivates warming
Skeletal musclesshiver
Skin bloodVessels constrict
Metabolic rate and Thermoregualtion
Behavioral adaptations allow animals to exist in different climates:
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 environments
Avoidance: 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 environments
Ectotherms in hot environments
Water 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.
Hypothermia
Body 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• Soiled or moistened hair coat• Restraint on a cold surface• Prolonged immobility• Chemical restraint drugs & anesthesia
• Surgery – Placement– shaving & wetting– cleaning solutions– open incision– vasodilatory drugs
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)
HyperthermiaPredisposing factors:• Increased ambient temperature• Humidity• Excessive muscle exertion or metabolic activity• Physical structure – think brachycephalic breeds• Dehydration• Trauma• Restraint
Hyperthermia
• The maximum body temperature compatible with life is ten degrees above normal.
• Hyperthermia increases metabolic rate and cellular oxygen consumption
Hyperthermia Clinical Signs
• Increased heart rate• Increased respiration• Open mouth breathing• Sweat & salivate profusely at beginning• Pulse weakens
• Restlessness• Slow responses• Uncoordinated• Convulsions• Collapse• Death
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
Next time…
• Chemical Restraint