Thermoregulation involves balancing heat production and heat loss to maintain a constant body temperature. Body temperature is affected by ambient temperature, activity level, digestion, and time of day or year. Endotherms internally regulate temperature through mechanisms like shivering, vasoconstriction, and brown fat metabolism. Ectotherms rely on external temperatures and behaviors like changing orientation. Hypothermia occurs when heat loss exceeds production, while hyperthermia is an excessive rise in core temperature. Both require warming or cooling treatments respectively.

1. THERMOREGULATION
Body temperature is determined by balance
between heat production and heat loss.
Normal function depends on a relatively
constant body temperature.

2. 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)

3. Metabolic rate and Thermoregualtion
Metabolic 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

4. 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)

5. 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

6. http://www.elmhurst.edu/~chm/vchembook/612citricsum.html

7. 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

8. Metabolic rate and Thermoregualtion
• Thermoneutral zone: range of ambient temperatures in which basal
metabolic rate maintained.

9. Rectal Temperatures
Species °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.

10. 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)

11. 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)

12. 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.

13. Endotherms in Hot Environments
Estivation: 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

14. • 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.

15. 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

16. Homeotherm (endotherm)
• When body heat production is stable, blood flow
into dermis (layer of the skin / integument)
depends on changes in ambient temperature

17. 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

18. 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

20. 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

21. 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

22. 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.

23. Heat from environment is via:
1. Radiation
2. Conduction
3. Convection

24. Radiation
• Electromagnetic waves transfer energy
without heating the intervening air.
Sun most important source
All warm objects (including animals) give
off radiant energy

25. Conduction
• Direct transfer of heat between an animal and an
object.
• The direction is from higher to lower temperature

26. Convection
• Transmission of heat by movement of a
medium surrounding or within an object:
– Air
– Water
– Blood

27. 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.

28. 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

29. Endotherms in Cold Environments
Insulation:
a. Hair, feathers;
• Pilomotor control.
• Shedding/molting allows seasonal
changes.
b. Blubber
• Metabolically inactive.
• Vasoconstriction reduces heat loss.

30. Heat Reduction
(cooling)
• Takes place through:
1. Conduction
2. Convection
3. Radiation
4. Evaporation
5. Excretion

31. Metabolic rate and Thermoregualtion
Negative 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

32. Behavioral adaptations allow animals to
exist in different climates:

33. 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.

34. 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.

35. 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.

36. 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

37. 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

38. Hypothermia Treatment
Warming 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 lamps
Monitor until you are sure they are thermoregulating
appropriately on their own

39. 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)

40. Hyperthermia
Predisposing factors:
• Increased ambient temperature
• Humidity
• Excessive muscle exertion or metabolic activity
• Physical structure – think brachycephalic breeds
• Dehydration
• Trauma
• Restraint

41. Hyperthermia
• The maximum body temperature
compatible with life is ten degrees above
normal.
• Hyperthermia increases metabolic rate and
cellular oxygen consumption

42. Hyperthermia Clinical Signs
• Increased heart rate • Restlessness
• Increased respiration • Slow responses
• Open mouth breathing • Uncoordinated
• Sweat & salivate • Convulsions
profusely at beginning • Collapse
• Pulse weakens • Death

43. 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)

44. Hyperthermia Treatment
• Provide diuresis, supportive care
• Hyperthermia may cause delayed organ dysfunction
• Watch P for several days, repeat CBC/Chemistry

45. Next time…
• Chemical Restraint