Thermoregulation Flashcards
Heat Transfer & Environment
Body temp will be constant if the:
SUM of heat gained (by conduction, thermal radiation, metabolism) per unit of time =SUM of heat lost (by convection, evaporation) per unit of time
Conduction: heat exchange between two surfaces in direct contact
• occurs when skin comes in contact with a cold or warm object
Convection: heat transfer brought about by flow of heat to the air (by wind)
• can also occur in water
• much faster than conduction
Evaporation: mechanism for heat transfer by changing of state of water from liquid to gas
- evaporating water (sweat) takes with it a great deal of energy/heat from the skin (means of cooling body)
Radiation: transfer of heat via infrared waves, such as radiator, sun
Wind Chill factor
• Rate of convection depends on both diff in temp b/w the surface and fluid surrounding it and velocity of that fluid with respect to the surface
• As convection from a warm surface heats the air around it, forms insulating boundary layer of warm air against the surface
• Moving air disrupts this boundary layer, allowing for cooler air to replace the warm air against the surface
The faster the wind speed, the more readily the surface cools
EX: air temp feels twice as cold at a wind velocity of 4 mph than if the wind velocity is 1 mph
Dry heat transfer
Heat transfer that does not involve evaporation or condensation of water
• occurs thru conduction, convection, and radiation
Rate of heat transfer: TB – TA
Heats move out of the body by dry heat transfer when: TB > TA
M=C (TB – TA)
C = Thermal conductance
TB = body temp; TA = ambient temp
Suppose that 2 mammals are in the same environment & have same driving force for dry heat loss (TB – TA) BUT one has higher thermal conductance than the other
–>The one w higher C will require higher metabolic rate to stay warm
- Animal w higher C has low resistance (insulation) to dry heat loss
M= 1/I (TB – TA)
Ecotherms vs Endotherms
Ectothermic animals rely on external environment to govern their body temp
Endothermic animals obtain heat energy primarily from internal reactions (metabolically)
At any temp, metabolic rates of endotherms always higher than those of ectotherms of comparable size
Endotherm: metabolic rate rises at low environmental temp, generating extra body heat
Ectotherm: metabolic rate falls at low environmental temp, conserving energy
Ectotherms
Found in all invertebrate groups (most fishes, amphibians, reptiles)
• Can compensate for seasonal variations in environmental temp
• Obtain heat energy externally (rely on external environment to govern body temp)
• Control body temp by regulating heat exchange with environment
Most aquatic invertebrates are limited thermoregulators BUT some use behavioral responses to regulate body temperature
Terrestrial invertebrates regulate body temp more closely
Warm-bodied Fish
Tunas, lamnid sharks, and billfish –endothermy in certain body regions
Opah displays whole-body endothermy
The tissues that are endothermic in tunas and sharks: red swimming muscles, stomach, other viscera, brain and back of the eyeball
In billfish, only brain and back of eyeball are endothermic
Countercurrent vascular array prevent heat from reaching gills (prevent heat loss)
Venous blood transfer heat to arterial blood gives off small arteries that penetrate into muscles
Ordinary metabolic heat production = source of heat for endothermy in all cases except the billfish, which have specialized “heater” organ for cranial endothermy (warms brain/eyes)
Advantages, Disadvantages, Consequences
Ecotherm
Advantages: less food needed, lower body energy expenditures
Disadvantages: optimal temp range limited, need to cope w fluctuating body temps
Consequences: limited body complexity, limited environmental range may be periods of inactivity
Advantages, Disadvantages, Consequences
Endotherm
Advantages: always an optimal temp for body biochemistry
Disadvantages: need constant supply of high-quality food; complex homeostasis to cool/warm body
Consequences: endothermic animals generate excess metabolic heat to thermoregulate
Endothermy
Birds, mammals & maybe some reptiles
• Behavior, homeostasis & anatomical adaptations used to maintain a stable body temp
- Body increases heat loss when core temperature rises above the set point
- Hypothalamus integrates info from thermoreceptors
- Skin controls heat transfer w environment
- Body reduces heat loss when core temperature falls below the set point
- Obtain heat energy primarily from internal reactions (metabolism)
- Maintain body temp over a narrow range
Homeothermy in Mammals & Birds
Mammals maintain deep body temp ~ 37 C; Birds = ~ 39 C (not absolutely constant)
Avg body temp of unstressed animal does not vary much w climate
Daily cycle occurs –1-2 degrees higher during active phases (day) than during resting phases (night)
Thermoregulation has high energy cost
Requires:
• neurons: sense body temp in the skin, scrotum, spinal cord, brain
• control center in brain (mainly hypothalamus)
Fever
Abnormal increase in body temp due to illness
• Effective way for body to fight infection
• Growth of pathogens decline in high temp
1) During infection, cytokines & prostaglandin PGE2 trick hypothalamus that the body is cold (although no change) = body produce heat (shivering & vasoconstriction)
2) When infection is cleared, hypothalamic thermostat is back to normal and start regulating cooling mechanisms (sweating & vasodilation)
Keeping Warm vs Cool
Keeping Warm:
When thermoreceptors signal fall in core temp below set point, hypothalamus triggers ANS =constriction of arterioles in skin (vasoconstriction) to reduce blood flow to skin and reduce heat conductance
Decrease blood flow to digestive system, shivering, stimulates thyroid to increase metabolic heat production
Keeping Cool:
When core temp rises above set point, hypothalamus sends signals thru ANS that trigger smooth muscles of arterioles in the skin to relax (vasodilation), increasing blood flow and heat lost from the body surface
Sweating, thyroid decrease stimulation of metabolic heat production
Human Thermoregulation
People burn external fuels (gas, coal) to keep warm in cold environments (exogenous)
Also keep warm by means of metabolic heat production & shivering (endogenous)
When naked, ppl exhibit typical mammalian relations b/w metabolic rate and air temp
When we add clothing, we change the slope of metabolism (temp)
• More insulating clothing=higher clo=lower cost of endogenous thermoregulation
Heat Production vs Heat Loss
Heat Production –by-product of metabolism
Basal metabolic rate (BMR) of all cells of the body
Effect of muscular activity on metabolic rate
Effect of endocrinology on metabolic rate (i.e., thyroxin, growth hormone, testosterone)
Effect of ANS on metabolic rate
Sympathetic=vasoconstriction & heat production
Parasympathetic=shivering
Heat Loss: how fast is heat transferred from deep tissues to skin OR from skin to surrounding environment
Study
significant increase in BMR associated with pharmacological testosterone treatment (mostly explained by increase in LBM)
In men w muscular dystrophy, small hypermetabolic effect of testosterone beyond that explained by increased LBM
