Thermoregulation

Temperature

Temperature is independent of amount of matter - Speed or intensity of random molecular movement.

Heat

Heat is directly proportional to amount of matter - Dependent on both speed of random molecular movement AND number of molecules.

Heat transfer and temperature

Transfer of heat to or from an object causes a change in temperature.

TA

Ambient or environmental temperature.

TB

Core body temperature (considered to be uniform).

TS

Temperature of body surface.

Conduction

Heat flows down a temperature gradient - Agitated molecules collide with other molecules, agitating them - Requires physical contact of molecules - H = (T1 - T2)/d × k.

Convection [

Heat flows down a temperature gradient - Macroscopic movement of molecules in water or air - Requires fluid flow (water or air) - H = (Ts - TA) hc.

Evaporation

Water must absorb heat to change from liquid to gas - Latent heat of evaporation required for water to change state - Heat remains with water vapor when it leaves the body - Sweating, panting, gular fluttering.

Radiation

Heat transfer does not require physical contact - All objects emit electromagnetic radiation - Higher temperatures → shorter wavelengths → greater energy - Major mechanism for heat exchange.

Ectotherm

Environment is the major source of body heat - Examples: most fish, amphibians, reptiles.

Endotherm

Internal generation is the major source of body heat - Tissues warmed by metabolic heat.

Homeotherm

Maintain relatively constant temperature by physiological mechanisms - Most birds and mammals.

Poikilotherm

Unregulated body temperature - Examples: most fish, amphibians, reptiles.

Advantages of homeothermy

Cellular functions specialized to take place especially reliably at certain temperatures.

Disadvantages of homeothermy

Require more food resources from environment.

Brown fat

Lipolysis: expression of uncoupling protein 1 (UCP1) results in oxidative phosphorylation producing heat instead of ATP - Select placental mammals - Newborns and cold-acclimatized adults - Especially small to moderate body size - Hibernators.

Shivering

Skeletal muscle contraction requires synthesis and hydrolysis of ATP, both yielding heat - Endotherms: birds and mammals.

Performance curve (ectotherms)

Laboratory-generated - Critical temperatures: beyond these, performance stops - Pejus temperatures: decreased performance with increased temperature - Components of performance are stand-ins for fitness.

Behavioral thermoregulation

Prominent in ectotherms - Regulating sun exposure, body surface exposed, layer of water, body surface contact with substrate.

Preferred body temperature

Ambient temperature resulting in greatest metabolic efficiency - Correlates with testicular development, sprint speed, hearing, digestion efficiency, immune responses.

Temporal heterothermy

Different thermal relations at different times - Includes hibernation, torpor, estivation, high amplitude cycling.

Hibernation

Controlled hypothermia - Biochemical downregulation for 2+ consecutive days during winter - Reduced body temperature and metabolic rate - Preceded by increased adipose stores.

Torpor

Reduced TB and metabolism - Length of time varies from hours to days - Triggered by environmental challenges resulting in calorie deficits.

Estivation

Reduced metabolism and body temperature (TB not as reduced as hibernation/torpor) - Can last days to months - Triggered by elevated temperatures and/or lack of water.

High amplitude cycling

Diurnal variations in core body temperature - Increase during day, store heat until night - Heat lost at night - Example: dromedary camels.

Regional heterothermy

Regional differences in metabolism or ambient temperature result in temperature differences - Maintained by countercurrent exchange.

Countercurrent exchange

Blood flowing in opposite directions through two close vessels - Heat equilibrates between vessels - Conserves heat.

Rete mirabile

Carotid rete in artiodactyla, cats, seals - Venous blood cooled by evaporative loss mingles with arterial blood to brain.

Tuna and sharks

Maintain swimming muscles above TA - Heat generated by muscles confined by countercurrent exchange - Form rete mirabile.

Billfish

Maintain brain and retinas above TA - Heater tissues from eye muscles - Rich in mitochondria - Heat generated by futile calcium cycling.

Supercooling

Avoidance of ice crystals - Extracellular fluid freezes first, ICF remains unfrozen - Seen in teleost fish, hibernating arctic ground squirrel.

Antifreeze compounds

Colligative: polyhydric alcohols (glycerol, sorbitol, mannitol) - Noncolligative: proteins/glycoproteins bind ice crystals to interfere with growth.

Freeze tolerance

Promote freezing of extracellular fluids (50-65%) - Synthesize ice-nucleating agents - Produce cryoprotectants (glucose, urea, glycerol) - Metabolic depression with disappearance of vital signs - Thawing involves antioxidant defenses and protein repair.