Thermoregulation *Extended Version*

Why does temperature matter?

—

#1

Temperature influences metabolic reactions of all cells— when body temperatures fall outside of their optimal ranges, physiological functions are compromised

Extreme temperature results in #2

Decreased rates of enzymatic reactions

#2a

Altered fluidity of lipids in cellular membranes

#2b

Denaturing of nucleic acids

Heat Transfer

—

#1

Animals produce heat as a by-product of metabolism

#2

Heat can either be lost or gained from the environment

Body Heat = #3

Heat produced metabolically +/- Heat gained/lost from the environment • Heat will always transfer from a higher temperature system to a lower temperature system

Heat Transfer Mechanisms:

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Radiation #1

Emission of electromagnetic waves that objects produce; transfers heat between object that are not in direct contact with each other

Evaporation #2

change of state of water from liquid to gas; breaking of hydrogen bonds requires energy, which comes from body heat, which results in heat loss (sweating/panting)

Conduction #3 (can be aided by convection)

direct transfer of heat from one surface to another (ex. absorbing heat from a warm rock)

Convection #4

heat transfer as air or water moves over a warm or cool surface (wind, moving water)

What is thermoregulation:

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#1

Thermoregulation allows an organism to maintain a relatively constant body temperature (within its optimal physiological range) despite fluctuations in temperature of the external environment

#2

Animals have adapted to temperature fluctuations in one or more of the following ways:

#2a

Occupy a place in the environment where the temperature remains constant and/or compatible with their physiological processes

#2b

Generate and trap heat internally to maintain a relatively stable body temp, despite fluctuations in the temp of the external environment

#2c

Anatomical or physiological adaptations to a specific environment

#3

Thermoregulation by animals is classified according to a) ability to maintain body temperature (heterotherm vs. homeotherm) and b) source of body heat (ectotherm vs. endotherm).

Body Temperature Maintenance: Heterotherms vs. Homeotherms:

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Heterotherms #1

have a body temperature that may vary with the environmental temperature

#1a

Thermoregulation generally relies on behavioral (versus physiological) mechanisms

Homeotherms #2

maintain a relatively constant body temperature despite fluctuations in environmental temperatures

#2a

Variable means of heat production and retention (physiological & behavioral)

Source of Body Heat: Ectotherms vs. Endotherms:

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Ectotherms #1

have a relatively low metabolic rate; amount of heat generated is negligible

#1a

High thermal conductivity; lack insulation

#1b

Many use specific behaviors to warm or cool their bodies

#1c

More common in temperate and tropical regions

Common in which animals? #1d

Invertebrates, fishes, amphibians, reptiles

Endotherms #2

Use metabolism to generate body heat

#2a

Lower thermal conductivity (fur, feathers, and/or fat insulating mechanisms)

#2b

Usually maintain a relatively stable body temperature

#2c

Allows some animals to inhabit regions with extreme temperatures

Common in which types of animals? #2d

Birds and mammals

Temperature Related Inactivity:

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Hibernation #1:

long-term decrease in metabolism and lowered body temperature in order to conserve energy; may last for weeks or months (bats, woodchucks, chipmunks and ground squirrels)

Torpor #2

short-term (often daily) drop in activity in which metabolism and body temp lower; allows animals to survive brief periods when not feeding (bats, hummingbirds, some other small birds and mammals)

Brumation #3

period of dormancy/inactivity during colder months; burrowing, huddling etc. (ectotherm version of hibernation; amphibians and reptiles)

Aestivation #4

period of inactivity in some animals that must withstand extended period of heat and drying; usually involves burrowing (many invertebrates, reptiles and amphibians)

Temperature Regulation in Fishes:

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#1

The temperature of the surrounding water determines the body temperature of most fishes

#2

Fishes that live in very cold temperatures have “antifreeze” proteins in their blood that lower the freezing point of blood plasma and other body fluids

#3

Some active fish (ex. bluefin tuna, great white shark) maintain a core temperature significantly above the temperature of the water by using a counter current heat exchanger

#4

Heat is transferred in rete mirabile (“wonderful net”); less is lost to the surroundings

Temperature Regulation in Amphibians:

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#1

Heterothermic ectotherms

#2

Most have difficulty controlling body heat because they produce little of it metabolically and rapidly lose most of it from their body surfaces

#3

Amphibians have an additional thermoregulatory problem because they must exchange oxygen and carbon dioxide across their skin surface, and this moisture layer acts as a natural evaporative cooling system

Behavioral adaptations #4

enable them to maintain a body temperature within a homeostatic range most of the time

#5

This problem of heat loss through evaporation limits the habitats & activities of amphibians to warm, moist areas

Temperature Regulation in Reptiles:

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#1

Mainly Heterothermic ectotherms

#2

Reptiles have dry, rather than moist skin, which reduces the loss of body heat through evaporative cooling of the skin

#3

Low metabolic rate; warm themselves by behavioral adaptations

#4

Many reptiles can maintain fairly constant body temperatures by changing position and location throughout the day

#5

These animals can also increase heat production in response to hormones— thyroxine and epinephrine

Heat production in Birds & Mammals:

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Production of heat #1

Thermogenesis

#2

Birds and mammals generate heat through:

#2a

Metabolic processes (making fuel)

Voluntary and Involuntary #2b

Shivering thermogenesis

#2c

Non-shivering thermogenesis (nervous/endocrine)

#2ca

Metabolic process associated primarily with brown adipose tissue (can also occur in muscle)

#2cb

Some mammals (bats, neonates) rely on increased metabolism in brown adipose tissue (in place of shivering thermogenesis)

Endocrine Negative Feedback & Thermoregulation:

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#1

Specialized neuronal receptors in the skin and other parts of the body sense temperature changes

#2

Warm neuronal receptors excite the cooling center and inhibit the heating center, cold neuronal receptors have the opposite effect

Negative feedback mechanisms #3

Control body temperature

Nervous & Endocrine Function in Thermoregulation:

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#1

Stimulation of the heating center results in:

#1 #1a

Vasoconstriction of superficial blood vessels

#2 #1b

Erection of hair and fur

#3 #1c

Shivering and non-shivering thermogenesis

#4 #1d

Increased release of thyroid hormones

#2

Stimulation of the cooling center results in:

#1 #2a

Vasodilation of superficial blood vessels

#2 #2b

Sweating

#3 #2c

Panting

#4 #2d

Decreased release of thyroid hormones

Endocrine Negative Feedback Loop

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#1

Low temp activates heating response:

#1a

Hypothalamus releases thyrotropin releasing hormone (TRH)

#1b

Ant. pituitary releases thyroid stimulating hormone (TSH)

#1c

Thyroid releases thyroxine (T3 & T4)

#1d

Metabolic activity increases body temp increases

#2

High levels of T3 & T4  neg feedback

#2a

Hypothalamus/ant. pituitary stop releasing TRH/TSH

#2b

Thyroid stops releasing thyroxine

#2c

Metabolic activity/body temp decrease