09 - Thermoregulation

four major types of heat exchange between an animal and it’s environment

Conduction

Convection

Radiation

Evaporative heat loss

Conduction

which is the transfer of heat from one object to another object when the objects are in direct contact.

Convection

in which there is the transfer of heat from an animal into a moving stream of either air or water

Radiation

which is the transfer of energy (heat) between objects that are not in physical contact.

Evaporative heat loss

in which the evaporation of water either across the skin or across the respiratory

system causes the loss of heat.

Poikilotherms:

Body temperature fluctuates with environmental temperature.

Homeotherms:

Maintain a constant body temperature.

Ectotherms:

Gain heat from the environment.

Endotherms:

Generate heat metabolically (internally)

typical pairings of Thermal Regulation

poikilothermy and ectothermy

homeothermy and endothermy

poikilothermy and ectothermy

crabs, whose body heat derives from and reflects environment

homeothermy and endothermy

humans, using metabolic heat to manitain constant body temp

homeothermy and poikilothermy

benthic fish in thermally stable env

ectothermy and endothermy

kangaroo rat whose body heat derieves from both metabolism and environemnt

terrestrial ectotherms

can use behavioural means to keep their body temperature slightly different from

the temperature of the ambient air,

aquatic ectotherms

always take on the temperature of the water that

they are in (except some large, active fish like sharks and tuna)

Ectotherms and Global Warming Implications

• Example: Bork (Pagothenia borchgrevinki), an Antarctic ice-fish

  • Lives at ~2°C, but a slight temperature increase could be fatal.

  • Reason: The enzyme acetylcholinesterase only functions at 1-2°C.

  • If temperature rises to 4°C, the enzyme stops working, leading to failure of neurotransmission.

Behavioral Thermoregulation in ectotherms

Although ectotherms cannot produce their own heat via metabolism, they are not completely at the mercy of the environmental temperature. Some animals use behavioural means to adjust their body temperature.

Behavioral Thermoregulation in ectotherms - example

Example: Lizards

• Night: Stay in burrows (cuz outside is colder) → Body temperature = burrow temperature.

• Day: Move into sunlight → Warm up.

• Regulation: Alternating between shade and sun to control body temperature.

When exposed to very cold temperatures, ectothermic animals can

freeze (and tolerate the freezing) or they

can employee strategies to avoid freezing.

Freeze Tolerance in Ectotherms

Freeze tolerant animals actually let their extracellular fluid freeze. However, they do not let their intracellular fluid freeze. This way cells avoid getting damaged by ice crystals.

Freeze Avoidance in Ectotherm

One strategy to avoid freezing is to produce antifreeze compounds.

These are compounds that lower the freezing point of body fluids.

Types:

• Sugars & alcohols: Increase solute concentration → Lower freezing point.

• Special antifreeze proteins: Bind to ice crystals and prevent further growth.

Vertebrate Thermostat

The hypothalamus in the brain functions as the body’s thermostat.

It receives information from temperature sensors (receptors) in the skin and within the core and then triggers physiological adjustments (via various effector systems) in order to induce heat loss or heat gain, depending upon what is required.

Vertebrate Thermostat experiment key findings

Cooling the hypothalamus → Brain thinks the body is cold → Increases metabolic rate → Body warms up.

Warming the hypothalamus → Brain thinks the body is hot → Decreases metabolic rate → Body cools down.

set-point

the body temperature that the hypothalamus works to maintain.

what happens when body temp increaes form set point

this is detected by temperature receptors in the core, the skin as well as by temperature-sensitive neurons in the hypothalamus

This activates the “heat-loss centre” within the hypothalamus and triggers mechanisms that cool the body.

neg feedback

mechanisms to cool down body

sweating

by increasing blood flow to the skin in order for heat to be lost across the skin surface.

what happens when body temp decreaes form set point

then the heat-promoting centre in the hypothalamus is activated. This triggers mechanisms that either produce heat or help the body retain heat.

neg feedback

mechanisms to heat up body

blood flow to the skin is reduced in order to lessen the amount of heat that is lost across the skin.

Shivering will occur in skeletal muscles and, in some cases non-shivering thermogenesis (heat production) can occur.

thermoneutral zone

There is a range of ambient temperatures called the thermoneutral zone in which metabolic rate is constant.

what happen When ambient (environmental) temperature goes below a lower-critical temperature or above an upper- critical temperature

metabolic rate increases

what is the cost of being homeothermic/ endothermic

The increase in MR is the cost of

being homeothermic / endothermic. It takes energy (ATP-driven processes) to keep body temperature

constant once you are out of the TNZ. Energy (increased MR) is required to both warm the animal when it is

below the TNZ and cool the animal when it is above the TNZ

heat exchange - within or bwlow tnz

via conduction, convection andradiation

heat exchange - above tnz

evaporation

temp in the tnz zone

The range of temperatures that comprise the thermoneutral zone depends upon the species.

In general, cold- acclimated species have a larger thermoneutral zone and a reduced lower-critical temperature compared to warm-acclimated species.

homeothermic / endothermic animals defenses against both heat and cold

Behaviour Defenses

animals can take shelter in the shade as a behavioural means to defend against the heat whist huddling together as a defense against the cold.

Insulatory Defenses

Animals can use insulation to either keep heat out or keep heat in. A camel uses its fur as an insulator to keep heat out while beavers live in insulated lodges as a means to keep warm during the winter.

Changes in Cutaneous Blood Flow

vasodilation

vasoconstriction

vasodilation

Blood flow to the skin can be increased by dilating blood vessels in the skin

cools animal down

vasoconstriction

Blood flow to the skin can be decreased by constricting

the blood vessels that flow to the skin

reduces tranfer of heat

Defenses against the Heat - homeothermic / endothermic

Temperature Cycling

Brain Cooling

Temperature Cycling

Camels

Daytime: Body temperature rises instead of using evaporative cooling (which would cause water loss).

Nighttime: Heat is lost through conduction, convection, and radiation when the environment is cooler.

Squirrels

Body temperature increases in the hot sun.

The squirrel retreats to a burrow to cool down before going back out again.

This cycling pattern helps it avoid overheating.

Brain Cooling

brain = most sensitive, thus has specilized cooling methods

Cool venous blood from the nasal region (cooled by air moving in/out of nostrils) pools in a sinus.

The carotid artery, which carries warm arterial blood to the brain, branches into tiny capillaries (carotid rete mirabile) within this sinus.

Heat exchange occurs: The cool venous blood lowers the temperature of the arterial blood before it reaches the brain.

This protects the brain from overheating while the rest of the body endures high temperatures.

Defenses against the cold - homeothermic / endothermic

Shivering Thermogenesis

Shivering Thermogenesis

Shivering produces heat. Shivering is the uncoordinated contraction of motor units such that motor units are contracting randomly.

This contraction produces heat and is called shivering thermogenesis.

Given that the muscle contraction is random, there isn’t actually any mechanical work being done by the muscle.

Non-Shivering Thermogenesis

When the hypothalamus detects cold temperatures it causes an activation of sympathetic nerves that cause the adrenal gland to release noradrenaline (and adrenaline). These hormones then activate a heat production mechanism in the brown fat.

Brown fat

Brown fat (or brown adipose tissue) has a unique mechanism to produce heat.

Brown fat is present in newborn (neonatal) animals and is also found in animals that hibernate.

The brown appearance comes from the many mitochondria present in the fat cells

Heterothermy

refers to a homeothermic animal (one with a constant body temperature) altering the level at which it holds its body temperature constant.

It is a controlled and deliberate reduction in body temperature.

examples of Heterothermy

Hibernation and torpor are examples.

Torpor and hibernation - simmilarity and differences

they rely on the same mechanisms to reduce metabolic rate

and body temperature. The differences are in the duration as well as the extent of the reduction of MR / body

temperature.

Torpor

occurs in small mammals and birds. It usually lasts less than 24 hours, metabolic rate doesn’t

decrease much more than 70% and body temperature usually remains above 12 °C

Hibernation

decrease in body temperature and metabolic rate is greater during hibernation than during torpor and the “bouts” of hibernation last longer than the bouts of torpor (which tend to occur daily).

How Hibernation Works:

• Animals hibernate for months during cold seasons.

• Instead of staying in a deep sleep the entire time, they go through bouts of hibernation.

bouts of hibernation.

• Step 1: The animal lowers body temperature & metabolic rate for 10-20 days.

• Step 2: It briefly increases temperature & metabolic rate for a day or two.

• Step 3: The cycle repeats throughout the hibernation period.

The Energy Savings during Hibernation

huge savings

Why is hibernation important?

If an animal didn’t hibernate, it would need to burn a massive amount of energy to survive winter.

Hibernation conserves energy, allowing survival with minimal food intake.