9- thermoregulation (temperature homeostasis)

What is another name for thermoregulation?

Temperature homeostasis

What species/ organisms are endotherms?

Mammals and birds

Why does the body temperature need to be kept regulated?

To allow enzymes to keeping working close to their optimum temperature and to prevent them from denaturing.

What are the two ways of thermoregulation and what type of animal does each?

Mammals and birds can generate their own heat so are called endotherms.

All other animals rely on gaining heat from their surroundings, so are called ectotherms.

Give some facts about endotherms and temperature regulation.

• Sometimes called warm-blooded animals (but this isn’t scientific as endotherms can get quite cold e.g. during hibernation).

• They don’t keep their whole body at the same temperature

• They maintain a constant core temperature, whilst allowing the peripheral temperature to be colder.

How many thermoregulatory centres are their in humans and where are they found?

Two, found in the hypothalamus.

Where do the thermoregulatory centres receive input from?

Thermoreceptors;

• receptors in the hypothalamus itself monitor the temperature of the blood as it passes through the brain (the core temperature)

• receptors in the skin monitor the external temperature

What are the two thermoregulatory centres?

• Heat loss centre- is activated when the core temperature rises. It sends impulses to several different effectors in the body to reduce the core temperature.

• Heat gain centre- is activated when the core temperature falls. It sends impulses to several different effectors in the body to increase the core temperature.

What are the different effectors responses to high and low temperatures?

Smooth muscles in peripheral arterioles in the skin:

Low- vasoconstrict so less heat is carried from the core to the surface of the body.

High- vasodilate, so more heat is carried from the core to the surface.

Sweat glands:

Low- no sweat produced.

High- sweat secreted onto surface of the skin, where it evaporates (endothermic process, takes heat from the body).

Skeletal muscles:

Low- shivering, muscles contract and relax repeatedly and involuntarily, generating heat from friction and metabolic reactions.

High- no shivering.

Erector pili muscles in skin (attached to skin hairs):

Low- muscles contract, raising skin hairs and trapping an insulating layer of still, warm air next to the skin.

High- muscles relax, lowering the skin hairs and allowing air to circulate over the skin, encouraging convection and evaporation.

Brown fat tissue:

Low- non-shivering thermogenesis. Increased respiration in brown fat tissue, producing heat but no ATP (especially important in babies and hibernating animals).

High- decreased respiration in brown fat tissue.

Adrenal and thyroid glands:

Low- glands secrete adrenaline and thyroxine, increasing metabolic rate in different tissues, generating heat.

High- glands stop releasing adrenaline and thyroxine, so metabolic rate slows.

Behaviour:

Low- curling up, huddling, finding shelter, layering with clothes etc.

High- basking, stretching out, finding shade, swimming, removing clothes etc.

What can the responses to high or low temperature also do?

• Some responses to low temperature actively generate heat (thermogenesis), while others just conserve heat.

• Some responses to warm temperatures actively cool down the body, while others just reduce heat production or transfer heat to the surface.

What are some examples of when mammals alter their set point on purpose?

Fever- white blood cells release chemicals called pyrogens as part of the inflammatory response to infection. These pyrogens raise the set point of the thermoregulatory centre causing the whole body temperature to increase by 2-3 degrees Celsius. This helps to kill bacteria and explains why patients shiver even though they are hot.

Hibernation- some mammals release hormones that reduce their set point to around 5 degrees Celsius while they hibernate. This drastically reduces their metabolic rate, which helps conserve their food reserves.

Torpor- bats and hummingbirds reduce their set point every day while they are inactive. They have a high surface area: volume ratio, so this reduces heat loss.

What occurs when temperature homeostasis fails, causing hypothermia?

When heat loss exceeds heat generation, due to prolonged exposure to cold temperatures. As the core temperature decreases the metabolic rate also decreases, leading to less thermogenesis. If the core temperature drops below 32 degrees Celsius shivering stops so the core temperature drops even further. If the core temperature falls below 30 degrees Celsius it is usually fatal.

This is an example of positive feedback.

What occurs when temperature homeostasis fails, causing hyperthermia?

When heat gain exceeds heat loss, usually due to prolonged exposure to high temperatures. This situation is often associated with dehydration, which reduces sweating, the only effective way to cool down. A rise in core temperature increases the metabolic rate, fuelling a further increase in temperature. If the core temperature rises above 40 degrees Celsius hyperthermia is usually fatal.

This is an example of positive feedback.

What is an advantage of being endotherms?

Animals can survive in a wide range of environmental temperatures, and so can colonise almost any habitat, and remain active at night and in cold weather. This gives endothermic predators an obvious advantage over ectothermic prey.

However, this requires a lot of energy, so endotherms need to eat far more than ectotherms.

What do ectotherms rely on to maintain their body temperature?

External heat sources.

What are examples of animals which are ectotherms?

Reptiles, such as lizards, iguanas and crocodiles.

List some facts about ectotherms.

• They do not have thermal insulation.

• Their body temperature varies with the environmental temperature.

• They are sometimes called cold-blooded (but this isn’t scientific as they can get very warm).

• They cannot warm up by shivering because if their temperature is low, they cannot respire fast enough to make ATP for rapid shivering because, if their temperature is low, they cannot respire fast enough to make ATP for rapid muscle contraction.

How do ectotherms regulate their body temperature?

Thermoregulatory behaviours

What thermoregulatory behaviours do lizards do to maintain their body temperature?

• Lizards lie down on warm ground to gain heat, and raise themselves off the ground if they get too hot.

• Sun lizards prevent overheating by taking shelter under rocks or vegetation.

• Some lizards can adjust the amount of heat they gain by changing their angle to the sun. Turning their backs to the sun presents the maximum surface area, while pointing towards the sun presents the minimum surface area.

• At night, lizards shelter in burrows, which provide insulation to reduce heat loss (and hide them from predators).

What thermoregulatory behaviours do crocodiles do to maintain their body temperature?

They move between the land and the water during the day to maintain a constant temperature.

What thermoregulatory behaviours do iguanas do to maintain their body temperature?

Iguanas, bask on rocks in the sun every morning until their metabolic rate is fast enough for them to become active.

What is the advantage(s) of being an ectothermic animal?

They use less energy.

At rest reptile’s metabolic rate is only 10% of that of a mammal of a similar size, and at night the their temperature drops with the temperature of the surroundings, their metabolic rate drops further.

So they need to eat far less, and can often survive for weeks without eating.

However, at certain times of the day, they can only move slowly and have reactions, which makes them easy prey and poor predators.