1. Thermoregulation

2. Why Regulate Heat
With the exception of some prokaryotes, life can only exist between temperatures of about -2ºC and about 50ºC.
There are several reasons for this:
Enzymes are temperature sensitive
Many membrane proteins can only function properly if they float freely in the lipid bilayer, which is only possible if it is liquid; at low temperature the bilayer freezes.

3. Why Regulate Heat
At around 85ºC, the hydrogen bonds holding the two strands of DNA together break, causing it to become single stranded.

4. Thermoregulation This is the regulation of body temperature.
Homeothermy is the ability to regulate body temperature.
Includes birds and mammals
Remember poikilothermic and homeothermic.
Homeothermic is hard to define as there are animals that can partially regulate their temperature by their behaviour.

5. Thermoregulation
The most useful distinction is based on how temperature is regulated.
Ectotherms – regulate their body temperature by their behaviour; they can only maintain their body temperatures above the ambient temperature by absorbing radiant heat.
Ecto means outside.

6. Thermoregulation
Endotherms keep warm using heat generated inside the body; they regulate their heat loss by physiological mechanisms in the skin.
Endo means inside.

7. Body Temperature in Endotherms
Most mammals have body temperatures between 37-39ºC
Birds have slightly higher temperatures between 40-42ºC
Our body temperatures varies slightly for a number of reasons.
In healthy humans the average is about 35.8ºC in the early morning and about 37.3ºC in the evening. (for precise comparisons temp should be taken at the same time each day)

8. Body Temperature in Endotherms
Temperature varies with level of activity, and may rise to 40ºC in vigorous exercise.
Women in the second phase of their menstrual cycle have temperatures about 0.3ºC higher than the first phase.
Body temperature varies from person to person suggesting that there may be genetic factors involved.

9. Body Temperature in Endotherms
Many endotherms hibernate in winter, body temperature falling to a degree or two above the ambient temperature.
Temperature also varies from one part of the body to another
Though the core temperature in deeper parts of the body does not fluctuate much , in the outer shell (especially the limbs) it varies considerably.
Different organs have slight differences, reflecting variation in heat production.

10. How We Lose and Gain Heat
Heat can be gained or lost from any place where the body is in contact with the environment (skin and lungs)
Heat can be gained or lost by:
Conduction
Convection
Radiation
Evaporation

11. Conduction from ground Evaporation from lungs
Evaporation from Skin
Conduction to ground
Conduction from ground
Evaporation from lungs
Radiation
Direct and reflected solar radiation
Conduction and Convection

12. Conduction
When your body is in direct contact with a cooler object you lose heat by conduction. Heat is also lost by conduction through the gut wall and the lungs.
The rate at which heat is lost by conduction is affected by 2 factors:
The thermal conductivity of the material next to the skin. A naked person is quite comfortable in air at 20ºC, but quickly feels cold in water at the same temperature. Water conducts heat faster than air.

13. Conduction
The temperature gradient, the steeper the thermal gradient, the faster the conduction.

14. Convection
This is the transfer of heat by currents in gases or liquids.
This is why cold air feels much colder when there is a wind blowing.
i.e. wind chill and how a fan makes you feel cooler.

15. 40°F = 4.4°C
10°F = -12.2°C
Wind Chill

16. Radiation
Absorption of heat from the sun or a heat source such as a fire.

17. Evaporation
The change from liquid to gas absorbs latent heat, which produces a cooling effect.
To keep cooler than the surroundings animals have to lose water – sweating.
In very hot weather a person may produce as much as 1.5L of sweat per hour.
Water is also lost from the respiratory system.

18. The Skin This is the largest organ in the human body.
The skin plays a vital role in thermoregulation in two ways:
It contains receptors that detect changes in the environmental temperature
It contains effectors that can vary the rate of heat loss from the body.

19. The Skin

20. The Skin Consists of two distinct layers: An outer epidermis
Has to withstand wear and tear.
The innermost cells are in constant mitotic division and form the Malpighian layer
As they differentiate the epidermal cells produce large amounts of the tough, fibrous protein, Keratin becoming strongly bonded together.
Cells near the surface make up the dead cornified layer.

21. The Skin An inner dermis
This is much thicker than the epidermis and contains a network of fibrous proteins, collagen and elastin.
It contains several structures that are important in thermoregulation.

22. The Inner Dermis Hair Follicles These are pits from which hairs grow.
The angle of the hair can be adjusted by contraction of a small erector muscle attached to each follicle.
In most mammals this allows the thickness of the fur to be increased.
In humans this thermoregulatory role has been lost, though the muscles still contract to produce “goose pimples” when the body is cold.

23. The Inner Dermis Sweat Glands
Deep in the dermis these are supplied by sympathetic nerve fibres.
In humans most sweat glands are eccrine glands and secrete a dilute salt solution which cools the skin when it evaporates.
In most mammals most sweat glands open into hair follicles and secrete pheromones and play no part in thermoregulation.

24. The inner Dermis Blood Vessels
Besides nourishing the skin, these bring heat to the body surface.
Stimulation of the smooth muscle in the arterioles causes them to constrict and thus reduce the delivery of heat to the skin.

25. Inner Dermis Afferent nerve endings
Some of these are sensitive to changes in temperature
Beneath the dermis is a layer of adipose tissue which contains fat that acts as an energy store and a thermal insulator.

26. Heat Production
Most energy taken into the body is eventually lost as heat produced in metabolism, this is the main source of heat in endotherms.
Respiration or ATP production occurs in the mitochondria.
There are three main steps.

27. Step 1
A series of enzymatic reactions known as the TCA Cycle or Kreb’s Cycle where carbon from glucose and lipids is converted into CO2, while electrons are transferred to the inner membrane of the mitochondria.
This CO2 is exhaled.

28. Step 1

29. Step 2
In the inner mitochondrial membrane a series of oxidation/reduction reactions take place (electron transport or oxidative phosphorylation).
Electrons are transferred through a series of specialized molecules in the membrane.
This results in the movement of H+ ions to the space between the inner and outer membranes – these will build up unless a channel opens up allowing them back across the membrane.

30. Step 2 Meanwhile the electrons are transferred to O2 to make H2O.
Importantly this chemical reaction releases heat and is a major site of heat production in the body.

31. Step 2

32. Step 3
A by-product of this process is that it creates an ion gradient across the membrane.
This difference in H+ ions drives a protein ATP Synthase that is embedded in the membrane, which adds phosphate to ADP to make ATP.
In the process H+ ions are transported back across the membrane so the electron transport in step 2 can only keep going if ATP synthase is active.

33. Step 3

34. Exercise and Heat Production
When a person exercises, muscles have increased demand for ATP.
Feedback signals are sent to increase the rate of oxidative phosphorylation. This means there is greater demand for O2 and increased production of CO2.
As a by-product of this heat is produced.
This is why you get hot when you exercise.

35. Shivering
This is a feedback mechanism that occurs when a mammal is faced with cold temperatures.
Shivering is a result of involuntary muscle contractions, causing a rise in oxidative phosphorylation and thus heat production.
This is known as shivering-induced thermogenesis.

36. Non-Shivering Thermogenesis
Babies are not able to shiver.
They use non-shivering thermogenesis to keep warm.
This happens in a special tissue called brown fat.
In brown fat, there are a large number of mitochondria that express a special protein called uncoupling protein (UCP).

37. Non-Shivering Thermogenesis
UCP is a channel that allows H+ ions to travel back across the membrane without having to produce ATP.
This means that the transfer of electrons to oxygen and the pumping of H+ ions outwards can happen without the need for ATP.
This means that heat production can happen without making extra ATP.

38. Non-Shivering Thermogenesis
Babies have a significant amount of brown fat, and they use it until they develop the ability to shiver.
Other animals, e.g. bears, rely on brown fat when they hibernate, as it allows them to produce heat without having to exercise.

39. Non-Shivering Thermogenesis

40. Feedback Mechanisms to Restrict Heat Loss
Skin is an area where animals can lose heat to the environment.
Blood is a major way to transfer heat around the body.
Animals can rapidly reduce blood flow to the skin, reducing heat transfer and thus reducing heat loss.
This is why your fingers and toes go blue when they are very cold.

41. Feedback Mechanisms to Restrict Heat Loss
In a cold environment, small muscles in the skin are activated that make the hair stand upright.
This is called piloerection (goosebumps).
This blocks the flow of air around the skin slowing heat loss.

42. How does the Body Cool Down when it is too Hot?
Sensors in the skin detect temperature changes, send signals to the brain and induce responses that lower temperature.
Blood Flow to the skin can be increased when body temperature is high, so increasing the rate of heat loss to the atmosphere by radiation.
e.g. large ears in elephants.

43. How does the Body Cool Down when it is too Hot?
Sweating occurs mostly in humans and other primates.
Sweating occurs as a feedback mechanism in response to a rise in external temperature or due to heat produced during exercise.
Sweating occurs when signals from the brain activate sweat glands throughout the skin.
The sweat is a saline solution.
It cools the body as it evaporates from the skin.

44. How does the Body Cool Down when it is too Hot?
Many animals use panting as a means of losing heat.
Panting transfers heat to the water in the lining of the airways leading to the lungs.
These are normally kept moist so by panting the animal is able to induce more water evaporation and thus more cooling.

45. How is it Controlled?
The major control centre for regulating body temperature is the hypothalamus.
This has its own sensors, which can sense very small changes in temperature and send out signals to regulate cooling mechanisms.
This is called the central regulatory mechanism as it is sensing temperature at the core of the body.

46. The Hypothalamus

47. How is it Controlled?
It takes time for core temperatures to be affected by the environment so the body also has temperature sensors in the skin that tell the brain what is happening to the environmental temperature, enabling the body to react more quickly.
These sensors are a special type of neuron found only in the skin. They contain a series of protein channels called TRPV channels.

48. How is it Controlled?
These TRPV channels are found in the plasma membrane of these nerve cells, and their structure is highly tuned to a particular temperature.
It the temperature changes, the structure of the TRPV proteins changes., then their function changes.
When TRPV proteins are at their optimum temperature, they make a channel through the membrane.

49. How is it Controlled?
This allows a number of ions to cross the membrane changing the electrical charge across the membrane.
This triggers an electrical pulse that moves along the nerve cell to the next nerve cell until it reaches the hypothalamus, where it adds to information provided by the central regulatory sensors.

51. Fever This is associated with infection.
The immune system produces fever-inducing chemicals called pyrogens, which travel through the blood to the hypothalamus, where they trick it into raising the body temperature.
Aspirin and paracetamol lower temperature by blocking the production of pyrogens.

52. Frostbite
The blood vessels in the skin constrict in very cold conditions to preserve heat.
If this is prolonged it starves the cells peripheral tissue of nutrient and heat. When nerve cells stop working no signals get sent to the brain, the fingers and toes go numb.
If this happens for a short time it can be reversed. If it happens for longer the cells die i.e. frostbite

53. Frostbite

54. Why do Chillis Burn?
Capsaicin, a chemical found in chillis binds directly to the TRPV channels in the lining of the mouth.
This activates these receptors in the same way as if the food were genuinely too hot, so the brain generates the same type of signal.