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(a) Homeostasis - introduction to temperature control in the human body

Homeostasis is a word that is sometimes used to describe your bodily functions that try to maintain a stable constant internal environment including the factors listed above.

Know that internal conditions that are controlled in the body include temperature control - thermoregulation.

The temperature of your body depends on the rate at which energy is released by respiration and the rate at which your body loses heat energy e.g. conduction through skin and clothing, radiation from your skin and convections as air moves over your skin. Therefore your body must balance the heat gained and the heat lost.

Thermoregulation is the maintaining of a steady body temperature (eg for us ~37.5^oC)

If you get too hot, you need to remove excess heat energy.

If you get too cold you need to retain heat and reduce heat loss.

The temperature is particularly important for enzyme action - most enzymes in the body have an optimum operating temperature of ~37^oC, normal body temperature when things work best, so that's what your biochemistry wants, ~37^oC!

If the temperature is too low, your biochemical reactions slow down - enzymes less efficient.

If the temperature is too high, the protein structure of enzymes can break down (denature) making the ineffective.

Your normal body temperature should be 37^oC +/- 0.5^oC.

Homeostasis maintains our core body temperature at a steady 'normal' ~37^oC by a negative feedback mechanism - described below where any change in body temperature triggers a response to counteract the rise or fall of temperature if it departs from the 'normal'.

The temperature is controlled by the brain to maintain the temperature at which enzymes work best (37^oC).

Somewhere in the complexity of the brain some kind of 'biochemical thermostat' is at work.

This 'thermostat' is sensitive to the blood temperature of the brain and the skin - there are two sets of receptors that aid thermoregulation.

(1) The thermoregulatory centre in the hypothalamus of the brain, contains receptors which are sensitive to the temperature of the blood circulating in the brain.

(2) It also receives nerve signals from receptors in the skin about the external temperature and these nerve endings are involved in a reflex arc action.

These receptors are found in the outer layer of the skin (epidermis) and in the deeper layers of the skin below the epidermis.

When the hypothalamus detects a change in temperature, it causes a response in the skin.

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(b) The negative feedback system for thermoregulation

The homeostasis negative feedback system for temperature control - thermoregulation of the body
(a) The homeostasis negative feedback system for too high a temperature	(b) The homeostasis negative feedback system for too low a temperature
1. Temperature receptors detects a stimulus that indicates the body temperature is too high.	1. The temperature receptors detects a stimulus that indicates the body temperature is too low.
2. The thermoregulatory coordination centre receives information from the temperature sensors and processes the stimulus information and then automatically organises a response by the effectors.	2. The thermoregulatory coordination centre receives information from the temperature censors and processes the stimulus information and then automatically organises a response by the effectors.
3. The effector produces a response from e.g. sweat from glands or blood vessel dilate to counteract the increase in temperature to lower it and restore the correct optimum body temperature ~37oC.	3. The effector produces a response e.g no sweat or hairs stand up to counteract the decrease in temperature and restores the optimum level by increasing the temperature to restore the correct optimum body temperature ~37oC.
4. The effectors will carry on producing the 'temperature reducing' response as long as the coordination centre is stimulated by the temperature receptors i.e. until the correct temperature is reached.	4. The effector will carry on producing the 'temperature increasing' response as long as the coordination centre is stimulated by the temperature receptors i.e. until the correct temperature is reached.
5. The effector response might be more than required, and the temperature becomes too low, if too far below the correct temperature, the receptors will detect this, and the negative feedback will stimulate the effectors to increase the level (1. - 3. on the right)	5. The effector response might be more than required, and the level becomes too high, if too far above the 'ideal' the receptors will detect this and the negative feedback will stimulate the effectors to decrease the level (1. - 3. on the left).
This is all automatically done by the organism's complex control systems and enables the organism e.g. your body, to maintain as near as possible the 'ideal' temperature conditions for healthy life! More details on what the effectors actually do is described below.

A simple graphical representation of the body's 'thermostat' homeostasis mechanism.

Note on antagonistic mechanisms - very smart biology stuff !!!

Some effectors work antagonistically e.g. at the same time two effectors produce opposite effects.

Most factors in your body's internal environment are controlled by several effectors, which often have antagonistic actions.

Control by antagonistic effectors is sometimes described as “push-pull,” where the increasing activity due to one effector is accompanied by decreasing activity due to a 2nd antagonistic effector and both are trying to restore the 'system' to normal.

In this case one effector produces a cooling response if the temperature is too high and another effector produces a heating effect if the temperature is too low.

This antagonistic mechanism gives the body a much more precise and sensitive control of its temperature.

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(c) Details on the 'mechanics' of how the body decreases or increases its temperature

The process begins when the thermoregulatory centre in the brain detects (i) the blood temperature is too high or too low OR (ii) receives nerve impulses from the temperature receptors in the skin.

(1) How you are cooled if you are too hot

(i) Your sweat glands produce sweat (water plus salt) through pores in the epidermis that evaporates from your skin surface.

This is a heat energy absorbing change involving the latent heat of evaporation of water.

So the process of sweating transfers heat from your energy store to the surrounding environment store and cools you down.

(ii) The blood vessels supplying the skin capillaries dilate (become wider) so more blood flows nearer to the skin.

This increases the efficiency of heat transfer from your body to the surroundings - rate of heat loss increased - more blood flow - more energy lost.

This process is called vasodilation and is effected by the muscles of the blood vessels.

In an exam don't say 'it increases blood pressure' or you will cause that in your teacher!

(iii) Your hair erector muscles relax and your hairs lie flat - reduces the thickness of the trapped layer of insulating air between the hairs, allowing more heat to be lost from the body by thermal conduction and convection.

 

(2) How you are warmed up if you are too cold

(i) Your hair erector muscles contract so your body hairs stand up and trap a thicker insulating layer of air between the hairs.

Air is a poor conductor and the effect is no different than mineral fibre insulation in the loft of a house and so keeps you a bit warmer by reducing heat losses by thermal conduction and convection from exposed skin.

(ii) Your sweat glands stop producing sweat.

Heat will no longer be transferred by evaporation of water from your skin.

(iii) The blood vessels supplying the skin capillaries constrict (become narrower) so less blood flows to the skin.

This decreases the efficiency of heat transfer from your body to the surroundings - reduces rate of heat loss - less blood flow - less energy lost.

This process is called vasoconstriction and is effected by the muscles of the blood vessels.

(iv) One last trick of the body is to get you to shiver!

Shivering is when your skeletal muscles automatically contract and this requires energy - so the rate of respiration increases to release more heat energy to warm up your body.

 

(3) What are your body's endurance limits!

I'm afraid there are limits to your body's response to changes in temperature

A very high temperatures makes you feel extremely uncomfortable as your body struggles to cope with the situation and you suffer from 'heat exhaustion' and then heatstroke - which can be fatal.

If the external temperature rises well above 37^oC it becomes difficult for your body to lose heat.

At the other extreme, particularly without adequate clothing, very low temperatures result in great heat loss from your body.

Your rate of respiration might not be enough to maintain the normal temperature of 37^oC - rate of respiration < heat loss from body

This can lead to hypothermia and finally death.

If body respiration can't replace the heat loss, then your body gradually cools and begins to malfunction and eventually ceases to function at all.

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(d) Summary and learning objectives and knowledge for how thermoregulation works

How your body controls its temperature

•  Be able to explain how thermoregulation takes place, with reference to the function of the skin, including:
  • a) the role of the dermis – sweat glands, blood vessels and nerve endings, hair, erector muscles and sebaceous glands
  • b) the role of the hypothalamus – regulating body temperature
• Be able to explain how thermoregulation takes place, with reference to:
  • a) vasoconstriction
  • b) vasodilation
  • c) negative feedback

• The body temperature is controlled by the brain to maintain the temperature at which enzymes work best (~37^oC).

  • Somewhere in the complexity of the brain (the hypothalamus) some kind of 'biochemical thermostat' is at work.

  • This 'thermostat' is sensitive to the blood temperature of the brain and via nerve impulse signals from temperature receptors in the skin.

  • When the hypothalamus receives nerve signals from the skin about its temperature, either its too cold or its too hot, response mechanisms are automatically triggered in the dermis - the deeper layer of the skin, this is an example of a 'negative feedback' mechanism.

    • This automatic temperature change response is an example of negative feedback.

    • If you are too cold, hair erector muscles contract, and your hairs stand upright trapping a layer of insulating air. Your sweating is reduced to a minimum since heat is absorbed and therefore lost in the process of evaporation. In vasoconstriction, the blood vessels near the skin surface constrict so less blood flows and therefore less heat energy is transferred to the cold surroundings.

    • If you are too hot the erector muscles relax allowing the hairs to lie flat on the skin, no longer trapping insulating air. You also begin to sweat which removes heat energy in the process of evaporation. The blood vessels near the skin surface widen (to dilate - process of dilation, vasodilation) which allows more blood to flow and hence transfer more heat to the surroundings.

    • I'm afraid there are limits to your bodies response ...

• A very high temperatures make you feel extremely uncomfortable as your body struggles to cope with the situation and you suffer from 'heat exhaustion' and then heatstroke - which can be fatal.

• At the other extreme, particularly without adequate clothing, very low temperatures resulting in great heat loss the body can lead to hypothermia and finally death. If body respiration can't replace the heat loss, then your body gradually cools, it begins to malfunction and eventually ceases to function at all.

• Know and understand that sweating helps to cool the body.

  • Know that more water is lost when it is hot, and more water has to be taken as drink or in food to balance this loss.

• Know and understand that body temperature is monitored and controlled by the thermoregulatory centre in the brain.

  • Know and understand that this centre has receptors sensitive to the temperature of the blood flowing through the brain.

    • The name of the centre in the brain (hypothalamus) is not required.

• Also know and understand that temperature receptors in the skin send impulses to the thermoregulatory centre, giving information about skin temperature.

• Know and understand that if the core body temperature is too high:

  • blood vessels supplying the skin capillaries dilate so that more blood flows through the capillaries and more heat is lost,

  • sweat glands release more sweat which cools the body as it evaporates.

• Know and understand that if the core body temperature is too low:

  • blood vessels supplying the skin capillaries constrict to reduce the flow of blood through the capillaries,

  • muscles may ‘shiver’ – their contraction needs respiration, which releases some energy to warm the body.

• Thermoregulation and the effect of temperature on enzymes
  • Thermoregulation is the maintaining of a steady body temperature (eg for us ~37.5^oC)
  • If you get too hot, you need to remove excess heat energy.
  • If you get too cold you need to retain heat and reduce heat loss.
  • The temperature is particularly important for enzyme action - most enzymes in the body have an optimum operating temperature of ~37^oC, normal body temperature, so that's what your biochemistry wants, ~37^oC

• The body temperature is controlled by the brain to maintain the temperature at which enzymes work best (~37^oC).

  • Somewhere in the complexity of the brain (the hypothalamus) some kind of 'biochemical thermostat' is at work.

  • This 'thermostat' is sensitive to the blood temperature of the brain and via nerve impulse signals from temperature receptors in the skin.

  • When the hypothalamus receives nerve signals from the skin about its temperature, either its too cold or its too hot, response mechanisms are automatically triggered in the dermis - the deeper layer of the skin, this is an example of a 'negative feedback' mechanism.

  • This automatic temperature change response is an example of negative feedback.

  • If you are too cold, hair erector muscles contract, and your hairs stand upright trapping a layer of insulating air. Your sweating is reduced to a minimum since heat is absorbed and therefore lost in the process of evaporation. In vasoconstriction, the blood vessels near the skin surface constrict so less blood flows and therefore less heat energy is transferred to the cold surroundings.

  • If you are too hot the erector muscles relax allowing the hairs to lie flat on the skin, no longer trapping insulating air. You also begin to sweat which removes heat energy in the process of evaporation. The blood vessels near the skin surface widen (to dilate - process of dilation, vasodilation) which allows more blood to flow and hence transfer more heat to the surroundings.

  • I'm afraid there are limits to your bodies response ...

    • A very high temperatures make you feel extremely uncomfortable as your body struggles to cope with the situation and you suffer from 'heat exhaustion' and then heatstroke - which can be fatal.

    • At the other extreme, particularly without adequate clothing, very low temperatures resulting in great heat loss the body can lead to hypothermia and finally death. If body respiration can't replace the heat loss, then your body gradually cools, it begins to malfunction and eventually ceases to function at all.

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Practical work to help develop your skills and understanding may have included the following:

Use surface temperature sensors to monitor skin temperature in different conditions

Planning an investigation to measure the cooling effect of sweating

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Homeostasis notes index:

Homeostasis - introduction to how it functions (negative feedback systems explained)

Homeostasis - control of blood sugar level - insulin and diabetes

Homeostasis - osmoregulation, ADH, water control, urea and ion concentrations and kidney function, dialysis

Homeostasis - thermoregulation, control of temperature

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WHERE NEXT?

Other general HUMAN BIOLOGY revision notes

Introduction to the organisation of cells => tissues => organs => organ systems (e.g. in humans)

Examples of surfaces for the exchange of substances in animal organisms  

See also Enzymes - section on digestion and synthesis 

The human circulatory system - heart, lungs, blood, blood vessels, causes/treatment of cardiovascular disease

The brain - what the different parts do and the dangers if damaged

An introduction to the nervous system including the reflex arc 

Hormone systems - Introduction to the endocrine system - adrenaline & thyroxine hormones 

Hormone systems - menstrual cycle, contraception, fertility treatments 

Respiration - aerobic and anaerobic in plants and animals. 

Keeping healthy - communicable diseases - pathogen infections  

Keeping healthy - non-communicable diseases - risk factors for e.g. cancers  

Keeping healthy - diet and exercise 

Keeping healthy - defence against pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies

See also Culturing microorganisms like bacteria - testing antibiotics/antiseptics 

Food tests for reducing sugars, starch, proteins and lipids 

The eye - structure and function - correction of vision defects 

Optics - lens types (convex, concave, uses), experiments, ray diagrams, correction of eye defects (gcse physics)