B4: Homeostasis

OCR 21st Century Additional Science

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Changing to stay the same...

Keeping internal conditions constant is called homeostasis.

In a cell...

  • Oxygen goes in for respiration
  • Water enters and leaves
  • Food goes in for respiration,glucose and repair
  • Carbondioxide and other waste products from reactions in the cell also leave, as the cell has to get rid of it

Your body must work hard to:

  • Keep a constant temperature
  • Keep correct levels of salt and water
  • Control amounts of nutrients (e.g. glucose)
  • Take in enough oxygen for respiration
  • Get rid of toxic waste
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Control Systems

The control systems keeping a steady state in your body work in a similar way to artificial control systems

All control systems have:

  • A receptor, which detects the stimuli (the change)
  • A processing center, which recieves the information
  • An effector, which produces an automatic response

What about your body?

  • Some of the temperature control in your body is automatic (e.g. when you sweat)
  • You can also consiously control your body temperature (e.g. taking a coat off when it's too hot)
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Feedback in control systems

Any change in the system results in an action that reverses the change. This is known as negative feedback


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Antagonistic Effectors

Control systems can also have more than one effector. In these circumstances, the effectors will have an opposite effect with one another. This is known to be called antagonistic effectors

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Chemical reactions in our bodies rely on enzymes, as they speed up chemical reactions.

  • Enzymes are biological catalysts that have emormous catalytic power.
  • They are proteins- large molecules made of amino acids
  • An enzymes shape is important to how it works

How do enzymes work?

  • Some enzymes break down large molecules into smaller ones. Others join small molecules together
  • In this case, molecules must fit exactly into a part of the enzyme called the active site
  • Scientists explain this in the form of a 'lock-and-key' mechanism
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Why do we need enzymes?

  • At 37 degrees, chemical reactions in your body are too slow
  • A way to increase the rate of reaction is to raise the temperature
  • As the temperature rises molecules: have more energy, move around faster and collide more often and react more easily when they do collide
  • However, higher temperatures can damage body cells
  • So, to keep warm, your body needs to warm up by releasing energy  from respiration
  • Enzymes are catalysts-so they speed up reactions without it getting used up
  • When enzymes are exposed to high temperatures, exceeding the optimum temp. the enzyme is said to be denatured
  • Optimum temperature=best temperature (and for enzymes=37 degrees)
  • This means that the temperature is too low for the enzyme to denature
  • Enzymes also have a optimum pH
  • So, if the pH is too high, the enzyme also denatures
  • Optimum temp. varies with where the enzyme is situated
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Getting hot, Getting Cold!

  • If the environment is hot, energy transferres heat to you body
  • Your body can also warm up by respiration: Glucose+Oxygen=Carbondioxide+water(+energy)
  • If the environment is cooler than you, energy will be transfered away from your body
  • For temperature to remain constant, energy gained must equal to energy lost so... Heat Gained=Heat Lost
  • Not your whole body has the same temp. You extremities (hands and feet) are cooler than your core (deeper parts)
  • Amount of energy released in respiration is greatest in muscles and liver  
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Warming up and Cooling down

  • When you are too hot, nerve impulses in from the brain stimulate your sweat glands- this makes sweat, and can only cool you down if the sweat evapourates quickly
  • Shivering is also one way to keep you body warm
  • When you shiver, your muscles contract quickly and they must respire fast to release energy
  • Shivering is an automatic response- you can also make a conscious decision by putting on a coat when you are cold
  • The hypothalamus is the processing centre in the brain for sleep, water balance, body temperature, appetite and other functions. The cerebral hemispheres are where you make conscious decisions 
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Too Cold?

When you get too cold, you should...

  • Start to shiver- energy warms the tissues when muscles contract
  • Consume warm food and drinks- energy is transferred from the warm substances to your body
  • Clothes and hair-raising - clothes and hair trap and insulating layer of air. This slows the loss of energy from your body to the environment
  • Heater- go to an area where is it warm, energy is tranferred from the environment to your body
  • Vasoconstriction- Loss of energy from the body's surface is reduced
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Too Hot?

When you get too hot, you should...

  • Start to sweat- energy is lost from skin molecules when water in sweat evaporates
  • Consume cold food and drinks- you are cooled as energy from food and drinks is transfered to your bidy
  • Wear protective clothing- in order to protect from UV rays from the Sun, and also to keep you cool
  • Fan- moving air in atmosphere increases rate of evaporation of sweat
  • Vasodilation- Energy loss from your body's surface is increased
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Vasoconstriction and Vasodilation


  • The blood vessels near the surface of the skin are filled with blood. Energy from the warm blood is transferred down the temp. gradient to the environment. So basically, it means, energy loss from your body's surface is increased. You can only vasodilate when you become hot


  • The muscles in the walls of blood vessels near the surface of the skin contract. Less blood flows near the surface of the skin, so less energy is lost to the environment. So basically, it means loss of energy from your body's surface is reduced. You can only vasoconstrict when you become cold

Vasodilation and Vasoconstriction are a good example of the effects of control by antagonistic effectors

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Diffusion and Osmosis

  • Diffusion is the overall movement of particles from an area of high concentration to an area of low concentration
  • Diffusion is a passive process-it does not require any energy
  • Cells let some molecules through-but not others
  • This is called a partially-permeable membrane
  • A solution which has high concentration of water molecules is a dilute solution
  • It is just like diffusion in water
  • This overall flow of water from a dilute to a more concentrated solution across a partially-permeable membrane is called osmosis
  • Osmosis is also a passive process   
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Active Transport

  • All cell membranes are partially-permeable
  • Some molecules cannot move by diffusion or osmosis as they might be too big
  • So cells have another way of moving into molecules
  • This is called active transport
  • Cells use energy from respiration to move the cells across the membrane
  • Glucose is a from of a molecule that is transported in and out of cells by using active transport
  • Unlike diffusion and osmosis- active transport is not a passive process
  • It requires energy from respiration 
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Water Homeostasis

  • Your body must balance water inputs and water output
  • Water inputs are: food, drink and water made in respiration
  • Water outputs are: exhaled air, sweat, urine and faeces
  • Kidney control water balance
  • They do this by changing the amount of urine you make
  • Less volume of urine=more concentrated
  • More volume of urine=more dilute
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How do your kidneys work?

  • Kidneys have 2 jobs: water homeostatsis and excretion
  • Excretion is getting rid of toxic waste from chemical reactions in your cells
  • These two jobs are linked because you use water to flush out waste products such as urea
  • Liver cells make urea when they break down amino acids your body cannot use
  • Kidneys work like sieves: small molecules are filtered out of the blood as it passes through the kidneys
  • Small molecules are filtered out of the blood consist of: urea, water, glucose and ions of salt. Bigger molecules (e.g. proteins) and blood cells are too large to be filtered out
  • Useful molecules are reabsorbed: all glucose, some water, and some ions of salt (this is done by diffusion and active transport)
  • The amount reabsorbed is controlled by ADH, which is released by the pituitary gland
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Getting the balance

  • Some of the small molecules are useful for the body, so you don't want to lose them
  • The kidneys reabsorb them
  • The useful chemical that go back into the blood are: all of the glucose for respiration, as much salt the body needs, as much water as the body needs
  • The rest filtered chemicals go to the body
  • They make up the urine
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More about water balance

  • The concentration and volume of your urine varies
  • The salt concentration of the blood how much water your kidneys reabsorb
  • The salt concentration of the blood can become higher than normal because of: excess sweating, not drinking enough water and eating salty food
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Drugs and Urine

  • Some drugs affect the amount of urine a person makes
  • Caffeine in tea and coffee causes a greater volume of dilute urine to be produced
  • Alcohol has an even greater effect, and can make the person very dehydrated
  • Ecstasy has an opposite effect
  • It produces more concentrated urine, and may lead to overheating
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Controlling water balance

  • Control system for water balance=negative feedback system
  • Receptors in the hypothalamus in the brain detect any changes in salt concentration in the blood
  • Hypothalamus is the processing centre
  • When the salt concentration is too high, it triggers the release of the hormone ADH from the pituitary gland. This gland is also in the brain, just below the hypothalamus
  • ADH (Antidiuretic hormone) is only released when salt concentration is too high, which allows the walls of the kidney tubules to become more permeable to water
  • So basically, you urinate less when ADH is present in the blood plasma, due to reabsorption 
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Drugs affect ADH control

  • Caffeine, alcohol and ecstasy change the volume of urine a person makes because they affect ADH production
  • Alcohol suppresses ADH production; less water is reabsorbed in the kidneys, so a larger volume of urine is made
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When it all goes wrong

  • Sometimes a person's control system cannot cope with extreme conditions
  • It can affect the persons homeostasis
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Heat Stoke

  • Heat stroke happens when your body cannot lose heat fast enough
  • Fever, prolonged exercise, and over-exposure to the sun and drugs like ecstasy can all lead to heat stoke
  • Core temperature of 42 degrees and above affect the hypothalamus
  • Your temperature control system fails, and you stop sweating
  • Your body rises out of control
  • The most common symptoms for heat stoke are: hot and dry skin, rapid pulse rate, dizziness and confusion
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Treatment for heat stroke

  • Sponging them with water
  • Wrapping them in wet towels
  • Use of a fan
  • Putting ice in their armpits and groin
  • Doctors may also use cool intravenous drips
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  • Hypothermia happens when your core body temperature falls below 35 degrees
  • A person's heat cannot be fast replaced
  • Hypothermia causes 30000 deaths in the UK in one year
  • The symptoms of hypothermia are as follows:
  • <35=shivering, confusion, drowsiness, slurred speech, loss of coordination
  • <30=coma
  • <28=breathing stops
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Treatment for hypothermia


  • Insulate them, particularly head, neck, armpits and groin
  • Handle them gently to keep blood flow to the limbs low
  • Warm them gently with warm towels
  • Give them warm drinks (not alcohol)


  • Give them food, as digesting food lowers metabolic rate
  • Use hot water bottles
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Minhajul Haque


Much better then a different one I saw

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