B4- Homeostasis

A brief summary of the OCR 21st century Biology module B4- Homeostasis

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  • Created by: R_Hall
  • Created on: 13-06-11 19:03

Changing to Stay the Same

  • Keeping conditions inside your body the same is called homeostasis.
  • Your body works hard to-
  • 1. Keep a constant body temperature.
  • 2. Keep correct levels of water and salt.
  • 3. Control amounts of nutrients (eg glucose).
  • 4. Take in enough O2 for respiration.
  • 5. Get rid of toxic waste (CO2 and urea).
  • All control systems have a receptor (detects the stimuli) , processing centre (receives the information) and an effector (produces an automatic response).
  • An incubator has a temperature centre (the receptor), a thermostat with a switch (processing centre) and a heater (effector).
  • Some of your temperature control is automatic (sweating) but some you control (putting a coat on).
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Feedback in Control Systems

  • Negative feedback- any change in a system that results in an action that reverses the change (for example, if the temperature in a fridge goes up, it is cooled down).
  • Antagonistic effectors are effectors with 2 opposite effects. For example, when you drive through a 30mph zone, you don't want to go fast or slow. The brake and the accelerator are antagonistic effectors.
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  • Enzymes are catalysts that speed up chemical reactions in living organisms. They are proteins- an enzyme's shape (determined by amino acid chains) is important.
  • Some enzymes break apart molecules and some join them together.
  • Molecules must fit exactly into the active site- key in a lock, the lock-and-key model.
  • At body temperature, chemical reactions would be too slow to keep you alive. The temperature must be raised- molecules have more energy, move faster, collide more often and react more easily.
  • High temperatures change the shape of an enzyme- the enzyme becomes denatured, and can never change back.
  • The temperature at which an enzyme works best is the optimum temperature- we rely on enzymes to works at our body temperature.
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Too Hot, Too Cold

  • If your environment is hotter than you, it transfers heat to you. You also gain heat through respiration.
  • For your temperature to remain constant , heat gained= heat lost.
  • Your extremities are colder than your core, they have a bigger surface area compared to their size.
  • When you are cold, circulation to extremities is redirected to the core- warm
  • There are sensitive receptors in the brain that detect blood temperature.
  • The Hypothalamus is the processing centre for temperature, the cerebral hemispheres make conscious decisions to warm up or cool down.
  • Shivering is an automatic response- when you shiver, muscle cells contract and respire, releasing energy.
  • When you are too hot, nerve impulses cause the sweat glands to secrete sweat, which cool you down when it evaporates from your skin.
  • To cool down, your blood vessels vasodilate- they allow more blood to flow into capillaries on the skin, so more energy is transferred.
  • Vasoconstriction is opposite, less blood reaches the capillaries, less energy lost.
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Diffusion and Osmosis

  • Molecules in gases and liquids move randomly- they spread out.
  • More molecules move away from where they are concentrated. The molecules diffuse from areas of high concentration to low concentration.
  • Diffusion is a passive process- it doesn't require energy.
  • Partially permeable membranes are cell membranes that let some molecules through, but not others.
  • The overall flow of water from a dilute to a more concentrated solution across a partially permeable membrane is osmosis.
  • Active transport is a way of moving molecules that cannot move through diffusion. Cells use energy from respiration to transport molecules across the membrane.
  • Land animals lose water to the environment and the water must be replaced. If not, the body fluids become too concentrated and the cells stop.
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Water Homeostasis

  • Keeping a steady water level is done by balancing water inputs and outputs.
  • Kidneys control the water balance by changing the amount of urine you make.
  • Kidneys deal with water homeostasis and excretion- getting rid of toxic waste products from chemical reactions in cells.
  • These 2 jobs are linked as you use water to flush out waste eg urea.
  • Liver cells make urea when they break down unusable amino acids. It diffuses into the blood and is carried around the body.
  • Urea is filtered out as it is poisonous in large concentrations.
  • Kidneys sieve small molecules out of blood (water, sugar, urea and salt ions), keeping in large molecules (proteins) and blood cells.
  • Kidneys reabsorb useful chemicals into the blood- glucose for respiration, as much salt and water as needed.
  • The rest of the filtered chemicals go to bladder to make up urine.
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Drugs and Water Balance

  • Salt concentration of blood determine show much water your kidneys reabsorb and how much is excreted in urine.
  • Salt concentration of blood can become high because of excess sweating, not drinking enough water or eating salty food.
  • Caffeine= lots of dilute urine. Alcohol has even greater effect.
  • Ecstasy= little urine-may make person drink too much water.
  • Receptors in hypothalamus detect salt concentration in blood changes.
  • If salt concentration is too high, the hormone ADH is released from the pituitary gland. If salt concentration is low, no ADH is released.
  • ADH travels in the blood to kidney tubules (the effectors). ADH affects the amount of water reabsorbed. More ADH= more water.
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When it All Goes Wrong

  • Heat stroke happens when your body cannot lose heat fast enough. You may have become dehydrated and produce too little sweat.
  • Fever, prolonged exercise, over-exposure to Sun and Ecstasy lead to heat stroke.
  • Core body temp. over 42c affect the hypothalamus, your temp. control system fails.
  • Rapid cooling is essential- sponging, fan, ice.
  • Hypothermia occurs when core body temp. falls below 35c- body heat cannot be replaced as it is lost.
  • Babies have large surface area compared to their volume, lose heat quickly.
  • Core must be heated but not skin and limbs; this increases blood flow to them, causing further heat loss.
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alex colton


very good Talullah , do you know how to get different colours?

alex :) 



Thank you :)

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