Homeostasis and response

• Homeostasis


  The regulation of conditions inside your body and cells in order to maintain a stable internal environment
• Control systems


  Systems that keep the conditions in your body steady
• Conditions controlled by control systems

  • Body temperature
  • Blood glucose level
  • Water level
• Control systems


  • They are automatic - you don't have to think about them
  • They can control conditions using the nervous system or hormones
• Parts of a control system

  • Receptors
  • Coordination centres (including the brain, spinal cord and pancreas)
  • Effectors
• How a control system works

  1. Stimulus (change in the environment) detected by receptors

  2. Information sent to coordination centre

  3. Coordination centre receives and processes information

  4. Coordination centre organises a response

  5. Effector produces the response to return the level to its optimum
• If the level of something is too high


  The control system decreases the level
• If the level of something is too low


  The control system increases the level
• The nervous system means that humans can react to their surroundings and coordinate their behaviour
• Central nervous system ( CNS)

  • made up of the brain and spinal cord
  • The CNS is connected to the body by sensory neurones and motor neurones
• Receptors

  • they detect the stimuli
  • different receptors detect different stimuli
  • receptors in the ears detect sound
• sensory neurone


  carry information as electrical impulses from the receptors to the CNS
• Effectors

  • effectors respond to electrical impulses and bring about a change
  • muscle and glands are effectors
  • they respond in different ways- muscle contract and glands release hormones
• motor neurones


  carry electrical impulses from the CNS to effectors
• The CNS coordinates the response

  1. CNS is a coordination centre
  2. it receives information from the receptors and then coordinates a response
  3. the response is carried out by effectors
• Synapse 


  Gap in between two neurones which connect together
• reflexes

  • Automatic responses- you dont have to think about them
  • this makes them really quick
  • help you stop getting injured
  • the neurones in the arcs go through the spinal cord or through an unconscious part of the brain
• being stung by a bee

  1. Bee stings your finger. Bee sting is detected by receptors
  2. impulses are sent along the sensory neurone to the CNS
  3. in the CNS, a relay neurone passes on the impulses from sensory neurone to motor neurone
  4. Impulses are sent along the motor neurones to the effector
  5. the effector here is a muscle. It contracts to move your hand away from the bee
• Reaction time is time taken to respond to a stimulus
• factors effecting reaction time

  • age
  • gender
  • drugs
• measuring reaction time

  • The person being tested should sit with arm resting at edge of table
  • hold ruler upright and hold at 0 level
  • let go without any warning
  • person tested should catch the ruler and the number on the ruler at the top of thumb is the result
  • repeat several times for mean
  • then give that person caffeine drink, after 10 min repeat
• The Brain is Responsible for Complex Behaviours
• The brain


  • Made up of billions of interconnected neurones
  • Complex behaviours include running, breathing,
  • The brain is in charge of all of our complex behaviours
• Different parts of the brain

  • Cerebral cortex
  • Medulla
  • Spinal Cord
  • Cerebellum
• Cerebral cortex


  Outer wrinkly bit

  its responsible for things like Language and memory
• medulla


  controls unconscious activities like breathing and your heartbeat
• cerebellum


  responsible for muscle coordination
• parts of eye

  • Iris
  • cornea
  • pupil
  • lens
  • ciliary muscle
  • sclera
  • optic nerve
  • suspensory ligaments
  • retina
• Sclera


  tough, outside wall of the eye
• cornea


  clear layer at the front of the eye. It refracts light into the eye
• iris


  has muscles which change the size of the pupil

  it controls how much light enters the eye
• retina


  contains receptor cells that detect light intensity and colour
• lens


  focuses the light onto the retina
• ciliary muscles and suspensory ligaments


  the shape of the lens is controlled by these
• optic nerve


  carries electrical impulses from the receptors on the retina to the brain
• in very bright light

  • light receptors in the eye detect very bright light
  • a reflex is triggered that makes the pupil smaller
  • the circular muscles in the iris contract
  • the radial muscles relax
  • this reduces the amount of light that can enter the eye
• in dim light

  • light receptors in the eye detect dim light
  • a reflex is triggered that makes the pupil larger
  • the radial muscles contract
  • the circular muscles relax
  • this increases the amount of light that can enter the eye
• looking at near objects

  • ciliary muscles contract
  • suspensory ligaments relax
  • the lens becomes fat
  • this increases the amount of light is refracted
• looking at distant objects

  • ciliary muscles relax
  • the suspensory ligaments tighten
  • this makes the lens go thin
  • it refracts light by a smaller amount
• Long sighted people cannot focus properly on near objects

  • Lens is wrong shape, doesn't refract light enough
  • light is focused behind the retina
  • corrected by wearing glasses
  • lens in glasses needs to be curved outwards, refracts light so light focuses on the retina
  • term for long-sightedness is hyperopia