Homeostasis and Response

• lens


  focuses light onto the retina
• optic nerve


  carries impulses from the receptors on the retina to the brain
• sclera


  the tough supporting wall of the eye
• cornea


  transparent outer layer at the front of the eye

  refracts light into the eye
• iris


  contains muscles that control the diameter of the pupil and how much light enters the eye
• what happens to the eye in bright light


  the circular muscles contract

  the radial muscles relax

  reduces the amount of light that can enter the eye
• what happens to the eye in dim light

  the circular muscles relax

  the radial muscles contract

  increases the amount of light that can enter the eye
• ciliary muscles and suspensory ligaments 


  controls the size of the lens
• myopia


  short sightedness
• what is long sightedness


  unable to focus on near objects
• what is short sightedness


  unable to focus on distant objects
• how can glasses fix long sightedness


  glasses have a convex lens (curves outwards)

  refracts the light rays so they focus on the retina
• how do glasses fix short sightedness


  glasses have concave lens (curves inwards)

  refract the light rays so they focus on the retina
• pros of contact lenses


  lightweight

  invisible

  convenient
• soft contact lenses are more comfortable than hard contact lenses
• soft contact lenses carry a much higher risk of infection than hard contact lenses
• how do contact lenses work


  sit on the surface of the eye

  shaped to compensate for the fault in focusing
• what does laser eye surgery change

  the shape of the cornea

  therefore changes how strongly it refracts light into the eye
• how does laser eye surgery improve short sightedness

  slims the cornea

  makes the refraction of the light less powerful so it focuses on the retina
• how does laser eye surgery improve long sightedness

  changes the shape of the cornea

  makes the light that refracts into the eye more powerful so it focuses on the retina
• pros of laser eye surgery

  able to precisely control how much tissue to take off
• cons of laser eye surgery

  risk of complications

  e.g infections or worsening the vision
• replacement lens surgery is effective for long sightedness
• process of replacement lens surgery

  natural lens is removed

  artificial lens of clear plastic is inserted
• cons of replacement lens surgery

  higher risks of damage to the eye

  possible damage to the retina

  could lead to loss of sight
• what is short sightedness

  unable to focus on distant objects
• blood glucose is too high

  insulin is added

  insulin and glucose move from the blood into the liver and muscle cells

  glucose converted into glycogen by the liver

  glucose level is reduced
• blood glucose is too low

  glucagon is added

  glucagon makes liver convert glycogen into glucose

  glucose is released into the blood by the liver

  blood glucose level is increased
• insulin

  the hormone that lowers the blood glucose level
• glucagon

  hormone that increases blood glucose level
• pancreas uses insulin and glucagon to monitor and control the blood glucose level
• excess glucose can be stored in liver and muscle cells
• diabetes effects the ability to control blood glucose level
• Type 1 Diabetes

  the body doesn't produce enough insulin

  requires insulin injects after meals which are effective
• Type 2 Diabetes

  body is resistant to the insulin that is produced

  controlled by regular exercise and a carbohydrate controlled diet

  obesity is a major risk factor in development
• optimum temperature for enzymes in the body

  37$^o$
• the body must balance the amount of energy gained and lost to keep the core temperature constant
• Thermoregulatory Centre

  contains receptors sensitive to the temperature of blood flowing through the brain

  receives impulses from temperature receptors on skin
• different responses are produced by different receptors to counteract the increase or decrease in body temperature
• when your too hot:

  sweat is produce by sweat glands

  this transfers energy to the environment

  blood vessels dilate (vasodilation)

  helps transfer energy from the skin to environment