Humans

• The CNS is made up of the brain and spinal cord
• When receptors detect a stimulus they send electrical impulses along sensory neurones to the CNS, which then sends electrical impulses through a motor neurone to the effector
• Hormones are chemical messengers sent in the blood all around the body but only affect certain parts
• Neurotransmitters stimulate the neuron to send electrical impulses along its axon
• Conjunctiva


  Lubricates and protects the surface of the eye
• Sclera


  • Tough outer layer that protects the eye
• Cornea


  Refracts (bends) light into the eye, transparent and has no blood vessels
• Iris


  Controls the diameter of the pupil and how much light enters the eye
• Lens


  Focuses the light onto the retina
• Retina


  Light-sensitive part covered in light receptors called rods and cones
• Rods


  • More sensitive in dim light but can't sense colour
• Cones


  • Sensitive to colours but aren't so good in dim light, found all over the retina but concentrated at the fovea
• Optic nerve


  Carries impulses from the receptors to the brain
• Iris reflex - adjusting for bright light


  1. Bright light triggers reflex to make pupil smaller, allowing less light in

  2. Dim light triggers reflex to make pupil bigger
• Focusing on near and distant objects

  1. For distant objects: Ciliary muscles relax, suspensory ligaments pull tight, lens becomes less curved

  2. For near objects: Ciliary muscles contract, suspensory ligaments slacken, lens becomes more curved
• Short-sightedness


  • Cornea or lens bends light too much or eyeball is too long, images of distant objects focused in front of retina
• Long-sightedness


  • Cornea or lens doesn't bend light enough or eyeball is too short, images of near objects focused behind retina
• Homeostasis


  So important for organisms (and for science students)
• When you're too hot

  1. Lots of sweat is produced

  2. Blood vessels close to the surface of the skin widen (vasodilation)

  3. Hairs lie flat
• When you're too cold

  1. Very little sweat is produced

  2. Blood vessels near the surface of the skin constrict (vasoconstriction)

  3. You shiver, which increases your rate of metabolism

  4. Hairs stand on end to trap an insulating layer
• Smaller organisms


  • Have bigger surface area to volume ratios
  • Can gain (or lose) heat faster because there is more area for the heat to transfer across
  • Can lose body heat more easily in hot climates and are vulnerable in cold environments
  • Organisms with smaller surface area to volume ratios gain (or lose) heat more slowly because there is less area for the heat to transfer across
• Animals living in cold conditions have a compact (rounded) shape to keep their surface area to a minimum, reducing heat loss
• That's why elephants have big ears - to increase their surface area to volume ratios and help them cool down
• Adrenaline is produced in the adrenal glands, and readies the body for a fight or flight response by increasing heart and breathing rate
• Insulin is produced in the pancreas, and lowers blood glucose levels by having muscles and the liver take in excess glucose and converting it into glycogen for storage
• Testosterone is produced in the testes, and is the primary sex hormone in males, causing the development of male reproductive organs and secondary sexual characteristics
• Progesterone is produced in the ovaries, and maintains pregnancy by maintaining the uterus lining to cushion the growing foetus
• Oestrogen is produced in the ovaries and is the main sex hormone in females, causing the regulation of the menstrual cycle and secondary sexual characteristics