Homeostasis

• Homeostasis


  The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes
• Enzymes and cells require very stable conditions in order to work
• The body has systems in place to keep the internal conditions optimum for the cells to function
• Internal conditions maintained by homeostasis

  • Blood glucose concentration
  • Body temperature
  • Water levels
• Automatic control system


  Involves the nervous system or hormones
• Automatic control system


  1. Receptor cells detect changes in the environment

  2. Receptor cells pass information to a Coordination Center

  3. Coordination Center receives and processes the information

  4. Coordination Center sends instructions to the effector

  5. Effector carries out the response
• Stimulus


  A change to the environment
• Receptor cells


  Detect changes in the environment
• Coordination Center


  Receives and processes information from receptor cells
• Effector


  A muscle or gland that carries out the response
• The optimum level is stored
• Nervous system


  Consists of two parts: central nervous system (brain and spinal cord) and other nerves running to and from the central nervous system
• Homeostasis


  1. Stimulus

  2. Receptor

  3. Coordination Center

  4. Effector

  5. Response
• Neuron


  Another way of saying nerve cell
• Reflex arc


  1. Stimulus detected by receptor

  2. Electrical impulse passed along sensory neuron to central nervous system

  3. Electrical impulse passed along relay neuron in central nervous system

  4. Electrical impulse passed along motor neuron to effector

  5. Effector (muscle) contracts, response occurs
• Reflexes are automatic and rapid, with no decision-making by the conscious part of the brain
• Reflexes help to protect us from danger
• Measuring a person's reaction time

  1. Person 1 sits on a stool with good upright posture

  2. Person 1 places the forearm of their dominant arm across the table with hand overhanging the edge

  3. Person 2 holds a ruler vertically with the 0 cm mark between Person 1's thumb and first finger

  4. Person 2 tells Person 1 to prepare to catch the ruler

  5. Person 2 drops the ruler at a random time

  6. Person 1 catches the ruler with thumb and first finger as quickly as possible

  7. Person 2 records the measurement on the ruler level with Person 1's thumb

  8. Repeat the test several times and calculate the mean
• Reaction time


  Time taken for a person to respond to a stimulus
• The two people then switch places with Person 1 testing the reaction time of Person 2
• Independent variable


  The variable that is changed in an experiment
• Dependent variable


  The variable that is measured for each change in the independent variable
• Control variables


  • Variables that are kept constant to avoid affecting the dependent variable
  • Examples: distance between thumb and first finger, measuring at top of thumb, room conditions
• Other independent variables that could be investigated: effect of practice, dominant vs non-dominant hand, effect of chemicals like caffeine
• Need to check for any medical issues that could be affected by chemicals given to test subjects
• Experiments should be carried out in a lab where hazardous chemicals are not normally used
• Eye


  A sense organ that contains receptors sensitive to both light intensity and color of light
• How light passes through the eye

  1. Light passes through the transparent cornea

  2. Light passes through the pupil in the center of the iris

  3. Light passes through the lens

  4. Light is focused onto the retina at the back of the eye
• Cornea


  • Starts the focusing of light rays
• Lens


  • Can change shape to focus light from distant or near objects (accommodation)
• Retina


  • Contains receptor cells that detect light intensity and color
  • Sends electrical impulses down the optic nerve to the brain
• Sclera


  • Tough outer structure that protects the eye
• Ciliary muscles and suspensory ligaments

  • Work with the lens to allow focusing on distant or near objects
• Iris


  • Colored part of the eye
  • Controls the size of the pupil to regulate the amount of light entering the eye
• Pupil size adjustment in response to light

  1. Dim light: Pupil dilates to allow more light to enter

  2. Bright light: Pupil constricts to reduce light entering
• Pupil size adjustment is a reflex action not involving the conscious part of the brain
• Accommodation


  The ability to change the shape of the lens to focus on near or distant objects
• Accommodation


  1. Ciliary muscle contracts

  2. Suspensory ligaments loosen

  3. Lens becomes thicker

  4. Lens refracts light more strongly
• Focusing on distant objects

  1. Ciliary muscle relaxes

  2. Suspensory ligaments are pulled tight

  3. Lens becomes thinner

  4. Lens refracts light less strongly
• Focusing on near objects

  1. Ciliary muscle contracts

  2. Suspensory ligaments loosen

  3. Lens becomes thicker

  4. Lens refracts light more strongly