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 systems


  • Involve the nervous system or hormones
  • Detect changes in the environment (internal or external) through receptor cells
  • Pass information to a Coordination Center (brain, spinal cord, pancreas)
  • Coordination Center sends instructions to an effector (muscle or gland) to carry out a response
• Stimulus


  A change to the environment detected by receptor cells
• 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
• 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. In dim light, the pupil becomes larger to allow more light to enter

  2. In bright light, the pupil becomes smaller to reduce the amount of light entering
• Pupil size adjustment is a reflex action, not controlled by 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
• Hyperopia (long-sightedness)


  People with long sight have no problem focusing on distant objects but cannot focus on nearby objects
• Causes of hyperopia


  • Eyeball too short so light focused behind retina
  • Lens becomes less elastic so cannot become thick enough
• Treatment for hyperopia


  Glasses with convex lenses to partially focus light before it enters the eye
• Myopia (short-sightedness)


  People with short sight can focus on nearby objects but cannot focus on distant objects
• Causes of myopia


  • Eyeball too long so light focused in front of retina
  • Lens too thick
• Treatment for myopia


  Glasses with concave lenses to partially unfocus light before it enters the eye
• Other treatments for myopia and hyperopia

  • Hard or soft contact lenses
  • Laser surgery to change cornea shape
• Artificial lens replacement is a treatment for long and short-sightedness
• Thermoregulatory center


  Part of the brain that monitors and controls body temperature
• Thermoregulatory center


  • Contains receptors sensitive to the temperature of the blood
  • Receives electrical impulses from temperature receptors in the skin
• Body temperature gets too high


  Sweat glands secrete sweat onto the skin surface, which evaporates and cools the body
• Body temperature gets too high


  Blood vessels in the skin dilate (vasodilation), allowing more blood flow and heat transfer out of the body
• Body temperature drops too low

  Blood vessels in the skin constrict (vasoconstriction), reducing blood flow and heat loss
• Body temperature drops too low


  Skeletal muscles contract (shivering) to generate heat through increased respiration