5: Homeostasis and Response

• Homeostasis


  The maintenance of a constant internal environment
• What homeostasis controls in the human body


  • Blood glucose concentration
  • Body temperature
  • Water levels
• Control systems


  • Receptors - cells that detect stimuli
  • Coordination centres - process the information received from the receptors
  • Effectors - bring about responses to bring the conditions in the body back to optimum levels
• Response to a stimulus

  1. Receptor cells convert a stimulus into an electrical impulse

  2. Electrical impulse travels along sensory neurons to the central nervous system

  3. Information is processed and the appropriate response is coordinated

  4. Electrical impulse is sent along motor neurons to effectors

  5. Effectors carry out the response
• Reflex


  Automatic responses which take place before you have time to think
• Reflex arc


  1. Stimulus is detected by receptors

  2. Impulses are sent along a sensory neuron

  3. In the CNS the impulse passes to a relay neuron

  4. Impulses are sent along a motor neuron

  5. The impulse reaches an effector resulting in the appropriate response
• Synapse


  The gaps between two neurons
• Reaction time


  How long it takes you to respond to a stimulus
• Components of the brain


  • Cerebral cortex
  • Cerebellum
  • Medulla
• Challenges in investigating brain function and treating brain damage/disease

  • It is complex and delicate
  • It is easily damaged
  • Drugs given to treat diseases cannot always reach the brain because of the membranes that surround it
  • It is not fully understood which part of the brain does what
• Methods used by neuroscientists to map brain function


  1. Studying patients with brain damage

  2. Electrically stimulating different parts of the brain

  3. Using MRI scanning techniques
• Structures within the eye


  • Retina
  • Optic nerve
  • Sclera
  • Cornea
  • Iris
  • Ciliary muscles and suspensory ligaments
• Accommodation


  1. To focus on a near object: Ciliary muscles contract, suspensory ligaments loosen, lens becomes thicker and more curved

  2. To focus on a distant object: Ciliary muscles relax, suspensory ligaments tighten, lens becomes thinner
• Myopia


  Short sightedness - the lens is too curved, so distant objects appear blurry
• Hyperopia


  Long sightedness - the lens is too flat, so it cannot refract light enough
• Treatment methods for eye defects


  • Spectacle lenses
  • Contact lenses
  • Laser eye surgery
  • Replacement lens
• Thermoregulatory centre


  • Monitors and controls body temperature
  • Has receptors that monitor the temperature of the blood
  • Has receptors in the skin that send impulses to the thermoregulatory centre
• Mechanisms to regulate body temperature when it is too high


  • Sweat is produced from sweat glands
  • Vasodilation - more blood flows closer to the surface of the skin
• Mechanisms to regulate body temperature when it is too low


  • Sweating stops
  • Skeletal muscles contract rapidly (shivering) to generate heat from respiration
  • Hairs stand on end to create an insulating layer, trapping warm air
  • Vasoconstriction - blood does not flow so close to the surface
• Endocrine system


  Communication system that sends hormones (chemical messengers) around the body
• Glands that make up the endocrine system


  • Pituitary gland
  • Pancreas
  • Thyroid
  • Adrenal gland
  • Ovary
  • Testes
• Compared to the nervous system, the hormonal system is much slower but it acts for longer
• Blood glucose concentration


  The concentration of glucose in the blood needs to be kept within a certain limit because glucose is needed by cells for respiration
• Pituitary gland


  Secretes anti-diuretic hormone (ADH)
• Adrenal gland


  • Secretes adrenaline
  • Involved in the 'fight or flight' response (the body's response to stressful situations)
• Ovary


  • Secretes oestrogen
  • Involved in the menstrual cycle and the development of female secondary sexual characteristics
• Testes


  • Secretes testosterone
  • Involved in the production of sperm and the development of male secondary sexual characteristics
• The blood transports the hormone to a target organ or tissue where it has an effect
• Glucose


  Needed by cells for respiration
• Control of Blood Glucose Concentration


  1. Eating foods that contain carbohydrates increases the glucose levels in the blood

  2. If the glucose levels are too high, the pancreas produces the hormone insulin

  3. Insulin binds to cell in target organs (muscles and liver) causing glucose to move from the blood into muscle cells for respiration and excess glucose to be converted into glycogen which is stored in the liver

  4. The blood glucose concentration is reduced

  5. If glucose levels decrease, the pancreas produces the hormone glucagon

  6. Glucagon binds to to the liver cells causing glycogen to be broken down into glucose

  7. Glucose is released into the blood, increasing the blood glucose concentration
• Blood glucose concentration


  Kept constant through using insulin and glucagon in a negative feedback loop
• Type 1 diabetes


  The pancreas cannot produce enough insulin, blood glucose level can rise to a fatal amount, glucose is excreted with urine and lots of urine is produced leaving the individual very thirsty, treated with insulin injections at meal times
• Type 2 diabetes


  The body cells no longer respond to insulin, blood glucose levels can rise to a fatal amount, obesity is a risk factor, treatments include reducing the number of simple carbohydrates in diet, losing weight and increasing exercise, drugs to make insulin more effective on body cells, help the pancreas make more insulin or reduce the amount of glucose absorbed from the gut
• Osmosis


  The process by which water molecules move from a place where they are in high concentration to a place where they are in low concentration
• If the water concentration of the blood increases

  Cells in the body take up water, cells expand and may burst
• If the water concentration of the blood decreases

  Cells in the body lose water, cells shrink
• Kidneys


  • Filter out waste products, selectively reabsorb useful substances such as glucose, ions and water
• Waste products processed by the kidneys


  • Water
  • Ions
  • Urea
• Anti-diuretic hormone (ADH)


  Hormone involved in the control of the loss of water as urine, released by the pituitary gland, increases the permeability of kidney tubules to water
• Dialysis


  The function of the kidneys is carried out using an artificial membrane, blood moves between partially permeable membranes surrounded by dialysis fluid, useful ions and glucose are not lost from the blood but urea, excess ions and water diffuse across the membrane