Topic 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 information from receptors
  • Effectors - bring about responses to return conditions to optimum levels
• Response to a stimulus

  1. Receptor cells convert stimulus into electrical impulse

  2. Impulse travels along sensory neurons to central nervous system

  3. Information processed and appropriate response coordinated

  4. Impulse sent along motor neurons to effectors

  5. Effectors carry out response
• Reflex

  Automatic response that takes place before you have time to think
• Reflex arc

  1. Stimulus detected by receptors

  2. Impulses sent along sensory neuron

  3. Impulse passes to relay neuron in central nervous system

  4. Impulses sent along motor neuron

  5. Impulse reaches effector resulting in response
• Synapse

  The gap between two neurons
• Reaction time

  How long it takes to respond to a stimulus
• Brain

  • Cerebral cortex - controls consciousness, intelligence, memory and language
  • Cerebellum - controls fine movement of muscles
  • Medulla - controls unconscious actions like breathing and heart rate
• Investigating brain function and treating brain damage/disease is difficult because the brain is complex, delicate, easily damaged, drugs cannot always reach it, and its functions are not fully understood
• 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
• Eye

  • Retina - layer of light sensitive cells
  • Optic nerve - carries impulses from retina to brain
  • Sclera - white outer layer that supports eye structures
  • Cornea - transparent layer that allows light through and focuses it
  • Iris - muscles that surround pupil and control its size
  • Ciliary muscles and suspensory ligaments - control shape of lens
• Accommodation

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

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

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

  Long-sightedness - lens is too flat so it cannot refract light enough
• Treatments for eye defects

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

  • Monitors and controls body temperature
  • Has receptors that monitor blood temperature and skin temperature
• Mechanisms to increase body temperature

  1. Sweating increases to evaporate from skin surface

  2. Vasodilation - more blood flows closer to skin surface
• Mechanisms to decrease body temperature

  1. Sweating stops

  2. Shivering to generate heat from respiration

  3. Hairs stand on end to trap warm air

  4. Vasoconstriction - blood does not flow as close to skin surface
• Endocrine system

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

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

  Needs to be kept within a certain limit as glucose is needed by cells for respiration
• Eating foods that contain carbohydrates increases blood glucose concentration
• 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
• Compared to the nervous system, the hormonal system is much slower but it acts for longer
• 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
• When blood glucose levels increase/decrease

  A hormone is secreted to oppose the change
• Your blood glucose concentration is kept constant through using these two hormones (insulin and glucagon). They work in a negative feedback loop
• Type 1 diabetes

  The pancreas cannot produce enough insulin
• Type 2 diabetes

  The body cells no longer respond to insulin
• 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 body cells lose or gain too much water by osmosis, they do not function properly
• The Kidneys

  1. Filtering out the waste products

  2. Selectively reabsorbing useful substances such as glucose, ions and water
• Waste products processed at the kidney

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

  A hormone involved in the control of the loss of water as urine