osmoregulation

• What is osmoregulation?

  The control of water potential in the body
• What is water potential?

  The tendancy of water to move from one place to another
• What does osmoregulation involve?

  The control of levels of both water and salt in the body.
• Why must the correct water balance between cells and the surrounding tissues be balanced?

  Because water can enter cells causing lysis, or water can leave cells causing them to crenate
• What are examples of the body gaining water?

  Food, drink and metabolic reactions.
• how is water lost in the body?

  Urination, sweating, exhalation, faeces
• what happens if a mammal has to conserve less water?

  The walls of the collecting duct become less permeable, so less water is reabsorbed and more urine is made
• What happens if a mammal has to conserve more water?

  The walls of the collecting duct are more permeable so more water can be reabsorbed into the blood- less urine is made
• What hormone are the cells in the collecting duct responsive to?

  ADH- anti diuretic hormone
• Why do the cells in the walls of the collecting duct react to ADH?

  They have membrane bound receptors for ADH- these receptors bind to ADH causing a chain of enzyme controlled reactions inside of the cell. The end result of the reactuions is to cause vesicles containing aquaporins to fuse with the cell surface membrane- making the walls more permeable to water.
• What happens to the walls of the collecting duct if there is more ADH in the blood?

  More aquaporins are inserted- allowing more water to be reabsorbted via osmosis into the blood. Less urine is produced and the water potential of urine is lower
• what happens to the walls of the collecting duct if there is less ADH in the blood?

  The cell surface membrane will invaginate to create new vesicles that remove aquaporins from the membrane/ This makes the walls less permeable, and less water is reabsorbed via osmosis into the blood. More water passes on down the collecting duct to create a greater volume of urine that is more dilute and has a higher water potential.
• What is the signficiance of the osmoreceptor cells in the hypothalamus?

  They are the sensory receptors that detect the water potential of the blood. They respond to the effect of osmosis. If the water potential is low, these cells will shrink via osmosis. As a result, they stimulate neurosecretory cells in the hypothalamus.
• What is the significance of the neurosecretory cells in the hypothalamus?

  They produce and release ADH. The ADH is manufactured in the cell body, lying in the hypothalamus. ADH moves down the axon to the terminal bulb in the posterior pituitary gland where it is stored in vesicles, they carry action potentials down their axons causing the release of ADH via exocytosis.
• How does ADH enter blood?

  ADH enters blood capillaries running through the posterior pituitary gland. It is transported around the body and acts on the collecting ducts. Once the water potential of the blood rises again, less ADH is released. ADH is slowly broken down with a half-life of about 20 minutes.