Osmoregulation: the control of water levels and ion levels in the body.
Water is gained from:
- Metabolic processes (e.g. respiration)
Water is lost in:
- Water vapour when breathing out
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Adjusting ADH levels in the blood...
- Water potential of the blood is monitored by osmoreceptors in the hypothalamus.
- Osmoreceptors respond to the effects of osmosis:
- When the water potential of the blood is low, water moves out of osmoreceptor calls which causes them to shrink.
- This stimulates neuroecretory cells in the hypothalamus.
- These are specialised neurones which produce and release ADH - ADH is produced in the body of these cells (in the hypothalamus) which is then stored in the posterior pituitary gland.
- When neurosecretory cells are stimulated they send action potentials down their axons, causing the release of ADH, which then enters the blood stream.
- The target cells of ADH are the cells in the wall of the collecting duct.
- ADH has a half life of roughly 20 minutes.
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If the blood has a low water potential...
- This is detected by osmoreceptors in the brain
- This stimulates the posterior pituitary gland to release more ADH
- This travels to the kidneys in the bloodstream
- ADH has the effect of making the collecting duct of the nephron more permiable to water
- Cells in the wall of the collecting duct have membrane-bound receptors for ADH
- Aquaporins are placed in the membranes of the cells that line the wall of the collecting duct
- Water moves out of the collecting duct and into these cells by osmosis, and then from these cells into the bloodstream
- The overall result is the production of more concentated urine and blood concentration returns to normal levels
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If the blood has a high water potential...
- Less ADH is produced
- The plasma membranes of the cells in the wall of the collecting duct fold inwards to remove aquaporins in vesicles
- This makes the walls less permeable to water
- Therefore, less water is reabsorbed into the blood by osmosis
- The urine produced is more dilute
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