The Kidney - Osmoregulation

Osmoregulation: the control of water levels and ion levels in the body.

Water is gained from:

• Food
• Drink
• Metabolic processes (e.g. respiration)

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Water is lost in:

• Urine
• Sweat
• Water vapour when breathing out
• Faeces

• 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.

• 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

• 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