Controlling Water Potential:
Osmoregulation is the control of water & salt levels in the body.
Water is gained through food, drink & metabolic products.
Water is lost through urine, sweat, water vapour & faeces.
Osmoreceptors in the hypothalamus monitor blood passing
through the hypothalamus – they are sensitive to water
potential.
When the water potential in the blood is too low, water moves
out of these cells into the blood vessels by osmosis – causing
the osmoreceptor cells to shrink.
This causes neurosecretory cells in the hypothalamus to be
stimulated to produce ADH.
When the neurosecretory cells are stimulated, they cause ADH
to be released into the blood capillaries in the posterior
pituitary gland.
ADH uses a secondary messenger system as it is too large &
polar, so cannot cross the plasma membrane since it is not lipid
soluble:
1. ADH molecules combine with receptors on the plasma
membrane on cells of the collecting duct.
2. This activates an enzyme, which in turn causes vesicles
within the cell to fuse with the cell surface membrane.
3. These vesicles contain aquaporins (water permeable
channels) making the collecting duct membrane more
permeable to water.
4. More water leaves the collecting ducts by osmosis & will
be reabsorbed into the blood, resulting in more
concentrated urine.
Osmoregulation is the control of water & salt levels in the body.
Water is gained through food, drink & metabolic products.
Water is lost through urine, sweat, water vapour & faeces.
Osmoreceptors in the hypothalamus monitor blood passing
through the hypothalamus – they are sensitive to water
potential.
When the water potential in the blood is too low, water moves
out of these cells into the blood vessels by osmosis – causing
the osmoreceptor cells to shrink.
This causes neurosecretory cells in the hypothalamus to be
stimulated to produce ADH.
When the neurosecretory cells are stimulated, they cause ADH
to be released into the blood capillaries in the posterior
pituitary gland.
ADH uses a secondary messenger system as it is too large &
polar, so cannot cross the plasma membrane since it is not lipid
soluble:
1. ADH molecules combine with receptors on the plasma
membrane on cells of the collecting duct.
2. This activates an enzyme, which in turn causes vesicles
within the cell to fuse with the cell surface membrane.
3. These vesicles contain aquaporins (water permeable
channels) making the collecting duct membrane more
permeable to water.
4. More water leaves the collecting ducts by osmosis & will
be reabsorbed into the blood, resulting in more
concentrated urine.
