16.7 - The Role Of Hormones In Osmoregulation Flashcards
Where does homeostatic control of osmoregulation in the blood take place
achieved by a hormone that acts on the distal convoluted tubule and the collecting duct.
what does the water potential of the blood depend?
The water potential of the blood depends on the concentration of solutes like glucose, proteins, sodium chloride, and other mineral ions as well as the volume of water in the body
What can cause a rise in solute concentration that lowers the water potential of the blood?
A rise in solute concentration lowers its water potential. This may be caused by:
• too little water being consumed
• much sweating occurring
• large amounts of ions, for example, sodium chloride, being taken in.
How do cells called osmoreceptors respond when the water potential of the blood falls?
- osmoreceptors in the hypothalamus of the brain detect the fall in water potential.
- when the water potential of the blood is low, water is lost from these osmoreceptor cells by osmosis.
- Due to this water loss the osmoreceptor cells shrink, a change that causes the hypothalamus to produce a hormone called antidiuretic hormone (ADH).
Once ADH is produced, what is its pathway and initial action?
- ADH passes to the posterior pituitary gland, from where it is secreted into the capillaries.
- ADH passes in the blood to the kidney, where it increases the permeability to water of the cell-surface membrane of the cells that make up the walls of the distal convoluted tubule and the collecting duct
How does ADH increase the permeability of kidney cells to water?
- Specific protein receptors on the cell-surface membrane of these cells bind to ADH molecules, leading to activation of an enzyme called phosphorylase within the cell.
- The activation of phosphorylase causes vesicles within the cell to move to, and fuse with, its cell-surface membrane.
- These vesicles contain pieces of plasma membrane that have numerous water channel proteins (aquaporins) and so when they fuse with the membrane the number of water channels is considerably increased, making the cell-surface membrane much more permeable to water
What additional effect does ADH have on the collecting duct and the surrounding fluid
- ADH increases the permeability of the collecting duct to urea, which therefore passes out, further lowering the water potential of the fluid around the duct.
- The combined effect is that more water leaves the collecting duct by osmosis, down a water potential gradient, and re-enters the blood.
- As the reabsorbed water came from the blood in the first place, this will not, in itself, increase the water potential of the blood, but merely prevent it getting lower.
- The osmoreceptors also send nerve impulses to the thirst centre of the brain, to encourage the individual to seek out and drink more water
How do the osmoreceptors and pituitary gland respond when the water potential of the blood rises
- The osmoreceptors in the hypothalamus detect the rise in water potential and send fewer impulses to the pituitary gland.
- The pituitary gland reduces the release of ADH and the permeability of the collecting ducts to water and urea reverts to its former state.
- This is an example of homeostasis and the principle of negative feedback
What causes a fall in the solute concentration of the blood
A fall in the solute concentration of the blood raises its water potential. This may be caused by:
• large volumes of water being consumed
• salts used in metabolism or excreted not being replaced in the diet.
how does the body respond to fall in the solute concentration of the blood?
- The osmoreceptors in the hypothalamus detect the rise in water potential (swell) and decrease the frequency of nerve impulses to the pituitary gland to reduce its release of ADH.
- Less ADH, via the blood, leads to a decrease in the permeability of the collecting ducts to water and urea.
- Less water is reabsorbed into the blood from the collecting duct.
- More dilute urine is produced and the water potential of the blood falls.
- When the water potential of the blood has returned to normal, the osmoreceptors in the hypothalamus cause the pituitary to raise its ADH release back to normal levels (= negative feedback)
