16.7 The role of hormones in osmoregulation

Question 1

What does water potential in the blood depend on

Answer 1

The concentration of solutes eg glucose, proteins, sodium chloride and other mineral ions as well as volume of water in the body

Question 2

A rise in solute concentration lowers its water potential.
This may be caused by:

Answer 2

• Too little water being consumed
• Too much sweating
• Large amounts of ions eg sodium chloride being taken in

Question 3

How does the body know when water potential is too low

Answer 3

• Osmoreceptors in hypothalamus of brain detect it
• When water potential of blood is low, it leaves these cells by osmosis
• This makes the cells shrink and this change causes hypothalamus to release ADH

Question 4

What does body do if water potential is too low

Answer 4

-Once ADH has been released from Hypothalamus, it passes to the posterior pituitary gland from where it is secreted into the capillaries

• ADH passes into the blood to the kidney where it increases permeability to water of the cell surface membrane of the cells that make up the walls of distal convoluted tubule and collecting duct.
• Specific protein receptors on cell surface membrane of these cells bind to ADH molecules leading to activation of an enzyme called phosphorylase within the cell
• This activation causes vesicles within the cell to fuse with membrane, and these vesicles contain pieces of plasma membrane that contain aquaporins
• When vesicles fuse, they increase number of water channels in membrane so it is more permeable to water
• ADH therefore increases permeability of collecting duct to urea so it passes out, lowering water potential to fluid around the duct so more water moves out by osmosis and reenters the blood.
• As the reabsorbed water came from the blood originally, it won’t increase water potential of the blood but will prevent it getting lower

Question 5

What other thing do osmoreceptors do if water levels are too low

Answer 5

Send nerve impulses to the brain to make the individual thirsty and drink more water

Question 6

What happens once the blood has the right amount of water

Answer 6

Osmoreceptors in 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.

Question 7

A fall in solute concentration of the blood raises its water potential.

What could this be caused by

Answer 7

• Large volumes of water being consumed
• Salts used in metabolism or excreted not being replaced in the diet

Question 8

How does body respond to a rising water potential

Answer 8

• Osmoreceptors in hypothalamus detect rise in water potential
• Increase frequency of nerve impulses to pituitary gland to reduce its release of ADH
• Less ADH via the blood leads to decrease in the permeability of the collecting ducts to water and urea
• Less water is reabsorbed into blood from collecting duct
• More dilute urine is produced and the water potential of the blood falls