The role of hormones in osmoregulation

Question 1

What causes low water potential?

Answer 1

• little water is consumed
• sweating occurs
• large amounts of ions that lower the water potential

Question 2

How does the body response to the fall of water potential?

Answer 2

• cells called osmoreceptors in the hypothalamus of the brain detect the fall in water potential
• they lose water in response to this which causes them to shrink
• this causes the hypothalamus to produce a hormone called ADH
• ADH passes to the pituitary gland, from where it is secreted into the capillaries
• specific protein receptors on the kidney bind to ADH molecules, leading to activation of phosphorylase within the cell
• phosphorylase causes vesicles to fuse with the cell-surface membrane which releases aquaporins which form water channels in the cell surface membrane of the collecting duct and the distal convoluted tubule which increases the permeability so urea passes out and water can be reabsorbed
• this prevents the water potential from getting any lower
• the osmoreceptors send nerve impulses to seek out and drink more water to raise the water potential

Question 3

How does a body respond to the rise in water potential?

Answer 3

• the osmoreceptors in the hypothalamus detect the rise in water potential and increase 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)