15.6 Kidney & Osmoregulation

Question 1

What cells control the water potential of blood?

Answer 1

Osmoreceptors

Question 2

Where is ADH produced?

Answer 2

Hypothalamus

Question 3

What releases anti-diuretic hormone (ADH) into the blood?

Answer 3

Posterior pituitary gland

Question 4

How is cyclic AMP (cAMP) formed?

Answer 4

1) ADH is released from the pituitary gland and carried in the blood to the cells of the collecting duct.
2) ADH binds to receptors on the cell membrane and triggers the formation of cyclic AMP (cAMP) as a second messenger inside the cell.

Question 5

What is a second messenger?

Answer 5

A second messenger is a molecule which relays signals received at cell surface receptors to molecules inside the cell.

Question 6

What cascade of events does cAMP cause?

Answer 6

1) Vesicles in the cells lining collecting duct fuse with the cell surface membranes on the side of the cell in contact with the tissue fluid of the medulla.
2) The membranes of these vesicles contain protein-based water channels called aquaporins, and when they are inserted into the cell surface membrane, they make it permeable to water.
3) This provides a route for water to move out of the tubule cells into the tissue fluid of the medulla and the blood capillaries by osmosis.

Question 7

What happens when more ADH is released?

Answer 7

• More water channels are inserted into the membranes of the tubule cells.
• This makes it easy for more water to leave the tubules by diffusion, resulting in the formation of a small amount of very concentrated urine.

Question 8

What happens when ADH levels fall?

Answer 8

• Levels of cAMP fall
• Water channels removed from tubule cell membranes, and enclosed in vesicles
• Collecting duct becomes impermeable to water, so no water can leave.
• Large amounts of dilute urine produced.

Question 9

What happens during dehydration?

Answer 9

1) Water content of blood drops, so its water potential drops.
2) This is detected by osmoreceptors in hypothalamus.
3) Posterior pituitary gland stimulated to release more ADH into blood.
4) More ADH means DCT and collecting duct more permeable, so more water reabsorbed into blood.
5) Small amount of highly concentrated urine produced and less water is lost.

Question 10

What happens during hydration? (Low blood water content)

Answer 10

1) Water content of blood rises, so its water potential rises.
2) This is detected by osmoreceptors in hypothalamus.
3) Posterior pituitary gland releases less ADH into blood.
4) Less ADH means that the DCT and collecting duct less permeable, so less water reabsorbed by osmosis.
5) Large amount of dilute urine produced.