15.6 The Kidney And Osmoregulation

Question 1

Describe the regulation of water potential

Answer 1

• water is lost during excretion and sweat
• the kidneys regulate the water potential of the blood (and urine), so the body has the right amount of water :
• if water potential is too low, more water is reabsorbed by osmosis into the blood from the tubules of the nephron. This means urine is more concentrated, so less water is lost from excretion
• if water potential is too high, less water is reabsorbed by osmosis from the filtrate into the blood. This means urine is more dilute, more water is lost via excretion.
• regulation of the water potential of the blood takes place in the loop of Henle, DCT and the collecting duct

Question 2

Describe structure of loop of Henle

Answer 2

• loop of Henle is made up of two limbs — the descending limb and the ascending limb
  These limbs help set up a countercurrent multiplier mechanism to help reabsorb water back into blood

Question 3

Describe and explain the countercurrent multiplier mechanism (in the loop of Henle that helps reabsorb water back into blood)

Answer 3

1) Near the top of the ascending limb, Na+ and Cl- ions are actively pumped out into the medulla. The ascending limb is impermeable to water, so water stays inside the tubule. This creates a low water potential in the medulla, because theres a high concentration of ions

2) Because there’s a lower water potential in the medulla than in the descending limb, water moves out of the descending limb into the medulla by osmosis. This makes the filtrate more concentrated (describe impermeable to ions). The water in the medulla is reabsorbed into the blood through the capillary network

3) near the bottom of the ascending limb Na+ and Cl- ions diffuse out into the medulla, further lowering the water potential in the medulla

4) the first three stages massively increase ions concentration in the medulla, which lowers the water potential. This cause water to move out of the collecting duct by osmosis

Question 4

Why is the loop of Henle length different in different animals

Answer 4

• the longer the animal’s loop of Henle, the more water they can reabsorb from the filtrate
• when theres a longer ascending limb, more ions are actively pumped out into the medulla, which creates a lower water potential in the medulla
• this means more water moves out of the nephron and collectin duct into the capillaries, giving concentrated urine.
• animals that live in areas where theres short water supply have long loops of Henle to reabsorb and save as much water as possible

Question 5

Describe and explain the action of ADH

Answer 5

• the water potential of the blood is monitored by cells called osmoreceptors in the hypothalamus
• when osmoreceptors are stimulated by a low water potential in the blood, the hypothalamus sends nerve impulses to the posterior pituitary gland to release anti-diuretic hormone (ADH) in the blood
• ADH molecules bind to receptors on the plasma membrane of cells in the DCT and the collecting duct
• when this happens, protein channels called aquaporins are inserted into the plasma membrane
• the channels allows water to pass through via osmosis, making the walls of the DCT and collecting duct more permeable to water
  This means more water is reabsorbed from these tubules and into the medulla and into the body by osmosis
• a small concentrated amount of urine is produced, less water lost from body

Question 6

What happens when blood water content is too high (negative feedback control)

Answer 6

• water potential of blood rises
• this is detected by osmoreceptors in the hypothalamus
• nerve impulses sent to the posterior pituitary to release less ADH into the blood
• less ADH bind to receptors on the CSM of cells in the DCT and collecting duct
• less ADH means walls of DCT and collecting duct are less permeable, so less water is reabsorbed into blood by osmosis
• large amount of dilute urine is produce, more water is lost from body