A2 Homeostasis

Question 1

What happens to absorption of water in tubules of nephron when dehydrated?

Answer 1

WP is lower - more water reabsorbed by osmosis into blood from tubules. Urine more concentrated - less water lost during excretion

Question 2

Where in the nephron is water potential regulated?

Answer 2

DCT, loop of Henle and collecting duct - volume absorbed controlled by hormones
Water is reabsorbed along almost all of nephron

Question 3

Describe how the ascending (DCT) and descending (PCT) limbs in the loop of Henle maintain a sodium ion gradient

Answer 3

1) Top of A-limb, Na ions pumped out into medulla using AT. A-limb impermeable to water - stays inside. Creates low WP in medulla - high ion conc.
2) Lower WP in medulla than D-limb - water moves out D-limb into medulla by osmosis - makes filtrate more concentrated (D-limb impermeable to ions). Water in medulla reabsorbed into blood by capillary network
3) Bottom of A-limb, Na ions diffuse out into medulla - lowers WP further in medulla. A-limb impermeable to water so stays in tubule
4) Water moves out DCT by osmosis and reabsorbed into blood
5) First 3 stages increase ion conc. in medulla - lowers WP. Causes water to diffuse out of CD by osmosis. Water reabsorbed into blood by capillary network

Question 4

What cells monitor water potential of blood?

Answer 4

Osmoreceptors in the hypothalamus of the brain

Question 5

What is antidiuretic hormone (ADH)

Answer 5

Released by posterior pituitary gland into blood - causes walls of DCT and CD to be more permeable to water

Question 6

How does ADH change water content of blood when dehydrated?

Answer 6

• Water content drops - so WP drops
• Detected by osmoreceptors in hypothalamus
• Posterior pituitary gland stimulated - releases more ADH into blood
• DCT and CD become more permeable to water - so more water reabsorbed into blood by osmosis
• Small amount of highly concentrated urine produced - less water lost

Question 7

How does ADH change water content of blood when hydrated?

Answer 7

• Water content rises - so WP rises
• Detected by osmoreceptors in hypothalamus
• Posterior pituitary gland releases less ADH into blood
• DCT and CD become less permeable to water - so less water reabsorbed into blood by osmosis
• Large amount of dilute urine produced - more water lost