L9: body fluid osmoregulation

Question 1

What variation in osmolality will trigger sensors?

Answer 1

• a variation in +-3mOsm/Kg H2O triggers sensors in the body to do something to bring osmolality back to normal
• changes in plasma/ECF osmolality can cause cells to increase or decrease their cell volume.
• all body fluids are isomolar at 285.

Question 2

How is body osmolality regulated?

Answer 2

• stimulus - a change in osmolality
• receptor/sensory - osmoreceptors in the hypothalamus
• control centre - ADH release into blood from posterior pituitary gland
• effector - change in the water permeability of renal DCT and collecting ducts

Question 3

Hypothalamic osmoreceptors

Answer 3

• receptors detect changes in plasma osmolality
• very sensitive to even small changes (graph at 21:55).
• regulate release of ADH
• modulates water retention or loss, no effect on solutes.

Question 4

Where is ADH produced and stored?

Answer 4

Produced by the hypothalamus, stored in the posterior pituitary gland.

Question 5

When is ADH released into the blood?

Answer 5

ADH is released into the blood when plasma osmolality increases but release is inhibited when plasma osmolality decreases.
(also stimulated by changes in plasma volume but mainly osmolality, e.g. when there are big derangements in blood volume).

Question 6

What does ADH do?

Answer 6

• causes kidneys to conserve water by stimulating passive water reabsorption from collecting ducts.
• ADH leads to insertion of aquaporins in collecting ducts which increases their permeability to water.
• increases urine concentration and increases urine osmolality.

Question 7

Diuretic vs anti-diuretic

Answer 7

Diuretics make you pee more ADH makes you pee less etc.

Question 8

What else happens when ADH is increased to conserve more water?

Answer 8

Thirst centres are activated so you drink more water.

Question 9

How are aquaporins inserted in collecting ducts?

Answer 9

• ADH travels in the bloodstream to the V2 receptors on the basolateral membrane of cells lining the collecting ducts.
• binding of ADH to the V2receptors raises cAMP levels causing intracellular vesicles containing AQP2 to fuse with the apical membrane
• water is reabsorbed from CDs by osmosis, the gradient bought about by the high solute concentration in surrounding medulla, into the vasa recta.

Question 10

What happens to aquaporins if no ADH is present?

Answer 10

(39:30)
If no ADH is present, there are no aquaporins in the luminal/apical surface of the columnar cells lining the collecting ducts, so no water is reabsorbed from these regions.

Aquaporins are always present. The basolateral surface to allow water into these cells for survival.