Control of plasma osmolarity

Question 1

Describe the two efferent paths stimulated by a raise in plasma osmolarity, and describe the receptor. (4)

Answer 1

OVLT receptors in hypothalamus has a fenestrated epithelium in contact with plasma to detect osmolarity. Cells of the supraoptic nucleus also lie close with the baroreceptor reflex.
ADH released from kidneys saves water from excretion.
Thirst prompts drinking behaviour of solute free water.

Question 2

Why does ADH have a much more precise action than the thirst reflex? (1)

Answer 2

1% increase in osmolarity prompts release of ADH. 10% increase in osmolarity prompts thirst reflex.

Question 3

Explain the two clinical syndromes of too little ADH. (3)

Answer 3

Central diabetes insipidus - too little ADH produced due to damage to HPA, infection, tumour, aneurysm.
Nephrogenic diabetes insipidus - insensitivity to ADH due to increases exogenous ADH.
Both lead to diuresis - too much water excreted - increase in plasma osmolarity.

Question 4

Explain the clinical syndrome of too much ADH. (2)

Answer 4

Syndrome of inappropriate ADH syndrome (SIADH) - too much ADH - too much ADH - too much water being held on to - decrease in plasma osmolarity.

Question 5

Describe the differences in permeability of the parts of the LoH. (2)

Answer 5

Descending LoH - highly water permeable, not at all Na+ permeable.
Ascending LoH - actively transports NaCl out of the filtrate, but not water permeable.

Question 6

Explain how urea is used in the kidney to concentrate urine. (4)

Answer 6

Urea in the kidney acts as an effective osmole because it cannot diffuse through the membrane but must move through an AQP with water. It circles, leaving the collecting duct with the water under ADH, contributing to the high osmolarity of the interstitium due to its high concentration, meaning it diffuses back into the descending LoH and begins again.

Question 7

Explain how the vasa recta acts as a counter-current exchanger. (5)

Answer 7

The vasa recta is the straight capillary that runs next to juxtamedullary nephrons, but the blood flows int he opposite direction to the filtrate. Blood in the vasa recta enters the concentrated medulla and takes on salts to match the interstitium, meaning the hairpin of the vasa recta is sluggish. As the blood ascends, it absorbs all the the water leaving the descending limb of the LoH due to the osmotic and oncotic pressures of the blood. This means all the water produced by the loop cannot dilute the medullary conc.