Urinary concentration and diluting

Question 1

Why is urine concentration control important?

Answer 1

Intracellular environments require tightly controlled extracellular environments
ECF is affected by substrate levels and amount of water that it is dissolved in
Body water volume affects circulating volume, tissue perfusion and blood pressure

Question 2

How do the kidneys produces dilute urine?

Answer 2

normal ion transport (so filtrate gets more dilute) but little ADH so water cannot follow out of lumen

Question 3

How does the kidney produce concentrated urine?

Answer 3

kidney reabsorbs lots of water

Question 4

Describe water handling in the proximal convoluted tubule

Answer 4

High water permeability
Solutes resorbed and water follows by osmosis = isotonic change
Filtrate volume reduced by 65% but osmolality does not change

Question 5

Describe water handling in the LoH

Answer 5

Variable water permeability depending on location
Descending limb = water permeable
Ascending limb = ions actively pumped out
Produces dilute urine and hypertonic medullary interstitium at base of LoH

Question 6

Describe water handling in the collecting tubules

Answer 6

Water permeability varies depending on ADH
Cells and tight junctions are impermeable to water
ADH increases water permeability through aquaporins
Allows water to leave down conc gradient => concentrated urine

Question 6

Describe waster handling in the distal convoluted tubule

Answer 6

Largely impermeable to water (some ADH effect)
Macula densa and DCT - lots of electrolyte active transport
Filtrate becomes dilute

Question 7

Describe the action of ADH/Vasopressin

Answer 7

Made in hypothalamus
Released from post. pit in response to osmoreceptors detecting rise in plasma osmolality
ADH binds o V2 receptors on collecting duct cells => aquaporins inserted from intracellular vesicles into cell membrane => water flows out of tubules down conc gradient => concentrated urine

ADH also causes:
- vasoconstriction
- +ve feedback on aldosterone resorbing sodium in DCT

Question 8

What is the effect of low ADH levels

Answer 8

No water reabsorption in collecting ducts
Na and Cl continue to be resorbed
Urine gets even more dilute

Question 9

What does hyposthenuria mean?

Answer 9

Kidneys are actively diluting urine
Mainly occurs in LoH where water follows solutes out of tubule

Question 10

What is isosthenuric urine?

Answer 10

USG is same as plasma
Kidney is not concentrating or diluting urine

Question 11

What is hypersthenuric urine and what does it require?

Answer 11

Concentrated urine
Requires:
- ADH
- DCT/collecting ducts to be responsive to ADH
- hypertonic medullary intersatitium