Kidney: water 2 (Urine Concentration & dilution)

Question 1

Enumerate requirement to produce concentrated urine

Answer 1

• High ADH: high water retention & low water excreted in urine
• Hyperosmotic/Hypertonic Medullary ISF

Question 1

Enumerate Mechanisms to produce Hyperosmotic ISF

Answer 1

1. Countercurrent Multiplier: adds solutes to Medullary ISF to create hyperosmolarity
2. CounterCurrent Exchange system (of U shaped Vasa recta): prevents the loss of produced hyperosmolarity
3. Urea Cycle (urea trapping)
4. Sluggish Medullary blood flow

Question 2

Describe role of Ascending limb of loop of henle in Contercurrent multiplier system

Answer 2

Solutes:
* Active co transport of Na+ K+ & Cl- along with Ca, HCO3 & Mg in thick segment
* Passive reabsorption of Na+ & Cl- down conc. gradient into ISF

Water: Impermiable to water, so not reabsorbed

Osmolarity: Osmolarity of medullary ISF increase to 1200m osmo/L & tubular fluid is diluted to 100m osmo/L

Question 3

Describe role of Descending limb of loop of henle in Contercurrent multiplier system

Answer 3

solutes:
* Impremiable to Na+, Cl- & urea

Water:
* Permiable to water
* osmosis of water from tubular fluid into medullary ISF down osmotic gradient

Osmolarity:
* Osmolarity of tubular fluid increases from 300- 1200 & reaches equilibrium with Medullary ISF

Question 4

Describe countercurrent Exchange system of Vasa Recta

Answer 4

At Descending Limb of Vasa Recta:
* Solutes diffuse from Medullary ISF into blood
* water diffuses from blood into medullary ISF

At Ascedning Limb of Vasa Recta:
* Solutes diffuse from Blood to Medullary ISF
* Water diffuses from Medullary ISF to Blood

At tip, blood is hyperosmotic at 1200m osmo/L, reaching equilibrium with Medullary ISF

Question 5

Describe urea trapping & urea Cycling

Answer 5

1. 40 % of Urea filtered is passively absorbed by PCT
2. Loop of Henle is slightly permiable to Urea
3. At Collecting duct, Urea diffuses passively into medullary ISF by facilitation of ADH (through insertion of UT-1 & UT-3), which adds 50% osmolarity to Medullary ISF
4. Urea cycle occurs when Urea in medullary ISF is transported back into Loop of henle, reaches Collecting duct, then again back to ISF, several times before excretion

Question 6

Describe requirements for excretion of Diluted urine

Answer 6

• Absence of ADH
• DCT, Collecting Duct becomes impermiable to water
• Descending limb Causes Reabsortion of Solutes, which decreases Tubular Osomarity (more dilution)