Part 4: Counter-Current Mechanism & Osmoregulation

Part 4: Counter-Current Mechanism & Osmoregulation

Q1. The counter-current mechanism in kidneys operates between:

a) PCT and DCT
b) Loop of Henle and vasa recta
c) Bowman’s capsule and collecting duct
d) Glomerulus and vasa recta

Answer: b) Loop of Henle and vasa recta

Explanation:

• (a) PCT & DCT – reabsorption, no counter-current.
• ✅ (b) Descending & ascending limbs of loop of Henle with vasa recta maintain osmotic gradient.
• (c) Wrong pair.
• (d) Glomerulus – filtration, not counter-current.

Q2. The main function of the counter-current system is:

a) Increase GFR
b) Concentration of urine
c) Dilution of urine
d) pH regulation

Answer: b) Concentration of urine

Explanation:

• (a) GFR – not affected by counter-current.
• ✅ (b) It maintains medullary gradient → concentrated urine.
• (c) Dilution – opposite effect.
• (d) pH regulation – tubular secretion.

Q3. Which part of the loop of Henle is permeable to water?

a) Ascending limb
b) Descending limb
c) Both limbs
d) Neither

Answer: b) Descending limb

Explanation:

• (a) Ascending – impermeable to water.
• ✅ (b) Descending – permeable to water.
• (c) Both – wrong.
• (d) Neither – wrong.

Q4. Which part of loop of Henle actively transports NaCl into medullary interstitium?

a) Descending limb
b) Ascending limb
c) Collecting duct
d) PCT

Answer: b) Ascending limb

Explanation:

• (a) Descending – water permeable, not NaCl.
• ✅ (b) Ascending limb – active NaCl transport, impermeable to water.
• (c) Collecting duct – water under ADH.
• (d) PCT – reabsorbs but not counter-current.

Q5. Which structure acts as a counter-current exchanger?

a) Loop of Henle
b) Collecting duct
c) Vasa recta
d) PCT

Answer: c) Vasa recta

Explanation:

• (a) Loop of Henle – counter-current multiplier.
• (b) Collecting duct – water reabsorption.
• ✅ (c) Vasa recta – counter-current exchanger.
• (d) PCT – reabsorptive, not exchanger.

Q6. The osmolarity of filtrate at the tip of loop of Henle is about:

a) 100 mOsm/L
b) 300 mOsm/L
c) 1200 mOsm/L
d) 600 mOsm/L

Answer: c) 1200 mOsm/L

Explanation:

• (a) 100 – DCT.
• (b) 300 – plasma-like in Bowman’s capsule.
• ✅ (c) 1200 – highly concentrated at loop tip.
• (d) 600 – intermediate.

Q7. The osmolarity of filtrate leaving ascending limb of Henle is:

a) Hypertonic
b) Hypotonic
c) Isotonic
d) Same as plasma

Answer: b) Hypotonic

Explanation:

• (a) Hypertonic – inside descending limb.
• ✅ (b) Ascending limb pumps NaCl, impermeable to water → filtrate becomes hypotonic.
• (c) Isotonic – only at PCT.
• (d) Not same as plasma.

Q8. The osmolarity of filtrate entering Bowman’s capsule is:

a) 100 mOsm/L
b) 300 mOsm/L
c) 600 mOsm/L
d) 1200 mOsm/L

Answer: b) 300 mOsm/L

Explanation:

• (a) Too low.
• ✅ (b) Plasma-like osmolarity (isotonic).
• (c) Too concentrated.
• (d) Too high.

Q9. Which hormone directly affects collecting duct water permeability?

a) Aldosterone
b) ADH
c) Renin
d) ANF

Answer: b) ADH

Explanation:

• (a) Aldosterone – Na+ reabsorption.
• ✅ (b) ADH inserts aquaporins in collecting duct.
• (c) Renin – regulates angiotensin.
• (d) ANF – promotes excretion.

Q10. What is the osmolarity of medullary interstitium at deepest region?

a) 300 mOsm/L
b) 600 mOsm/L
c) 900 mOsm/L
d) 1200 mOsm/L

Answer: d) 1200 mOsm/L

Explanation:

• ✅ (d) Highest osmolarity ~1200 mOsm/L in inner medulla.
• Others are incorrect.

Q11. Which part of nephron is impermeable to water but permeable to NaCl?

a) PCT
b) Descending limb of Henle
c) Ascending limb of Henle
d) Collecting duct

Answer: c) Ascending limb of Henle

Explanation:

• (a) PCT – permeable.
• (b) Descending – water permeable, NaCl not.
• ✅ (c) Ascending – impermeable to water, permeable to NaCl.
• (d) Collecting duct – under ADH, permeable to water.

Q12. In counter-current mechanism, urea recycling contributes to:

a) pH regulation
b) Increased medullary osmolarity
c) Increased GFR
d) Glucose reabsorption

Answer: b) Increased medullary osmolarity

Explanation:

• (a) pH – tubular secretion.
• ✅ (b) Urea diffuses into medulla, raises osmolarity.
• (c) GFR – independent.
• (d) Glucose – not related.

Q13. The term “counter-current multiplier” refers to:

a) Vasa recta
b) PCT
c) Loop of Henle
d) Collecting duct

Answer: c) Loop of Henle

Explanation:

• (a) Vasa recta – exchanger, not multiplier.
• (b) PCT – no counter-current.
• ✅ (c) Loop of Henle – counter-current multiplier.
• (d) Collecting duct – not counter-current.

Q14. Which region of nephron creates hyperosmotic medullary interstitium?

a) Bowman’s capsule
b) Loop of Henle
c) DCT
d) Collecting duct

Answer: b) Loop of Henle

Explanation:

• (a), (c), (d) – not primary.
• ✅ (b) Loop of Henle pumps solutes → hyperosmotic medulla.

Q15. In absence of ADH, urine is:

a) Concentrated
b) Dilute
c) Same as plasma
d) Acidic

Answer: b) Dilute

Explanation:

• (a) Concentrated – requires ADH.
• ✅ (b) No ADH → collecting duct impermeable → dilute urine.
• (c) Not plasma-like.
• (d) Acidity not dependent.

Q16. In presence of ADH, urine osmolarity may reach up to:

a) 300 mOsm/L
b) 600 mOsm/L
c) 900 mOsm/L
d) 1200 mOsm/L

Answer: d) 1200 mOsm/L

Explanation:

• ✅ (d) Max concentration = 1200 mOsm/L (same as medullary interstitium).

Q17. Which type of nephron plays a major role in counter-current mechanism?

a) Cortical nephron
b) Juxtamedullary nephron
c) Both equally
d) None

Answer: b) Juxtamedullary nephron

Explanation:

• (a) Cortical – short loops, little role.
• ✅ (b) Juxtamedullary – long loops reach deep medulla.
• (c), (d) – wrong.

Q18. Counter-current mechanism helps in:

a) Glucose absorption
b) Water conservation
c) Protein digestion
d) Urea formation

Answer: b) Water conservation

Explanation:

• (a), (c), (d) – unrelated.
• ✅ (b) Conserves water by concentrating urine.

Q19. The osmolarity of filtrate in DCT is about:

a) 100 mOsm/L
b) 300 mOsm/L
c) 600 mOsm/L
d) 1200 mOsm/L

Answer: a) 100 mOsm/L

Explanation:

• ✅ (a) Filtrate entering DCT is hypotonic (~100 mOsm/L).

Q20. Which part of nephron plays a role in both counter-current and hormone action?

a) PCT
b) Loop of Henle
c) Collecting duct
d) Bowman’s capsule

Answer: c) Collecting duct

Explanation:

• (a), (d) – not involved.
• (b) Loop of Henle – counter-current only.
• ✅ (c) Collecting duct – counter-current + ADH action.

Q21. The medullary osmotic gradient is mainly due to:

a) Glucose
b) NaCl and urea
c) Proteins
d) Bicarbonate ions

Answer: b) NaCl and urea

Explanation:

• (a) Glucose – reabsorbed.
• ✅ (b) NaCl & urea – establish gradient.
• (c) Proteins – not filtrate solute.
• (d) Bicarbonate – pH role.

Q22. Which condition would occur if counter-current mechanism fails?

a) Urine becomes very dilute
b) Urine becomes very concentrated
c) No urine formation
d) Glucose appears in urine

Answer: a) Urine becomes very dilute

Explanation:

• ✅ (a) Failure → no gradient → dilute urine.
• (b) Concentration requires gradient.
• (c) Filtration still occurs.
• (d) Glucoseuria – unrelated.

Q23. Counter-current mechanism ensures:

a) High blood pressure
b) High medullary osmolarity
c) Protein synthesis
d) Increased GFR

Answer: b) High medullary osmolarity

Explanation:

• ✅ (b) Maintains osmolarity of medulla (~1200 mOsm/L).
• Others unrelated.

Q24. Which aquaporins are inserted into collecting ducts under ADH action?

a) AQP1
b) AQP2
c) AQP3
d) AQP4

Answer: b) AQP2

Explanation:

• (a) AQP1 – PCT, descending limb.
• ✅ (b) AQP2 – ADH-regulated in collecting duct.
• (c) AQP3 & (d) AQP4 – basolateral membranes, not regulated by ADH.

Q25. The ability of mammals to produce hyperosmotic urine is due to:

a) Cortical nephrons
b) Juxtamedullary nephrons and counter-current system
c) Glomerular filtration
d) Tubular secretion

Answer: b) Juxtamedullary nephrons and counter-current system

Explanation:

• (a) Cortical – cannot concentrate urine effectively.
• ✅ (b) Juxtamedullary + counter-current → hyperosmotic urine.
• (c) Filtration – initial process only.
• (d) Secretion – waste elimination, not concentration.

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