Questions: Renal Filtration and Tubular Processing

Question 1

A patient receives a drug that completely blocks NKCC2 (the Na-K-2Cl cotransporter in the ascending limb of the loop of Henle). Even with normal ADH levels, the patient fails to concentrate urine above plasma osmolarity. Why?

• A. NKCC2 blockade prevents water reabsorption in the proximal tubule, flooding the collecting duct
• B. Without NKCC2 activity, the medullary concentration gradient cannot be established, so ADH has no osmotic gradient to exploit when it opens aquaporins in the collecting duct
• C. NKCC2 is required for ADH to bind its receptor in the collecting duct principal cells
• D. Blocking NKCC2 prevents filtrate from entering the descending limb of the loop of Henle

Answer: (B)

NKCC2 is the molecular engine of countercurrent multiplication: by actively pumping solute out of the impermeable ascending limb, it loads the medullary interstitium and establishes the osmolarity gradient (~300 to ~1200 mOsm/kg from cortex to papilla). ADH works by inserting aquaporin channels in the collecting duct, allowing water to follow this gradient into the hypertonic medulla. Without the gradient (NKCC2 blocked), there is nothing for water to follow regardless of how much ADH is present. This is precisely the mechanism of loop diuretics (furosemide), which block NKCC2 clinically.

Question 2

What is the approximate osmolarity of the tubular fluid leaving the thick ascending limb of the loop of Henle and entering the distal tubule?

• A. ~1200 mOsm/kg — highly concentrated after traversing the hypertonic medulla
• B. ~300 mOsm/kg — isosmotic with plasma, unchanged from the proximal tubule
• C. ~100 mOsm/kg — hypotonic, because the ascending limb pumped out solute without allowing water to follow
• D. ~600 mOsm/kg — intermediate, reflecting partial concentration by the loop

Answer: (C)

The ascending limb actively pumps NaCl out via NKCC2 but is impermeable to water — so solute leaves but water cannot follow. The tubular fluid becomes progressively diluted as it ascends. By the time it exits at the cortex, the fluid is actually hypotonic (~100 mOsm/kg), less concentrated than plasma. This is counterintuitive but critical: the loop is a concentration-gradient generator for the medullary interstitium, not for the tubular fluid itself. The hypotonic fluid then passes to the collecting duct, where ADH determines final concentration by allowing water to flow out into the hypertonic medulla.

Question 3

The ascending limb of the loop of Henle is impermeable to water while actively transporting solutes outward — this asymmetry is what allows countercurrent multiplication to work.

• T. True
• F. False

Answer: True

Countercurrent multiplication requires the functional asymmetry between the two limbs. The descending limb is water-permeable but solute-impermeable: descending into the hypertonic medulla, water exits and solute enters, concentrating tubular fluid. The ascending limb is the opposite: NKCC2 pumps solute out but water cannot follow, loading the interstitium and diluting the tubular fluid simultaneously. If the ascending limb were water-permeable, water would follow transported solute into the interstitium, dissipating the gradient — countercurrent multiplication would fail entirely.

Question 4

Under normal conditions, glucose is present in urine because the proximal tubule can primarily reabsorb approximately 80% of filtered glucose.

• T. True
• F. False

Answer: False

Under normal conditions, essentially 100% of filtered glucose is reabsorbed in the proximal tubule via SGLT2 (and SGLT1 distally). Glucose does not appear in urine under normal physiological conditions. Glucosuria occurs only when plasma glucose is so high that the filtered load exceeds the transport maximum of the SGLT transporters — classically in uncontrolled diabetes mellitus (plasma glucose above ~180–200 mg/dL). Detecting glucose in urine is a reliable indicator of hyperglycemia precisely because normal renal glucose reabsorption is essentially complete.

Question 5

Explain why the loop of Henle is essential for producing concentrated urine, even though ADH acts on the collecting duct, not the loop itself.

Model answer: The loop of Henle builds the medullary osmolarity gradient through countercurrent multiplication — pumping solute into the medullary interstitium against a water-impermeable barrier. This pre-established gradient is what ADH exploits: when ADH inserts aquaporins in the collecting duct, water flows out passively along the osmotic gradient into the hypertonic medulla. Without the loop's gradient, ADH has no osmotic force to drive water reabsorption and concentrated urine cannot be produced.

The loop and the collecting duct perform complementary, interdependent functions: the loop creates the gradient (active, energy-requiring, ADH-independent); the collecting duct exploits it (passive water flow, gated by ADH). Neither alone can produce concentrated urine. This is also why loop diuretics (furosemide) are so potent: blocking NKCC2 erases the medullary gradient, preventing any urinary concentration regardless of ADH levels — demonstrating that the gradient, not ADH alone, is the rate-limiting factor.