Renal physiology and urine formation

9 DOH Abu Dhabi (HAAD) lab licensing practice questions on Renal physiology and urine formation, part of Urinalysis and Body Fluids. The first 9 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   A patient has a GFR of 120 mL/min and a plasma glucose of 25 mmol/L (450 mg/dL). Taking the tubular maximum for glucose as 2.0 mmol/min (360 mg/min) and ignoring splay, how much glucose is excreted each minute?

   1. A3.0 mmol/min (540 mg/min)
   2. B2.0 mmol/min (360 mg/min)
   3. C0 mmol/min (none excreted)
   4. D1.0 mmol/min (180 mg/min)

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   Answer: D · 1.0 mmol/min (180 mg/min)

   Filtered load = plasma glucose x GFR = 25 mmol/L x 0.120 L/min = 3.0 mmol/min (540 mg/min). The tubules can reabsorb at most 2.0 mmol/min, so the excess, 3.0 - 2.0 = 1.0 mmol/min (180 mg/min), is excreted. Convert mL/min to L/min before multiplying by a concentration in mmol/L. In real kidneys some glucose spills a little before the Tm is reached (splay), because nephrons differ.

   • A: 3.0 mmol/min is the filtered load, before any reabsorption.
   • B: 2.0 mmol/min is the amount reabsorbed (the Tm), not the amount excreted.
   • C: Nothing would be excreted only if the filtered load were below the Tm; here it is above it.

2. Question 2 · Medium

   A woman on long-term lithium passes about 6 L of urine a day with an osmolality of 120 mOsm/kg, and her plasma vasopressin (ADH) is high. The defect is most likely in the:

   1. AGlomerular filtration barrier
   2. BHypothalamic thirst control
   3. CPituitary release of ADH
   4. DCollecting duct response to ADH

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   Answer: D · Collecting duct response to ADH

   Lithium enters the principal cells of the collecting duct and reduces their aquaporin-2 water channels, so the cells respond poorly to ADH. The result is nephrogenic diabetes insipidus: large volumes of dilute urine despite a high ADH. In central diabetes insipidus ADH is low and desmopressin raises the urine osmolality; in the nephrogenic form it does not. Primary polydipsia also gives dilute urine, but ADH is suppressed.

   • A: Glomerular damage causes proteinuria, not dilute polyuria with a high ADH.
   • B: Excess drinking from a thirst disorder suppresses ADH; here ADH is high.
   • C: Failure of pituitary release would give a low ADH, not a high one.

3. Question 3 · Easy

   Normal glomerular filtrate has nearly the same concentration of small solutes as plasma, but it contains almost no:

   1. AProtein
   2. BGlucose
   3. CSodium
   4. DUrea

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   Answer: A · Protein

   The glomerular barrier (fenestrated endothelium, basement membrane and podocyte slit diaphragms) holds back cells and most plasma proteins by size and charge. Water and small solutes such as glucose, urea, sodium and amino acids pass freely, so the filtrate is essentially protein-free plasma with a specific gravity of about 1.010. Glucose is then reabsorbed in the proximal tubule, which is why normal urine has none even though the filtrate does.

   • B: Glucose is filtered freely and then reabsorbed in the proximal tubule.
   • C: Sodium is filtered freely; most of it is reabsorbed later along the tubule.
   • D: Urea is a small molecule that is filtered freely.

4. Question 4 · Medium

   Moderate constriction of which vessel raises glomerular capillary pressure and so increases the GFR?

   1. APeritubular capillaries
   2. BAfferent arteriole
   3. CRenal vein
   4. DEfferent arteriole

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   Answer: D · Efferent arteriole

   The glomerular capillaries sit between two arterioles. Constricting the efferent (outflow) arteriole holds blood back in the glomerulus and raises filtration pressure, whereas constricting the afferent (inflow) arteriole lowers both pressure and flow. Angiotensin II acts mainly on the efferent arteriole, which is why ACE inhibitors can lower GFR sharply in renal artery stenosis. Very strong efferent constriction eventually cuts plasma flow, so the rise in GFR is limited.

   • A: Peritubular capillaries lie after the efferent arteriole and take up reabsorbed fluid; they do not set glomerular pressure.
   • B: Afferent constriction reduces blood flow into the glomerulus and lowers GFR.
   • C: Renal vein obstruction raises pressure in the kidney and lowers GFR.

5. Question 5 · Medium

   A man with type 2 diabetes started dapagliflozin a month ago. His urine glucose is 4+ on the reagent strip, but his plasma glucose is 6.5 mmol/L (117 mg/dL). What best explains the glycosuria?

   1. ABlocked glucose reabsorption in the proximal tubule
   2. BGlucose secretion into the collecting duct
   3. CA false-positive glucose pad from the drug
   4. DPlasma glucose above the renal threshold

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   Answer: A · Blocked glucose reabsorption in the proximal tubule

   Dapagliflozin and the other gliflozins inhibit SGLT2, the sodium-glucose cotransporter in the early proximal tubule that reclaims most of the filtered glucose. Glucose is lost in the urine even at normal plasma levels, which is how the drug lowers blood glucose. Urine glucose is therefore of no use for monitoring patients on these drugs.

   • B: Glucose is not secreted by any part of the nephron.
   • C: The glucose oxidase pad detects real glucose; the drug does not react with it.
   • D: 6.5 mmol/L is below the usual renal threshold of about 9-10 mmol/L.

6. Question 6 · Easy

   Glucose usually starts to appear in the urine once the plasma glucose rises above about:

   1. A7.0 mmol/L (126 mg/dL)
   2. B5.5 mmol/L (100 mg/dL)
   3. C9-10 mmol/L (160-180 mg/dL)
   4. D20 mmol/L (360 mg/dL)

   Answer and explanation in LearnFree account

7. Question 7 · Easy

   When renal perfusion pressure falls, renin is released from which cells?

   1. AJuxtaglomerular cells
   2. BMesangial cells
   3. CMacula densa cells
   4. DGlomerular podocytes

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8. Question 8 · Easy

   At rest, about what share of the cardiac output flows through the two kidneys?

   1. AAbout 50%
   2. BAbout 75%
   3. CAbout 5%
   4. DAbout 25%

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9. Question 9 · Easy

   In the countercurrent system, which part of the nephron is freely permeable to water but reabsorbs little salt?

   1. ADistal convoluted tubule
   2. BProximal convoluted tubule
   3. CDescending limb of the loop of Henle
   4. DThick ascending limb of the loop of Henle

   Answer and explanation in LearnFree account