Questions: Tubular Reabsorption, Secretion, and Selective Transport

Question 1

A patient with uncontrolled diabetes has a blood glucose of 250 mg/dL, and glucose appears in their urine. What is the correct explanation?

• A. Glomerular filtration is impaired in diabetics, allowing glucose to leak into the filtrate
• B. Glucose is actively secreted into the tubule lumen in diabetic patients
• C. The filtered glucose load exceeds the transport maximum of SGLT cotransporters — all carriers are saturated and excess glucose passes unreabsorbed into the urine
• D. The kidneys cannot detect glucose as a useful substance above a certain plasma concentration

Answer: (C)

SGLT2 and SGLT1 transporters in the proximal tubule reabsorb glucose by active transport, but they have a finite number of binding sites (the transport maximum, Tm). At normal blood glucose, the filtered load is below Tm and all glucose is reabsorbed — none appears in urine. At 250 mg/dL, the filtered load exceeds Tm (~180 mg/dL threshold); all carriers are saturated, and the surplus glucose passes through unreabsorbed as glucosuria. Glomerular filtration is not impaired in typical type 2 diabetes — the problem is at the tubular reabsorption step, not filtration.

Question 2

A researcher measures the clearance of substance X and finds it equals three times the glomerular filtration rate. What must be true about how the nephron handles substance X?

• A. X is completely reabsorbed — high clearance reflects high plasma concentration causing more filtration
• B. X is only filtered at the glomerulus and not reabsorbed or secreted; its clearance exceeds GFR by chance
• C. X is filtered at the glomerulus and then actively secreted into the tubule lumen by proximal tubule cells, adding substance from peritubular blood to the filtrate
• D. X must be an endogenous waste product like creatinine that bypasses the glomerulus

Answer: (C)

Clearance of a substance exceeds GFR only when the nephron adds the substance to the filtrate beyond what was filtered — that is, by secretion. If clearance = GFR, the substance is filtered and not reabsorbed or secreted (like inulin). If clearance < GFR, the substance is net-reabsorbed. If clearance > GFR, the substance must be secreted. PAH is the classic example: at low plasma concentrations, PAH clearance approaches renal plasma flow because virtually all PAH is removed from blood in a single pass through filtration plus secretion.

Question 3

Most useful substances that pass through the glomerular filter are eventually reabsorbed by the tubule, because the kidney's primary function is conservation.

• T. True
• F. False

Answer: False

Inulin, creatinine, and various organic acids pass through the filter but are only minimally reabsorbed or not reabsorbed at all — and PAH is actively secreted, making its clearance far exceed GFR. The kidney's function is not blanket conservation but selective handling: each substance has a characteristic profile determined by its transport proteins and their maxima. Useful substances like glucose and amino acids are fully reabsorbed under normal conditions, but the same carrier-mediated system means they can spill when concentrations are high. Waste products may be neither reabsorbed nor secreted (creatinine) or actively secreted (PAH, drugs, organic acids).

Question 4

A substance whose renal clearance exactly equals the glomerular filtration rate is neither reabsorbed nor secreted by the tubule.

• T. True
• F. False

Answer: True

This is the defining property of inulin and the basis for using inulin clearance to measure GFR. If clearance = GFR, then the amount excreted in urine equals exactly the amount filtered — no tubular reabsorption or secretion has occurred. This relationship (clearance = GFR ↔ no net tubular handling) is the reference standard for inferring what happens to other substances: clearance below GFR means net reabsorption, clearance above GFR means net secretion. Creatinine is a practical clinical approximation to inulin, with minimal secretion making its clearance a slight overestimate of GFR.

Question 5

How does the transport maximum (Tm) explain why glucose appears in the urine of diabetic patients but not in healthy individuals, even though both use the same carrier-mediated reabsorption system?

Model answer: Both healthy and diabetic kidneys use SGLT2/SGLT1 transporters with the same Tm. The difference is the filtered load. In healthy individuals, blood glucose (~90 mg/dL) × GFR produces a filtered load well below the Tm — all glucose can be bound by available transporters and reabsorbed. In uncontrolled diabetes, high blood glucose (>180 mg/dL) × GFR generates a filtered load that saturates all available transporters. Once every SGLT binding site is occupied, no additional glucose can be reabsorbed regardless of how much remains in the tubule lumen. The excess passes into the urine. The Tm defines a threshold: below it, complete reabsorption; above it, spillage proportional to the excess load.

This Tm threshold concept generalizes to any carrier-transported substance. Amino acids, phosphate, and bicarbonate each have their own Tm, and each can spill into urine when plasma concentrations are pathologically elevated (aminoaciduria in Fanconi syndrome, phosphaturia in certain kidney diseases). Understanding the Tm framework — finite transport capacity, saturation kinetics, threshold phenomenon — is more powerful than memorizing which substances are reabsorbed, because it predicts behavior under any condition once you know the Tm and the filtered load.