Substances in Glomerular Filtrate Reabsorbed by Kidney Tubules
The glomerular filtrate is an ultrafiltrate of plasma containing water, electrolytes, nutrients, and waste. The renal tubules reclaim essential substances—normally <1% of filtered load appears in urine.
Major Reabsorbed Substances
1. Glucose
- Site: Proximal convoluted tubule (PCT) — SGLT2 (early), SGLT1 (late)
- Mechanism: Secondary active transport (Na⁺-glucose cotransport)
- Threshold: ~180-200 mg/dL (TmG); above → glycosuria
- Normal: 100% reabsorbed; urine glucose negative
2. Amino Acids
- Site: PCT (multiple Na⁺-dependent transporters)
- Mechanism: Secondary active transport; shared and specific carriers
- Normal: ~100% reabsorbed
- Disorders: Hartnup (neutral AA), cystinuria (dibasic AA + cystine)
3. Vitamins and Minerals
- Water-soluble vitamins (B, C): PCT reabsorption via specific carriers
- Vitamin D binding protein: Megalin/cubilin-mediated endocytosis
- Electrolytes: Na⁺ (65% PCT, 25% TALH, 5% DCT, 3% CD), K⁺, Cl⁻, Ca²⁺, Mg²⁺, PO₄³⁻, HCO₃⁻
- Bicarbonate: PCT (80-90% via Na⁺/H⁺ exchanger NHE3); critical for acid-base
4. Water
- PCT: 65% (isosmotic, follows solute)
- Descending thin limb: 15% (passive, aquaporin-1)
- Collecting duct: Variable (ADH-regulated via aquaporin-2)
- Total: ~99% reabsorbed (180 L filtered → 1-2 L urine)
5. Low-Molecular-Weight Proteins
- Examples: β₂-microglobulin, α₁-microglobulin, lysozyme, retinol-binding protein
- Site: PCT — megalin/cubilin receptor-mediated endocytosis
- Normal: >99% reabsorbed; trace in urine
- Tubular proteinuria: Elevated urinary LMW proteins = proximal tubule dysfunction
6. Drugs and Metabolites
- Many drugs filtered then reabsorbed (passive diffusion or transporters)
- pH-dependent reabsorption (weak acids/bases)
- Probenecid inhibits organic anion secretion (used to prolong penicillin)
- Urinary alkalinization/acidification manipulates excretion
Clinical Significance
- Fanconi syndrome: Generalized PCT dysfunction → glycosuria, aminoaciduria, phosphaturia, bicarbonaturia, LMW proteinuria
- Diabetes: Filtered glucose exceeds TmG → osmotic diuresis
- Pregnancy: Increased GFR + unchanged TmG → physiological glycosuria/aminoaciduria
- Drug interactions: Transporter competition (e.g., SGLT2 inhibitors block glucose reabsorption therapeutically)
Quantitative Perspective
| Substance | Filtered Load/day | Excreted/day | % Reabsorbed |
|---|---|---|---|
| Water | 180 L | 1-2 L | 99% |
| Na⁺ | 25,000 mEq | 100-200 mEq | 99.5% |
| Glucose | 180 g | 0 | 100% |
| HCO₃⁻ | 4,300 mEq | 0-2 mEq | >99.9% |