Notes on Proximal Convoluted Tubule (PCT) – Reabsorption & Secretion MCQ with answer and Lecture for NEET, GPAT

Notes on Proximal Convoluted Tubule (PCT) – Reabsorption & Secretion

Overview of the PCT

• Location: First segment of the nephron tubule, following Bowman’s capsule.
• Structure: Composed of simple cuboidal epithelium with extensive microvilli (brush border) to maximize surface area for reabsorption and secretion.
• Function: Primary site for reabsorption of filtered substances and secretion of waste products into the tubular fluid.

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Reabsorption in the PCT

• Definition: Process of moving substances from the tubular fluid back into the peritubular capillaries (blood).
• Key Characteristics:
  • Reabsorbs ~65-70% of the glomerular filtrate.
  • Bulk reabsorption occurs here, driven by active and passive transport mechanisms.
• Substances Reabsorbed:
  1. Water: ~65% reabsorbed via osmosis, following solute reabsorption.
  2. Glucose: 100% reabsorbed (normally) via Na+/glucose cotransporters (SGLT) in the apical membrane; secondary active transport.
  3. Amino Acids: 100% reabsorbed via Na+-dependent cotransporters.
  4. Sodium (Na+): ~65-70% reabsorbed via Na+/K+ ATPase (basolateral membrane) and cotransporters/channels (apical).
  5. Bicarbonate (HCO3-): ~80-90% reabsorbed; linked to H+ secretion and carbonic anhydrase activity.
  6. Chloride (Cl-): Reabsorbed passively (paracellular) or via Cl-/HCO3- exchangers.
  7. Potassium (K+): ~65% reabsorbed, mostly passive following Na+.
  8. Urea: ~50% reabsorbed passively due to concentration gradient.
• Mechanisms:
  • Active Transport: Na+/K+ ATPase on basolateral membrane creates a Na+ gradient, driving cotransport of glucose, amino acids, etc.
  • Passive Transport: Water, Cl-, and urea follow osmotic/electrochemical gradients.
  • Transcellular Pathway: Through cells (e.g., Na+, glucose).
  • Paracellular Pathway: Between cells (e.g., Cl-, water).

Secretion in the PCT

• Definition: Process of moving substances from the peritubular capillaries (blood) into the tubular fluid.
• Purpose: Eliminates waste, regulates blood pH, and clears drugs/toxins.
• Substances Secreted:
  1. Hydrogen Ions (H+): Secreted via Na+/H+ exchanger (NHE) to regulate pH and reclaim HCO3-.
  2. Ammonium (NH4+): Secreted as part of acid-base balance.
  3. Organic Acids/Bases: e.g., penicillin, PAH (para-aminohippurate), creatinine; via organic anion/cation transporters (OAT/OCT).
  4. Drugs/Toxins: Cleared via specific transporters.
• Mechanisms:
  • Active transport (e.g., H+ secretion via NHE3).
  • Carrier-mediated transport for organic compounds.

Regulation

• Starling Forces: Reabsorption enhanced by high oncotic pressure in peritubular capillaries (due to filtration in glomerulus).
• Hormones:
  • Angiotensin II: Stimulates Na+/H+ exchange → ↑ Na+ and HCO3- reabsorption.
  • PTH (Parathyroid Hormone): Inhibits phosphate reabsorption.
• Transport Maximum (Tm): Limits reabsorption of glucose/amino acids (e.g., glucosuria in diabetes when Tm exceeded).

Clinical Relevance

• Diabetic Nephropathy: Glucose spills into urine if blood levels exceed Tm (~180 mg/dL).
• Metabolic Acidosis: Impaired HCO3- reabsorption or H+ secretion.
• Drug Clearance: PCT secretion is key for eliminating medications.

Chart: Reabsorption & Secretion in the PCT

Multiple-Choice Questions (MCQs)

1. What percentage of the glomerular filtrate is typically reabsorbed in the PCT?
   • A) 25-30%
   • B) 65-70%
   • C) 90-95%
   • D) 100%
   • Answer: B) 65-70%
2. Which transporter is primarily responsible for glucose reabsorption in the PCT?
   • A) Na+/K+ ATPase
   • B) Na+/glucose cotransporter (SGLT)
   • C) Na+/H+ exchanger (NHE3)
   • D) Organic anion transporter (OAT)
   • Answer: B) Na+/glucose cotransporter (SGLT)
3. A patient with uncontrolled diabetes mellitus is likely to exhibit:
   • A) Increased glucose reabsorption in the PCT
   • B) Glucose in the urine due to exceeding Tm
   • C) Decreased Na+ reabsorption in the PCT
   • D) Increased H+ secretion in the PCT
   • Answer: B) Glucose in the urine due to exceeding Tm
4. The secretion of hydrogen ions (H+) in the PCT is primarily linked to:
   • A) Reabsorption of glucose
   • B) Reabsorption of bicarbonate (HCO3-)
   • C) Secretion of organic acids
   • D) Reabsorption of chloride
   • Answer: B) Reabsorption of bicarbonate (HCO3-)
5. Which hormone enhances Na+ reabsorption in the PCT?
   • A) Parathyroid hormone (PTH)
   • B) Atrial natriuretic peptide (ANP)
   • C) Angiotensin II
   • D) Aldosterone
   • Answer: C) Angiotensin II