Question 46:
In kidneys the glomerular filtration rate (GFR) depends upon:
Choose the correct answer from the options given below:
The correct answer is (A) (A), (B) and (C) only. Glomerular filtration rate (GFR) primarily depends on net filtration pressure from hydrostatic/oncotic forces, glomerular basement membrane properties, and filtration surface area.
Option Explanations
(A) The difference in hydrostatic and oncotic pressures between the glomerular capillaries and the lumen of the nephron: Correct. GFR follows Starling forces: net filtration pressure = (P_GC – P_BS) – (π_GC – π_BS), where P_GC is glomerular capillary hydrostatic pressure (~55 mmHg, favors filtration), P_BS is Bowman’s space hydrostatic (~15 mmHg, opposes), π_GC is glomerular oncotic (~28 mmHg, opposes), and π_BS is near 0. This difference drives ~125 mL/min filtration.
(B) The nature of glomerular basement membrane: Correct. This acts as the filtration barrier (fenestrated endothelium, GBM, podocyte slit diaphragms). Its permeability (Kf, hydraulic conductivity) and selectivity for size/charge directly affect GFR; damage (e.g., diabetes) reduces it.
(C) Total available filtration area: Correct. GFR = Kf × net pressure, where Kf incorporates capillary surface area (~1-2 m² total). More area (e.g., more glomeruli) increases filtration capacity.
(D) Relative concentration of ions: Incorrect. Ion concentrations influence tubular reabsorption or autoregulation indirectly (e.g., via macula densa NaCl sensing), but not GFR directly—it’s not a primary Starling force or Kf factor.
Glomerular filtration rate (GFR) determinants in kidneys are crucial for understanding renal physiology and filtration of ~180 L/day plasma. Primarily, GFR depends on hydrostatic/oncotic pressure differences, glomerular basement membrane nature, and total filtration area, as per Starling’s law: GFR = Kf × net filtration pressure.
Core GFR Determinants
Hydrostatic pressure in glomerular capillaries (~60 mmHg) minus Bowman’s space pressure (~15 mmHg), opposed by rising oncotic pressure (~30 mmHg), yields ~10-15 mmHg net drive. The glomerular basement membrane’s permeability and podocyte slits form Kf, while capillary surface area scales filtration proportionally. Ion concentrations play minimal direct role.
Why Option (A)?
Only (A), (B), (C) directly govern GFR; (D) affects reabsorption/downstream regulation like tubuloglomerular feedback. Autoregulation maintains GFR at 125 mL/min despite blood pressure changes.
Exam Tips for GATE/NEET
Focus on Starling forces equation and Kf components for PYQs. Common trap: overlooking GBM as “nature” implying permeability. Visualize pressures dropping along glomerular capillary.
