Renal Physiology: Glomerular Filtration covers renal physiology — glomerular filtration for INI CET (AIIMS PG).
Functional Anatomy:
- Kidneys: ~10–12 cm, retroperitoneal; ~1 million nephrons per kidney
- Nephron: Basic functional unit
- Renal corpuscle: Glomerulus + Bowman’s capsule
- Proximal tubule: Reabsorbs ~65% of filtered Na⁺, water, glucose, amino acids
- Loop of Henle: Countercurrent multiplier; diluting segment
- Distal tubule: Fine-tuning of Na⁺/K⁺/H⁺; aldosterone-sensitive
- Collecting duct: ADH-sensitive; final water reabsorption
Glomerular Filtration:
- GFR (Glomerular Filtration Rate): ~125 mL/min = 180 L/day
- ~1 mL/min per 1.73 m² (normalized for body surface area)
- Used to assess kidney function (creatinine clearance estimates GFR)
- Filtration fraction (FF): GFR / RPF (normal ~20%) — fraction of renal plasma flow that becomes filtrate
Starling Forces in Glomerulus:
- Net Filtration Pressure (NFP): P_GC (glomerular capillary hydrostatic pressure ~55 mmHg) – [P_BC (Bowman’s capsule hydrostatic pressure ~15 mmHg) + π_GC (glomerular capillary oncotic pressure ~30 mmHg)]
- NFP ≈ 10 mmHg (favors filtration)
- π_BC (Bowman’s capsule oncotic pressure) = 0 (no protein normally in filtrate)
Regulation of GFR:
1. Autoregulation (maintains GFR constant across MAP 80-180 mmHg):
- Myogenic mechanism: ↑BP → afferent arteriole stretches → myogenic contraction → ↓blood flow → protects glomerulus
- Tubuloglomerular feedback (TGF): ↑NaCl at macula densa → adenosine release → afferent arteriolar constriction → ↓GFR (prevents excessive flow)
- Afferent arteriole: Dilates with PGE₂, NO; constricts with angiotensin II (AT II)
- Efferent arteriole: Constricts with AT II → ↑P_GC → maintains GFR despite ↓RPF
2. Hormonal/Vascular Regulation:
- Angiotensin II (AT II): Constricts efferent arteriole > afferent → maintains GFR despite ↓RBF
- PGE₂, bradykinin, NO: Vasodilators → maintain RBF and GFR
- Sympathetic activation (severe hypovolemia): α₁ constriction → ↓RBF and ↓GFR
Filtration Barrier:
- Layers: Fenestrated endothelium (70–100 nm pores) → basement membrane (negatively charged) → podocyte foot processes (slit diaphragm — nephrin)
- Negatively charged barrier: Glycocalyx on endothelium and podocytes → repels negatively charged proteins (albumin is repelled → minimal filtration under normal conditions)
- Proteinuria: When barrier is damaged (e.g., nephrotic syndrome — loss of negative charge → increased albumin filtration)
Creatinine Clearance:
- CrCl ≈ GFR (creatinine is freely filtered, not reabsorbed, slightly secreted)
- Estimate GFR: Cockcroft-Gault equation (age, weight, gender, creatinine); MDRD or CKD-EPI formulas
- Normal: 90–120 mL/min/1.73m²; falls with age (~1 mL/min/year after 40)
⚡ Exam Tip for INI CET (AIIMS PG): In heart failure, ↓CO → ↓RPF → ↑angiotensin II → efferent arteriolar constriction → maintains GFR despite low RPF. ACE inhibitors interrupt this → can precipitate acute kidney injury (especially with bilateral renal artery stenosis or volume depletion). ACE inhibitors are beneficial in chronic heart failure but must be used cautiously in acute decompensation.