🧬 Glomerular Filtration Rate (GFR)

📑 Contents

❤️ The kidney – a vital organ

• Receives about 20% of cardiac output.
• Cardiac output ≈ 72 mL/beat × 72 beats/min ≈ 5200 mL/min.
• Renal blood flow ≈ 1‑1.2 L/min enters the kidney; ~90% goes to the cortex.
• Renal plasma flow = Renal blood flow × (1 – haematocrit) ≈ 500‑600 mL/min.
• Glomerular filtration rate (GFR) ≈ 20% of renal plasma flow (filtration fraction) = 100‑120 mL/min.
• The kidneys maintain a semi‑constant GFR through several autoregulatory mechanisms.

Forces involved in glomerular filtration (illustrative).

📊 Determinants of GFR

GFR depends on three main factors:

📈 Pressure gradient across the filtration barrier

• Glomerular hydrostatic pressure = 60 mmHg (promotes filtration).
• Hydrostatic pressure in Bowman’s capsule = 18 mmHg (opposes filtration).
• Colloid osmotic pressure of glomerular plasma proteins = 32 mmHg (opposes filtration).
• Net Filtration Pressure (NFP) = 60 – 18 – 32 = 10 mmHg.

Effect of glomerular hydrostatic pressure

Increased by (↑ GFR): ↑ arterial BP (slight, due to autoregulation), afferent arteriole vasodilation, moderate efferent arteriole vasoconstriction.

Decreased by (↓ GFR): afferent arteriole vasoconstriction, efferent arteriole vasodilation.

🧪 Effect of hydrostatic pressure in Bowman’s capsule

Increased by (↓ GFR): urinary obstruction (e.g., kidney stones), kidney oedema.

💧 Effect of colloid osmotic pressure of glomerular plasma proteins

Increased by (↓ GFR): dehydration, ↓ renal blood flow, severe efferent vasoconstriction.

Decreased by (↑ GFR): hypoproteinaemia, ↑ renal blood flow.

🕳️ Permeability of the filtration barrier

Three layers: capillary endothelium, basement membrane (negatively charged due to proteoglycans), and podocyte filtration slits. Damage to any layer increases filtration of proteins.

Filtration membrane surface area can be reduced in glomerular diseases (e.g., glomerulosclerosis), lowering GFR.

🔄 Autoregulation of GFR and renal blood flow

• Effective in blood pressure range 75–160 mmHg.
• If BP drops below 75 or rises above 160, GFR changes by ~10%.
• Mechanism: High BP → afferent arteriole constriction + efferent dilatation. Low BP → afferent dilatation + efferent constriction.

📌 Clinical relevance

Understanding GFR determinants is essential for:

• Interpreting eGFR changes in disease (diabetes, hypertension, glomerulonephritis).
• Managing drug dosing (e.g., metformin, contrast media).
• Recognising causes of acute kidney injury (pre‑renal, intrinsic, post‑renal).