Determinants of GFR

GFR Fundamentals - The Filtration Formula

• GFR is governed by Starling forces across the glomerular filtration barrier.
• Formula: $GFR = K_f \times [ (P_{GC} - P_{BS}) - (\sigma (\pi_{GC} - \pi_{BS})) ]$
  • $K_f$: Filtration coefficient (surface area × conductivity).
  • $P_{GC}$: Glomerular capillary hydrostatic pressure (favors filtration, ~55 mmHg).
  • $P_{BS}$: Bowman's space hydrostatic pressure (opposes, ~15 mmHg).
  • $\pi_{GC}$: Glomerular capillary oncotic pressure (opposes, ~30 mmHg).
  • $\pi_{BS}$: Bowman's space oncotic pressure (favors, normally 0).
  • $\sigma$: Reflection coefficient (normally 1).

⭐ $P_{GC}$ is the most important physiological regulator of GFR. It is directly influenced by the tone of the afferent and efferent arterioles.

Starling's Forces - Pressure Cooker Crew

Governs fluid movement across glomerular capillaries. GFR is the product of the filtration coefficient ($K_f$) and net filtration pressure (NFP).

• Formula: $GFR = K_f \times [(P_{GC} - P_{BS}) - \pi_{GC}]$
  • $K_f$: Filtration coefficient (capillary permeability & surface area).
  • $P_{GC}$: Glomerular capillary hydrostatic pressure (favors filtration, ~55 mmHg). The primary determinant of GFR.
  • $P_{BS}$: Bowman's space hydrostatic pressure (opposes filtration, ~15 mmHg).
  • $\pi_{GC}$: Glomerular capillary oncotic pressure (opposes filtration, ~30 mmHg).

📌 Mnemonic: Pressure Cooker Crew = Pressures in the Glomerular Capillaries.

⭐ During dehydration, ↑ plasma protein concentration leads to ↑ $\pi_{GC}$, which ↓ GFR. Conversely, in nephrotic syndrome, ↓ plasma protein concentration ↓ $\pi_{GC}$, leading to an ↑ in GFR.

Arteriolar Resistance - The Push-Pull Levers

• Afferent Arteriole (AA): The "inflow" tap. Its resistance primarily alters Renal Plasma Flow (RPF).

  • Constriction (e.g., NSAIDs, Norepinephrine): ↓ RPF, ↓ Glomerular Capillary Pressure ($P_{gc}$), ↓ GFR.
  • Dilation (e.g., Prostaglandins, ANP): ↑ RPF, ↑ $P_{gc}$, ↑ GFR.

• Efferent Arteriole (EA): The "outflow" drain. Its resistance creates back-pressure to maintain GFR.

  • Constriction (e.g., Angiotensin II): ↓ RPF, ↑ $P_{gc}$, ↑ GFR (initially). Severe constriction eventually ↓ GFR.
  • Dilation: ↑ RPF, ↓ $P_{gc}$, ↓ GFR.

• Filtration Fraction (FF): The proportion of plasma filtered. $FF = GFR / RPF$.

  • EA constriction causes a significant ↑ FF.

⭐ In low-flow states (e.g., dehydration, heart failure), prostaglandins are vital for afferent dilation to preserve GFR. NSAID use in these settings blocks this protection, risking acute kidney injury.

Filtration Fraction - GFR's Efficiency

• Definition: The fraction of renal plasma flow (RPF) that is filtered across the glomerular capillaries; a measure of filtration efficiency.
• Formula: $FF = GFR / RPF$
• Normal Value: ~20%.
• Regulation:
  • ↑ Angiotensin II (preferential efferent constriction) → ↑ FF.
  • ↓ RPF (e.g., renal artery stenosis) → ↑ FF to maintain GFR.

⭐ ACE inhibitors prevent efferent arteriolar constriction, thus ↓ FF. This can precipitate acute renal failure in patients with bilateral renal artery stenosis.

High‑Yield Points - ⚡ Biggest Takeaways

• GFR is primarily driven by glomerular capillary hydrostatic pressure (P_GC), which forces fluid into Bowman's capsule.
• Afferent arteriole constriction (e.g., NSAIDs) ↓ RPF and ↓ P_GC, causing a ↓ GFR.
• Efferent arteriole constriction (e.g., Angiotensin II) ↑ P_GC and ↑ Filtration Fraction, initially increasing GFR.
• Increased plasma protein concentration (e.g., multiple myeloma) ↓ GFR by increasing capillary oncotic pressure.
• Ureteral obstruction increases Bowman's capsule hydrostatic pressure, directly opposing filtration and thus ↓ GFR.