Glomerular structure and function

Glomerular Filtration Barrier - The Kidney's Sieve

The filtration barrier is selectively permeable, determined by size and charge.

• Three Layers:

  • Fenestrated capillary endothelium: Blocks cells (RBCs, platelets). First size barrier.
  • Glomerular basement membrane (GBM): Fused basal laminae with Type IV collagen and heparan sulfate (negative charge). Main charge barrier.
  • Podocyte foot processes (visceral layer): Slit diaphragms between pedicels. Final size barrier.

• Selectivity:

  • Size Barrier: Restricts molecules > 70 kDa (e.g., albumin).
  • Charge Barrier: Repels anionic molecules (e.g., albumin) due to negative charges in the GBM.

⭐ In Alport syndrome, a mutation in Type IV collagen disrupts the GBM, leading to hematuria and progressive renal failure. 📌 Mnemonic: "Can't see, can't pee, can't hear a bee."

Starling Forces - Pressure Cooker Physics

Net filtration pressure (NFP) dictates glomerular filtration, governed by opposing hydrostatic and oncotic pressures.

• Outward Force (Favors Filtration)
  • Glomerular Hydrostatic Pressure (PGC): Blood pressure in glomerular capillaries. The primary driver of filtration. (≈ 55 mmHg)
• Inward Forces (Oppose Filtration)
  • Bowman's Capsule Hydrostatic Pressure (PBC): Fluid pressure in Bowman's space. (≈ 15 mmHg)
  • Glomerular Oncotic Pressure (πGC): Osmotic pull from proteins in capillary blood. (≈ 30 mmHg)

Filtration occurs because PGC > (PBC + πGC).

Net Filtration Pressure (NFP) is calculated as: $NFP = P_{GC} - (P_{BC} + oldsymbol{\pi}_{GC})$

⭐ GFR is directly proportional to the Net Filtration Pressure. Any factor that alters Starling forces (e.g., ↓ RBF, ureteric obstruction) will directly impact GFR.

GFR & Clearance - Bean Counter's Math

• Glomerular Filtration Rate (GFR): Volume of plasma filtered per unit time.

  • Normal GFR ≈ 100-125 mL/min.
  • Gold standard for measurement is Inulin clearance because it is freely filtered and is not reabsorbed or secreted.

• Renal Clearance ($C_x$): Volume of plasma completely cleared of a substance (X) per unit time.

  • Formula: $C_x = (U_x \times V) / P_x$
  • $U_x$ = urine concentration, $V$ = urine flow rate, $P_x$ = plasma concentration.

• Interpreting Clearance:

  • $C_x < GFR$ → Net tubular reabsorption.
  • $C_x > GFR$ → Net tubular secretion.
  • $C_x = GFR$ → No net reabsorption or secretion (Inulin).

⭐ A doubling of serum creatinine (e.g., from 1 to 2 mg/dL) suggests an approximate 50% reduction in GFR.

• 📌 Creatinine is Casually Secreted, so its Clearance slightly overestimates GFR.

GFR Regulation - Tapping the Brakes

• Myogenic Mechanism: Intrinsic reflex of the afferent arteriole to changes in blood pressure.
  • ↑ Stretch from high BP → arteriolar vasoconstriction → ↓ RBF & GFR.
  • ↓ Stretch from low BP → arteriolar vasodilation → ↑ RBF & GFR.
• Tubuloglomerular Feedback (TGF): Macula densa cells in the JGA sense distal tubular flow.
  • ↑ GFR → ↑ NaCl delivery to macula densa → release of adenosine → afferent arteriole vasoconstriction → ↓ GFR.

⭐ NSAIDs inhibit prostaglandins, which normally dilate the afferent arteriole. This causes unopposed constriction, leading to a ↓ GFR, especially in CKD or heart failure patients.

High‑Yield Points - ⚡ Biggest Takeaways

• The glomerular filtration barrier has three layers: fenestrated endothelium, the glomerular basement membrane (GBM), and podocyte foot processes.
• A negative charge barrier, from heparan sulfate in the GBM, is crucial for repelling proteins like albumin.
• The barrier is also size-selective, restricting molecules larger than ~70 kDa.
• Podocyte effacement (flattening) is a key pathologic finding in nephrotic syndrome.
• Mesangial cells provide structural support, control filtration area, and perform phagocytosis.