GFR — Determinants & Measurement

GFR

GFR:

• “The amount of glomerular ultrafiltrate formed divided by the time of filtration”
• Normal value is approx 125 mL/min or 180 L/day
• Renal blood flow 1.25l/min in 70kg male → Filtration fraction 0.2

Functional anatomy:
3 Distinct Layers:

• glomerular capillary endothelium
  • highly specialised endothelium with fenestrations to ↓ filter thickness
  • prevents cellular components of blood from coming into contact with BM
• glomerular BM
  • made of CT; -vely charged
  • acts as filter
• bowmans epithelial cells (podocytes)
  • epithelial cells with foot processes → large SA
  • negatively charged
  • maintain BM + phagocytic functions

Direct GFR Determinants

The GFR (Net flux across the membrane) is the balance of hydrostatic pressure and oncotic pressure, as defined by the Classic Starling Equation.

where
K_f = Filtration coefficient
P = hydrostatic pressure
π = oncotic pressure
σ = Staverman’s reflection coefficient ie. Permeability of membrane to protein

• Normally NFP = 17mmHg
• Tubular oncotic pressure = zero throughout
• GC oncotic pressure varies from 21 to 33 mmHg as filtrate is removed.
• Thus less GFR produced at distal end of tubule.

Factors influencing GFR

Circulating Factors

• Prostaglandins
  • PGI₂ and PGE₂ → vasodilates → ↑ renal blood flow → ↑ GFR
  • ↓ PGE2, PGI2 and NO levels → inhibits afferent arteriolar vasodilation → ↓ GFR
• Long term → ↓ renin release → ↓ ATII-induced efferent arteriolar vasoconstriction → ↓ GFR
• Noradrenaline / Adrenaline
  • constrict renal afferent and efferent arterioles → ↓ renal blood flow and reducing filtration → ↓GFR
  • Constrict mesangial cells to → ↓ GFR.
• Mesangial cell tone:
  • Angiotensin 2, Endothelin, vasopressin → ↓ glomerular surface area and GFR
  • ANP, PGE2, Dopamine and cAMP → all increase GFR

Solute factors

• Size
  • < 7 kDa (Eg. glucose, ions, urea, H2O) filtered freely,
  • > 70 kDa (Eg. albumin) are not filtered;
  • neutral particles < 4 mm diameter filtered freely, > 8 mm are excluded
• Charge
  • Negatively charge (Anionic) particles repulsed
  • Cationic substances filtered more readily
• Protein binding
  • Albumin excluded

Disease Factors

• Filtration decreases
  • Shock → decreased glomerular pressure
  • Obstruction → increased bowman’s capsule hydrostatic pressure
  • Hypoproteinaemia → hepatic failure, nephrotic syndrome

JC / Gladwin / Sakurai / Kerr 2020

GFR Measurement

Clearance:

• Renal clearance = vol of plasma completely cleared of a given substance by the kidneys per unit time (ml/min)
• Involves: glomerular filtration, secretion, reabsorption, and rarely tubular metabolism
• Renal clearance = V x [U]/[P]
  • V = volume of urine or urine flow rate in ml/min
  • [U] = urinary concentration of substance in mg/ml
  • [P] = plasma concentration of substance in mg/ml
    

Estimating GFR:

• GFR = renal clearance of a substance if it is:
  • Freely filtered at glomerulus
  • Not secreted
  • Not reabsorbed
  • Not synthesized
  • Not metabolised
• The amount excreted in the urine = amount filtered
• i.e. [plasma] x GFR = [urine] x urine vol
• Rearrange: GFR = urine vol x [urine] / [plasma]

Substances used to estimate GFR:

• Criteria
  • easily measured in urine + plasma
  • non-toxic
  • easily administered
• Inulin
  • Protein; polymer of fructose
  • MW 5800 Da
  • freely filtered
  • Not reabsorbed, secreted, metabolised, or synthesized by the kidney
  • Therefore clearance of inulin = GFR
  • Requires continuous infusion for several hours to achieve steady state
• Creatinine
  • used to approximate GFR as is more practical
  • Released at a steady state from skeletal muscle cells (phosphocreatine)
  • Freely filtered + not reabsorbed
  • *Small amount secreted → overestimates GFR by small amount
  • Cockcroft Gault formula used

JC / Gladwin / Sakurai / Kerr 2020

Answer by Gladwin, JC, Kerr, Sakurai