What is the Glomerular Filtration Rate (GFR)? Discuss the physiological factors that can influence it.
Generally this question was well answered. Almost all answers gave the correct value for GFR
and the correct formula for Starling’s forces across the glomerular membrane.
Better answers discussed the physiological factors affecting each force (hydrostatic and
osmotic) across the membrane in a stepwise logical manner.
Some answers discussed the control of renal blood flow; this was not expected and therefore
was not rewarded.
If this question was well answered, how come 60% of the candidates fail it?
Glomerular filtration rate (GFR) is the sum of the ultrafiltrate produced by all nephrons
- This is about 20% of renal blood flow, which itself is 20% of cardiac output
- Therefore normal GFR = about 200ml/min, or 90-120 ml/min/1.73m2
Glomerular ultrafiltration is described by the Starling equation:
GFR = Kf [ (Pgc - PBC) - σ(Πgc - Πi) ]
- GFR is the glomerular filtration rate,
- Kf is the filtration coefficient of the glomerular filtration surface,
which is itself a product of:
- k, the hydrostatic permeability constant of the membrane, and
- S, the surface area of the glomerular filtration surface, which can be affected by the contraction of glomerular mesangial cells
- Pgc is the glomerular capillary hydrostatic pressure
- PBC is the hydrostatic pressure of fluid in Bowman's capsule
- σ is the reflection coefficient for blood protein
- Πgc is the oncotic pressure in the glomerular capillary blood,
- ΠBC is the oncotic pressure of the fluid in Bowman's capsule (usually zero)
Glomerular filtration of solutes is affected by:
- Molecule size: glomerular size barrier resists the passage of large molecules (>7000 Da)
- This size barrier consists of:
- Endothelial glycocalyx (mainly filters proteins)
- Endothelial fenestration (60-80 nm, mainly filter cells)
- Glomerular basement membrane (gel diffusion barrier, filters molecules by size, variable)
- Podocyte filtration slits (3-4 nm, mainly filters proteins)
- This size barrier consists of:
- Molecule charge: anionic charge of the glomerular filtration surface may resist the passage of anionic molecules
- This consist of probably only the endothelial glycocalyx and the glomerular basement membrane, both of which contain a large amount of heparan sulfate proteoglycans)
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