2005B10 Describe
the forces acting across the glomerular capillary membrane.
Explain how afferent and efferent arteriolar tone affect glomerular filtration
rate.
List:
· Corpuscular anatomy
· Starling hypothesis
· Starling forces at the renal corpuscle
· Effect of renal arteriolar tone
Corpuscular anatomy:
|
Components |
· Capillary tuft with afferent and efferent arterioles either side · Bowman’s capsule (epithelium) · Bowman’s space in between |
|
Barrier |
· Layers between blood and filtrate o Glycocalyx o Endothelium o Basement membrane (highly negatively charged) o Podocyte foot processes with slit diaphragms |
Starling hypothesis:
|
Traditional |
· Filtration rate governed by balance of hydrostatic and oncotic pressures either side of the membrane |
|
Revised |
· Subglycocalyceal space (normally protein-free) replaces interstitial space · Glycocalyx is a layer of proteoglycans and glycosoaminoglycans · Disrupted by inflammation, ↑mAP, ↑glucose, ANP · Protected by corticosteroids, antithrombin 3, NAC, volatile anaesthetics · (I don’t know the significance of the glycocalyx at the glomerulus) |
Starling forces:
|
· GFR = Kf[(PGC – PBC) – σ(πGC – πBC)] |
|
|
Filtration coefficient (Kf) |
· = GFR per mmHg filtration pressure · ∝ surface area x hydraulic conductivity · Very high at glomerulus · ↓ Kf -> ↓GFR (e.g. nephrectomy -> ↓surface area) |
|
Staverman reflection coefficient (σ) |
· Corrects actual oncotic pressure difference to observed effect · ∝ 1 / (permeability to protein) · Usually ~1 at glomerulus, ~0 at liver sinusoid · ↓ σ -> ↑↑GFR (e.g. nephrotic syndrome) |
|
Glomerular capillary hydrostatic pressure (PGC) |
· ↑PGC -> ↑GFR (e.g. ↑↑mAP) |
|
Bowman’s space hydrostatic pressure (PBC) |
· ↑PBC -> ↓GFR (e.g. ureteric obstruction) |
|
Plasma oncotic pressure (πGC) |
· ↓ πGc -> ↑GFR (e.g. ↓albumin |
|
Bowman’s space oncotic pressure (πBC) |
· ↑πBC -> ↑GFR (normally zero) |
Starling forces at the renal corpuscle:
|
|
Afferent arteriolar end |
Efferent arteriolar end |
|
PGC |
60 |
58 |
|
PBC |
15 |
15 |
|
πGC |
21 |
33 |
|
πBC |
0 |
0 |
|
Net |
+24 |
+10 |
i.e. predicts net filtration across the length of the capillary
Effect of renal arteriolar tone:
|
Afferent |
|
|
Efferent |
|
|
Hormonal effects |
· Angiotensin 2: constrict E>A -> ↑GFP ± GFR · Sympathetic nervous system: constrict E>A -> ↑GFP ± GFR · Atrial natriuretic peptide: dilate A&E -> ↑GFR · Cortisol: dilate A&E -> ↑GFR (?) |
