Expansion
Glomerular capillary pressure drives filtration; oncotic and capsular pressures oppose it
Expansion
Net filtration pressure = glomerular capillary hydrostatic pressure - (Bowman’s capsule hydrostatic pressure + glomerular capillary oncotic pressure)
Typical values:
- Glomerular capillary hydrostatic pressure: 45 mmHg, unusually high and maintained along the capillary
- Bowman’s capsule hydrostatic pressure: 10 mmHg
- Capillary oncotic pressure: 25 mmHg, rising to about 35 by the efferent end
- Net: about 10 mmHg at the afferent end, falling towards zero distally
There is normally no oncotic pressure in Bowman’s space, since the filtrate is protein free.
Clinical applications follow directly:
- Ureteric obstruction raises capsular pressure and reduces filtration
- Hypoalbuminaemia lowers oncotic pressure and transiently raises filtration
- Hypotension lowers capillary pressure, the commonest cause of prerenal failure
GFR = filtration coefficient x net filtration pressure, and the coefficient itself falls when glomeruli are lost or the barrier is damaged.