Glomerular Filtration

Question 1

Glomerular ultrafiltration

Answer 1

Filtration in the kidney occurs in the glomerulus, where a ‘semipermeable membrane’ separates the cells and plasma in the capillaries from the ‘filtrate’ which forms in the Bowman’s space. The filtration involves bulk flow, rather than strictly due to diffusion.

The two ‘forces’ which drive the flow are identical to those determining movement in all capillary beds:

Hydrostatic: higher hydrostatic pressure (50 mmHg) in the capillaries drives fluid out; much higher than in most capillaries.

Osmotic/oncotic: higher osmotic pressure in the capillaries (particularly due to plasma proteins) impedes the flow.
The osmotic force increase along the length of the capillaries, but equilibrium is not normally achieved (in people).

Question 2

Osmotic and Oncotic pressure

Answer 2

The total osmotic pressure of plasma is very high; that due to the proteins is very low. However, most of the constituents of plasma are of low molecular mass and size, and hence of equal distribution across the glomerular capillaries, it is the osmotic pressure of the proteins (the oncotic pressure) that affects the driving force.
Osmotic pressure can be estimated by one form of the Morse equation (derived from the van’t Hoff equation):
Osmotic pressure = nCRT
Where nC is the osmolality (0.28osm.kg-1), R=ideal gas constant (0.082 L.atm.mol-1.K-1), T=temperature (310 K).