Physical Forces Contributing to the Glomerular Filtrate  

 As in other instances of ultrafiltration, the rate of ultrafiltration is influenced by inherent factors in the filtration membrane (surface area and permeability) as well as pressure differences between the two sides of the membrane. These different pressures consist of the oncotic and hydosatatic pressures.

In the glomerulus the following are the pressures present and their contribution towards filtration:

1. Mean hydrostatic pressure in the capillary tuft - higher than in normal capillaries as capped by two arterioles. Enhances ultrafiltration
2. Mean colloid pressure in the capillary tuft Resists ultrafiltration
3. Mean hydrostatic pressure in the Bowman's Capsule Resists ultrafiltration
4. Mean colloid pressure in the Bowman's Capsule - normally zero (no proteins present) Enhances ultrafiltration


In real terms ultrafiltration at the glomerulus is the result of the difference in the hydrostatic pressure between the capillary and the Bowman's capsule (usually positive towards filtration, 55mmHg-15mmHg = 40mmHg) and the capillary colloid hydrostatic pressure (usually negative towards filtration, 30mmHg). Thus a net filtration pressure of 10mmHg exists that favours movement of fluid from the capillaries to the Bowman's capsule.