Walls of glomerular capillaries and Bowman’s capsule are very thin and are semipermeable due to the presence of pores in the former and slit-pores in the latter. They allow water and small molecules in the blood to pass through them. Fluid containing these materials is forced out of the glomerular capillaries into the Bowman’s capsule by the high pressure of the blood in the glomerular capillaries.

This pressure is about 70 mm Hg in man. The fluid tends to move in the reverse direction due to (i) the osmotic pressure of plasma proteins in the glomerular capillaries, and (ii) hydrostatic pressure of the fluid in the urinary tubule. These pressures in man are about 30 mm Hg and 20 mm Hg respectively. The net force moving the fluid from the glomerular capillaries, called the filtration pressure, is 70 – (30

• 20) or 20 mmHg.