As already explained elsewhere, the epithelial structure of the glomerulus is ideally suited for ultrafiltration of plasma. Each glomerulus is composed of 20 to 40 capillary loops with a surface area of around 15,000 cm2/100g. A high ultrafiltration coefficient (product of the filtration surface area and the hydraulic permeability), and a net outward filtration pressure, results in the ultafiltration of relatively large volumes of plasma (180L/day).
The filtration slits of the glomerulus are about 250A. While the size of these slits can partially explain the selectivity of the glomerulus, it does not explain why almost no albumin (size, 36A) is filtered through the glomerulus. The answer to this question lies in the fixed negative charge (probably at the basement membrane) that does not permit relatively large, negatively charged molecules (especially proteins) to pass through.