Ultrafiltration (kidney)

The high hydrostatic pressure forces small molecules in the tubular fluid such as water, glucose, amino acids, sodium chloride and urea through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman's capsule and into the renal tubules. This process is called ultrafiltration; the resulting fluid, virtually free of large proteins and blood cells, is referred to as glomerular filtrate, or ultrafiltrate. Further modification of ultrafiltrate, by reabsorption and secretion, transforms it into urine.

Glomerular pressure is about 75 millimeters of mercury (10 kPa). It is opposed by osmotic pressure (30 mmHg, 4.0 kPa) and hydrostatic pressure (20 mmHg, 2.7 kPa) of solutes present in capsular space. This difference in pressure is called effective pressure (25 mmHg, 3.3 kPa).

In hemodialysis centers, ultrafiltration takes place in a hemofilter on the hemodialysis machines, when the blood pressure is greater than the dialysate pressure (difference = transmembrane pressure (TMP)). This removes fluid from the blood while keeping its blood cells intact.

Selectivity

The structures of the layers of the glomerulus determine their permeability-selectivity (permselectivity). For instance, small ions such as sodium and potassium pass freely, while larger plasma proteins, such as hemoglobin tetramers, haptoglobin bound hemoglobin and albumin have practically no permeability at all. Also, negatively charged molecules will pass through far less frequently than positively charged ones.