In biological terms, ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowman's capsule in the kidneys. The Bowman's capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries through a wide afferent arteriole and leaves through a narrower efferent arteriole. The blood pressure inside these capillaries is high because:

1. The renal artery contains blood at very high pressure which enters the glomerulus via the short afferent arteriole.
2. The efferent arteriole has a smaller diameter than the afferent arteriole.

The high pressure forces small molecules 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 nephron. This type of high pressure filtration is ultrafiltration. The fluid formed in this way is called glomerular filtrate.

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).

It is also used in hemodialysis to clean whole blood while keeping its composition 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 and albumin have practically no permeability at all.

Slow Continuous Ultrafiltration

Slow Continuous Ultrafiltration (SCUF) is an artificial method which approximately mimics the ultrafiltration function of the kidneys. SCUF is a continuous renal replacement therapy (CRRT) generally used to remove fluid from fluid overloaded patients suffering acute renal failure. During SCUF blood is removed from the body and is passed through an extracorporeal circuit through a hemofilter and a predetermined percentage of plasma water is removed based upon a prescription. Typically, no more than 2 liters an hour of fluid is removed. The remaining blood is returned to the patient. Unlike hemodialysis, hemofiltration and hemodiafiltration, no dialysate or replacement fluids are used in SCUF.

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