Ultrafiltration is a filtration process in chemistry that can retain solutes with relative molecular masses in the order of 100s to 1000s.
It is also used in haemodialysis to clean whole blood while keeping its composition intact. In biological terms this 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 glomerus. 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 i.e. 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. Large molecules such as plasma proteins and blood cells do not pass through the filter because they are too big. glomerular pressure is about 75 mm of mercury.It is opposed by osmotic pressure(30 mm of mercury) and hydroststic pressure(20 mm of Hg) of solutes present in capsular space. this difference in pressure is called effective pressure(25 mm of Hg).