What is ultrafiltration?

Ultrafiltration is a process in the kidney by which urea, salt, water and glucose etc. is extracted from the blood. When blood passes through the top of the nephron, it enters a structure called the glomerulus which is a network of tiny capillaries. This causes the pressure to increase and fluid is forced through the "sieve-like" walls of the vessels into the Bowman's capsule. This fluid is called the filtrate. Blood cells and larger proteins do not pass through the capillaries as they are too large and so are not found in the filtrate. A large proportion what enters the filtrate is valuable to the body and so needs to be reabsorbed into the blood. This is a process called selective reabsorption.