Ultra filtration is a process in the kidney by which urea, salt, water, and glucose etc. is extracted from the blood. The blood flows through the glomerulus under great pressure which is much greater than in the capillaries elsewhere. The reason for this greater pressure is that the efferent arteriole is narrower than the efferent arteriole. This high pressure causes the liquid part of the blood to filter out from the glomerulus into the renal tubule. This filtration under extraordinary force is called ultra filtration.

The mechanism of occurrence:

● When blood passes through the top of the nephron, it enters a structure called the glomerulus (Bowman’s capsule) 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 by the process of selective re-absorption.

Explanation: Blood flows into the glomerulus from the afferent arteriole, which is wider in diameter than efferent article. This difference in diameter ensures the blood in the capillaries is under increased pressure. The high-pressure forces liquid and small molecules out into the Bowman’s capsule. The endothelium of the capillaries has small gaps to ensure substances can pass through. The basement membrane is made of glycoprotein’s which ensure large proteins cannot pass through-it is a selective barrier. Epithelial cells of the Bowman’s capsule called podocytes to have finger-like projections which ensures fluid can pass into the lumen of Bowman’s capsule. Once the blood is filtered in this way only blood cells and large plasma proteins remains in the blood and continue into the proximal convoluted tubule.