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A podocyte is a special type of cell found inside the kidney. These cells make up the outer wall of Bowman's capsule, which encloses a knot of extremely tiny blood vessels known as a glomerulus. There are many Bowman's capsules and many glomeruli inside the kidneys, and blood is filtered through them in the initial stages of urine formation. Blood filters through holes in the cells lining the glomerular blood vessels and then through the layer of podocytes which surrounds the blood vessels. Each cell is specially shaped so that, when the cells fit together, they create slits which form part of the blood filter.
The podocyte has a rounded cell body which has several long, arm-like projections coming from it. These are known as primary processes, and numerous slender feet, called secondary processes or pedicels, project from them. These cells fit together so that the pedicels of one podocyte slot into the gaps between the pedicels of another, like two halves of a zipper. When blood is filtered through the glomerulus and into Bowman's capsule, only small molecules and water are able to pass through the leaky blood vessel lining and the cell layer. Blood cells and proteins are too large and they remain behind.
As well as forming part of the blood filtering process, the podocyte cell is also thought to be involved in keeping the filter clean, although it is not known exactly how it does this. Blood inside the glomerulus is at high pressure, and these cells help support the blood vessel walls and prevent them from expanding. A number of conditions are associated with podocyte damage, including diabetes mellitus and kidney diseases. When cells are injured and deformed, this can mean that they no longer form filtration slits correctly and protein molecules may pass into the urine. If these cells are lost, or if they multiply until there are too many, this can affect the structure of the glomerulus, causing it to degenerate or collapse.
After entering Bowman's capsule through its outer layer of podocyte cells, the filtered blood, which is known as filtrate, travels through a long tube that loops through different parts of the kidney. Various waste substances are added and useful ones are removed until the fluid known as urine is formed. The function of the kidney is to remove waste and toxic particles from the body, and these are the substances that end up leaving the body during urination.