This is a filtering unit that encloses a network of 'leaky' capillaries called the glomerulus. The walls of the Bowman's capsule are made from specialised epithelial cells called podocytes. Podocytes have finger-like extensions called pedicels that wrap around the capillaries and form slits to allow blood plasma from the glomerulus to pass through to the lumen of the Bowman's capsule. A basement membrane creates a fine mesh of collagen and glycoproteins to ensure molecules with a relative molecular mass of greater than 69,000 do not get through. The process of filtration that takes place in the Bowman's capsule is called ultrafiltration.