Podocytes are cells in the Bowman's capsule of the kidneys, which wrap around the capillaries of the glomerulus. Long feet-like projections of the podocytes leave filtration slits between the leaky capillaries and Bowman's space, explains Wikipedia. As the Bowman's capsule filters blood, large molecules, such as proteins, are held back and small molecules, such as water, salts and amino acids, move through filtration slits to begin the formation of urine.
As explained by Richard Ransom of the University of Michigan, one of the most important functions of the kidneys is the filtration of blood. The kidneys must excrete fluid and waste products, while retaining blood cells and proteins within the bloodstream.
The first functional unit in this process is the glomerulus, which is a network of leaky capillaries surrounded by Bowman's capsule. The capillary walls are the barrier between the bloodstream and the urinary space of Bowman's capsule, and the podocyte processes maintain the filtration barrier. As described by Ransom, the podocytes of the glomerulus have a cell body, major processes that extend outward and distal foot processes. Neighboring foot processes integrate to form a network of filtration slits. Podocytes are also involved in the regulation of the glomerular filtration rate, or GFR. By contracting, podocytes restrict the filtration slits and decrease GFR.