The renal tubules are part of the kidneys and the main job of the kidneys is to produce urine. Under normal circumstances, the production of urine by the kidneys relies on the glomeruli and tubules working together. A glomerulus and a tubule make up a renal unit, and one of our kidneys contains one million renal units. The glomerulus is a network of capillaries through which blood passes, and this capillary filters out metabolic wastes and excess water from the blood, as well as some nutrients, which is called the initial urine. The initial urine is so large that it needs to be reprocessed through the renal tubules, which, through the functions of dilution and concentration, ultimately produce the final urine, which is the urine that we excrete.