A knotted network of capillaries within a nephron, the glomerulus, receives arterial blood and is porous enough to allow much of the fluid portion of the blood to flow out of it along with smaller molecules such as vitamins, nutrients, and waste products. Every time a muscle is used creatinine is produced. In addition proteins are broken down mostly in the liver, and a by-product of this is urea. These materials enter the surrounding Bowman's capsule, thus moving into the nephron's tubule as a fluid called filtrate. At the nephron's next structure, called the proximal tubule, much of the original water and almost all the original nutrients are moved back into blood circulation. Waste products remain in the nephron tubule, however, because of their chemical composition. This general process continues over the length of the nephron tubule: water and nutrients move back into circulation, while waste products become ever more concentrated within the tubule. By the time the filtrate has reached the collecting duct, it has become urine. The body is able to control how much water the kidneys send to the bladder (in urine) or retain in circulation.