The kidneys filter blood through a two-step process. Blood first enters a filter called the glomerulus, where excess fluid and waste products are redirected into the second area of filtration, known as the tubule. The tubule extracts any needed minerals that make it through the first filter and sends them back into the bloodstream, while the final product emerges from the tubule as urine.
The glomerulus and the tubule are two parts of a larger filtration unit called the nephron. The nephron is the working unit of the kidney. Each kidney contains up to a million nephrons, which filter a combined 120 to 150 quarts of blood a day, creating one or two quarts of urine. The glomerulus prevents larger molecules such as red blood cells and proteins from passing into the tubule, sending only fluid, waste products and smaller mineral molecules into the second phase of the process. Once urine leaves the kidneys, it passes through a tube of muscle called the ureter and into the bladder, where it is stored until the bladder is full. Every hour, the entire blood supply of the body comes down the renal artery and passes through the kidneys about 12 times. The kidneys also help control blood pressure and regulate the balance of salts and acids in the bloodstream.