The kidneys consist of roughly one million tiny filtering units called nephrons, which contain a filter, known as a glomerulus, and a tubule. The kidneys filter between 120 and 150 quarts of blood each day, producing urine, which contains excess fluid and waste.The urine then travels to the bladder before being expelled from the body.
In each nephron, the glomerulus allows fluid and most waste products to pass through it, while stopping blood cells and most protein from getting through. The fluid then passes through the tubule, which removes waste and sends needed minerals, such as sodium, potassium and phosphate, back to the body. The tubules also regulate the amount of fluid the kidneys retain, thus helping to maintain the stable composition of blood.
Urine contains the waste removed by the kidneys such as toxins and by-products of metabolism and fluid. From the tubules, it travels to the bladder, which fills until it sends a signal to the brain that tells the person he needs to urinate.
The kidneys also produce hormones that help to regulate blood pressure and stimulate the body's creation of red blood cells. The filtering process that occurs in the kidneys also creates a form of vitamin D, which helps develop and maintain healthy bones.