The urinary system controls the composition, volume and pressure of the blood by removing and restoring water and solutes. Its components are the two kidneys, two ureters, the urinary bladder and urethra. The kidneys eliminate nitrogenous waste materials from the blood, as well as inorganic salts, heat and carbon dioxide. Waste materials, if allowed to remain in the body, become toxic. Blood is pressed through a unique filtration system in the kidneys, where its cells and fluids are separated. Most of the fluids are then returned to the blood, leaving waste products and water behind. This urine is then channelled into the urinary bladder, a holding vessel that must be emptied through the urethra when it reaches a certain capacity. The urethra has a sphincter muscle, which controls urination. If this muscle becomes weak or damaged [as sometimes happens during childbirth], incontinence, the inability to hold urine in the bladder, or occurs. Besides detoxification, the kidneys control the amount of water and minerals leaving the body. Every hour, your kidneys remove up to 1 1/2 gallons of liquid from your blood. This liquid is filtered and any useful substances are returned to the blood. The leftover liquid and waste products are expelled from the body. They also regulate blood pressure and help maintain the acid-alkaline balance in the blood. For this reason, chronic kidney disease in particular has serious implications for the rest of the body. The functional part of the kidney, where filtration, secretion and reabsorption take place, is the nephron. If one kidney is removed, the other will enlarge, and can function at 80% capacity of two kidneys.