Ultrafiltration is how urea, salt, water and glucose are taken out of the blood in the kidneys. The things that are filtered out need to be returned to the blood through a process called selective reabsorption.
The kidneys filter the waste out of the blood. They are about the size of a fist and located below the rib cage. One kidney is on each side of the spine. The kidneys filter about 120 to 150 quarts of blood to produce about 1 to 2 cups of urine each day. They prevent the buildup of wastes, keep electrolytes stable and make hormones that help regulate blood pressure and make red blood cells. Each kidney is made up of nephrons.