Tubular reabsorption allows the body to reclaim any needed materials from the kidneys after glomerular filtration. Glomerular filtration is the process of forcing water, salts, urea and glucose from the blood through a membrane via pressure. Most of the materials filtered by the kidneys are returned to the blood, depending on the body's needs.
The initial filtering process of the kidneys is rather imprecise and claims a large amount of materials in excess of the actual wastes. The materials that enter the kidneys or stay in the blood are selected simply based on size. Thus, large particles such as blood plasma proteins and red blood cells do not pass into the kidneys, while water and smaller solutes do.
Tubular reabsorption occurs, as the name implies, in the renal tubules. Depending on the materials involved, it is accomplished via diffusion and active transport. Sodium ions, for instance, require active transport, and indeed moving sodium back into the blood constitutes most of the kidney's energy use. Water, a large majority of the filtered materials in the kidneys, follows the sodium ions out through osmosis.
The vast majority of the materials the kidneys absorb is released back to the body. Only 1 percent of the filtered materials are actually excreted through urine.