Why do we need the tubule system in our kidneys?

The most important factor determining whether something will be filtered out the glomerulus is its size. Charge is also important, but size is critical. Because the glomerulus is allowing filtration based on size, waste products will be removed from the blood, but so will essential ions and molecules. Larger molecules such as proteins and the even larger blood cells won't pass through a normal glomerular capillary. We need the tubule system in order to adjust the excretion of essential ions and molecules. The tubules can reabsorb ions and molecules that have been filtered, and they can adjust the amount of reabsorption to match the body's requirements.