Ultrafiltration is the process of filtration on a molecular level. Solvent and some solutes are forced across a selectively permeable membrane. This process is usually specific to particle size, but may also be selective to other characteristics, such as charge. Ultrafiltration is important in the production of filtrate, which becomes urine in the nephron, where blood in the glomerulus is ultrafiltrated into the Bowman's capsule.