What features of the nephron allow ultrafiltration to take place in the kidneys?

The glomerulus is a knot of cappilaries inside the Bowman's capsule. The afferent arteriole (small blood vessel leading into the glomerulus) has a much larger diameter than the efferent arteriole (small blood vessel leaving the glomerulus). This creates a build up of pressure, forcing the small molecules in the plasma like water and urea out of the glomerulus and into the Bowmans capsule via podocytes. Podocytes are cells that have 'foot-like extensions' which are wrapped around a capillary. This increases the surface area for filtration.