The basic function of the kidney is the formation of urine for elimination through the urinary excretory system. Two different processes determine this formation: the filtration of fluid through the glomerular capillaries into Bowman's space and the modification of the volume and composition of the glomerular filtrate in the renal tubules. The fluid passes from the glomerular capillaries to Bowman's capsule due to the existence of a pressure gradient between these two areas. This process is favored by two structural characteristics that make renal corpuscles particularly effective filtration membranes: glomerular capillaries have a much higher number of pores than other capillaries, and the efferent arteriole has a smaller diameter than the afferent arteriole, causing greater resistance to outflow of blood flow from the glomerulus and increasing glomerular hydrostatic pressure. Increased glomerular hydrostatic pressure (due to increased blood flow through the glomerulus) increases filtration, while increases in Bowman's hydrostatic pressure or urinary space (which remains constant, unless there is disease at that level, usually due to fibrosis) and plasma P. oncotic (determined by proteins, which tend to "drag" plasma into the glomerulus) decrease filtering. Resulting in a filtering pressure of 10 mmHg.
