<P> Cortical nephrons (the majority of nephrons) start high in the cortex and have a short loop of Henle which does not penetrate deeply into the medulla . Cortical nephrons can be subdivided into superficial cortical nephrons and midcortical nephrons . </P> <P> Juxtamedullary nephrons start low in the cortex near the medulla and have a long loop of Henle which penetrates deeply into the renal medulla: only they have their loop of Henle surrounded by the vasa recta . These long loops of Henle and their associated vasa recta create a hyperosmolar gradient that allows for the generation of a concentrated urine . </P> <P> The nephron uses four mechanisms to convert blood into urine: filtration, reabsorption, secretion, and excretion of numerous substances . The structure and function of the epithelial cells lining the lumen change during the course of the nephron, and have segments named by their location and which reflects their different functions . </P> <P> The proximal tubule as a part of the nephron can be divided into an initial convoluted portion and a following straight (descending) portion . Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular capillaries, including approximately two - thirds of the filtered salt and water and all filtered organic solutes (primarily glucose and amino acids). </P>

Where is the filtrate produced in the nephron