<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> <P> The loop of Henle is a U-shaped tube that extends from the proximal tubule . It consists of a descending limb and an ascending limb . It begins in the cortex, receiving filtrate from the proximal convoluted tubule, extends into the medulla as the descending limb, and then returns to the cortex as the ascending limb to empty into the distal convoluted tubule . The primary role of the loop of Henle is to concentrate the salt in the interstitium, the tissue surrounding the loop . </P> <P> Considerable differences aid in distinguishing the descending and ascending limbs of the loop of Henle . The descending limb is permeable to water and noticeably less permeable to salt, and thus only indirectly contributes to the concentration of the interstitium . As the filtrate descends deeper into the hypertonic interstitium of the renal medulla, water flows freely out of the descending limb by osmosis until the tonicity of the filtrate and interstitium equilibrate . The hypertonicity of the medulla (and therefore concentration of urine) is determined in part by the size of the loop of Henle . </P> <P> Unlike the descending limb, the thin ascending limb is impermeable to water, a critical feature of the countercurrent exchange mechanism employed by the loop . The ascending limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives countercurrent exchange . In passing through the ascending limb, the filtrate grows hypotonic since it has lost much of its sodium content . This hypotonic filtrate is passed to the distal convoluted tubule in the renal cortex . </P>

Why do we have two types of nephrons