The limb of the nephron loop is relatively impermeable to solutes but freely permeable to water. The limb of the loop is permeable to solutes, but not to water; its segment, in particular, has an active pump that moves salt (NaCl) out of the filtrate into the medullary interstitial fluid. The medullary regions of the collecting ducts are permeable to , a small solute that enhances the osmolality of the medullary interstitial fluid. The acts as a countercurrent exchanger to maintain the osmotic gradient while delivering a nutrient blood supply. The purpose of this mechanism is to maintain a high solute content in the of the medullary pyramids of about mOsm/L. The permeability of the distal portion of the DCT and the collecting ducts to water is controlled by the hormone .