Reabsorption of water and other materials as well as the process of producing a concentrated urine is made possible by an active transport mechanism. The ascending limb of the loop of Henle, which is impermeable to water, pumps sodium into the tissue fluid surrounding the nephron. A sodium gradient is created in which the concentration of sodium is lowest in the tissue fluid of the cortex and highest in the tissue fluid of the medulla. As the filtrate passes through the collecting tubule, which is permeable to water, water moves by osmosis from the filtrate (with its low sodium concentration) into the tissue fluid (with its high sodium concentration), leaving behind a urine that is isotonic to the tissue fluid of the medulla but that is very hypertonic to the original blood plasma.
