Mammals and birds produce urine hyperosmotic to their tissues. Most salt and water is absorbed across the membranes of the proximal convoluted tubule. The remaining filtrate is isosmotic to blood plasma. The descending arm of the Loop of Henle is most freely permeable to water. The base of the loop is freely permeable to salt, and the thick region of the ascending loop is where active transport of sodium ions occurs.
Freshwater fishes release a continuous dilute ammonia to the surroundings.
Both fresh and salt water fishes release nitrogen from the gills, as well. Antidiuretic hormone makes collecting ducts permeable to urea, which further allows for the reabsorption of water. Decreasing the levels of ADH means that ducts are not permeable to urea, which lowers the osmotic concentration in the medulla so that less water diffuses outward and is reabsorbed, and urine volume is increased. Aldosterone stimulates the kidneys to resorb salt, ultimately functioning in retention of water.