Only birds and mammals can produce urines that have higher osmolalities than their plasma. In both of these vertebrate groups this is accomplished through the operation of a countercurrent multiplier system although the details of the system may differ somewhat between the two groups. In most mammals the loop of Henle has a thin ascending limb of variable length. In birds, the loop of Henle does not have a thin ascending portion as the tubule epithelium always thickens before the hairpin turn. In mammals, both urea and sodium chloride contribute to the medullary interstitial osmotic gradient although the exact contribution of each osmolyte can vary in time and from species to species. In birds the interstitial osmotic gradient is made up almost entirely of sodium chloride. The development of the avian renal medulla (medullary cone) is very similar to the outer medulla of mammals.