Adaptations

Mammals that live in different habitats have kidneys adapted to the amount of water available. The key feature is the length of the loop of Henle.A longer loop of Henle can build up a greater concentration of ions in the medulla (a lower water potential), because the countercurrent multiplier works over a longer distance. More water can then be reabsorbed from the collecting duct, so the urine can be more concentrated. To hold long loops, the medulla must be thick, so the relative medullary thickness (the thickness of the medulla compared with the size of the kidney) is a good predictor of how concentrated an animal's urine can be.Desert mammals, such as the kangaroo rat and the camel, have very long loops of Henle and a thick medulla. The kangaroo rat can produce urine about five times as concentrated as human urine, and it rarely needs to drink: it gets most of its water as metabolic water from respiring the dry seeds it eats. It is also active at night, lives in cool, humid burrows and produces very dry faeces.Freshwater mammals, such as the beaver, have short loops of Henle and a thin medulla. Water is plentiful, so they do not need to conserve it and they produce large volumes of dilute urine.Humans are in between, with a mixture of long and short loops.Worked example: maximum urine concentration is about 500 mOsm kg⁻¹ in a beaver, 1200 mOsm kg⁻¹ in a human and 5500 mOsm kg⁻¹ in a kangaroo rat. Kangaroo rat urine is 5500 ÷ 1200 ≈ 4.6 times as concentrated as human urine.