Maintaining a stable internal environment, or homeostasis, is essential for the survival of complex animals. Living organisms must maintain a salt and water balance, and they must continually remove the toxic byproducts generated by metabolism.
Organisms have evolved a variety of strategies to maintain a more or less stable internal environment. In animals that have circulatory systems, the blood typically passes through excretory organs, commonly termed kidneys. In terrestrial animals, the kidneys not only play a major role in the removal of wastes but are also the primary organs of osmoregulation.
In this animation, we look at the function of the mammalian kidney.
The function of the mammalian kidney may be summarized as follows:
The glomeruli filter large volumes of blood plasma. Most of this volume, along with valuable molecules such as glucose and amino acids, is reabsorbed from the proximal convoluted tubules.
The loops of Henle create a concentration gradient in the medulla of the kidney by a mechanism called the countercurrent multiplier. As the filtrate flows up through the thick segment of the ascending limb, NaCl is transported out of the filtrate and into the extracellular spaces of the medulla. The resulting increase in the osmotic concentration of the extracellular fluids draws water out of the filtrate in the thin descending limb making it more concentrated. As this more concentrated filtrate flows up through the thick ascending limb, more NaCl is transported out and the extracellular fluid becomes even more concentrated. Thus, the opposing—or countercurrent—directions of flow through the two limbs of the loop of Henle, along with differential permeability and transport capacities of these limbs results in "multiplying" the concentration difference between the filtrate and the extracellular fluid of the medulla.
The concentration difference is not uniform throughout the medulla, but forms a gradient that increases from the region next to the cortex to the tip of the renal pyramid. In response to this gradient, as the urine flows down through the collecting duct from the cortex to the tip of the renal pyramid, the urine can lose water osmotically and become more concentrated. This countercurrent multiplier mechanism makes it possible for mammals to produce a urine that is hypertonic to their blood plasma.
Mammals inhabit an enormous range of environments on Earth, including some of the most arid. The major adaptation that allows mammals to maintain homeostasis in the face of a wide range of osmotic stress is the variable ability of their kidneys to concentrate urine. Thus, when they take in lots of water in their food, they can excrete the excess water by producing dilute urine, and when they are exposed to very arid conditions, they can conserve water by producing a highly concentrated urine.