Comparative Physiology of the Kidney
Variations in nephron structure and physiology equip the kidneys of different vertebrates for osmoregulation in their various habitats. We have seen, for instance, that nephrons of the mammalian kidney can concentrate urine and conserve water. Among mammals, those able to excrete the most hyperosmotic urine, such as kangaroo rats and other mammals adapted to the desert, have exceptionally long loops of Henle that maintain steep osmotic gradients in the kidney. This results in urine becoming very concentrated as it passes from cortex to medulla in the collecting ducts. In contrast, beavers, which rarely face problems of dehydration, have nephrons with very short loops, resulting in a dilute urine.
Birds, like mammals, have kidneys with juxtamedullary nephrons that specialize in conserving water. However, the nephrons of birds have much shorter loops of Henle than those typical of mammalian nephrons, and birds are unable to concentrate urine to the osmolarities achieved by mammalian kidneys.
The kidneys of reptiles, having only cortical nephrons, produce urine that is, at best, isosmotic to body fluids. However, the epithelium of the cloaca (see Chapter 30) helps conserve fluid by reabsorbing some of the water present in urine and feces. Also, most terrestrial reptiles excrete NITROGENOUS WASTES . in an insoluble form known as uric acid, which helps to conserve water because it does not contribute to the osmolarity of the urine (this adaptation is discussed in more detail in the next section).
In contrast to mammals and birds, freshwater fishes face the problem of excreting excess water because the animal is hyperosmotic to its surroundings. Ins of conserving water, the nephrons use cilia to sweep a large volume of very dilute urine from the body. Freshwater fishes conserve salts by efficient reabsorption of ions from the filtrate in the nephrons.
Amphibians' kidneys function much like those of freshwater fishes. When in fresh water, the skin of the frog accumulates certain salts from the water by active transport, and the kidneys excrete a dilute urine. On land, where dehydration is the most pressing problem of osmoregulation, frogs conserve body fluid by reab-sorbing water across the epithelium of the urinary bladder.
Bony fishes that live in seawater, being hyperosmotic to their surroundings, face the opposite problem to that of their freshwater relatives. In many species, nephrons lack glomeruli and capsules, and a concen-trated urine is formed by secreting ions into the renal j tubules. Thus, the kidneys of marine fishes excrete very little urine and function mainly to get rid of divalent ions such as Ca2+, Mg2 + , and SO42-, which the fish takes in by its incessant drinking of seawater. As mentioned previously, monovalent ions such as Na+ and CI- are excreted mainly by the gills, as is most nitrogenous waste in the form of NH4+ (ammonium), Osmoregulation-control of salt and water balance-was the original function of the kidney. In the course of evolutionary development, the excretion of NITROGENOUS WASTES . became a second function.