Unit Six. Animal Life
26. Maintaining the Internal Environment
The kidney, first evolving in freshwater fish, is a complex organ consisting of up to a million repeating disposal units called nephrons. The nephron pictured below is representative of those found in mammals and birds; the nephron of other vertebrates lacks a looped portion. Blood pressure forces the fluid in blood through a capillary bed, called the glomerulus, at the top of each nephron. The glomerulus retains blood cells, proteins, and other useful large molecules in the blood but allows the water, and the small molecules and wastes dissolved in it, to pass into a cuplike structure surrounding the glomerulus and then into the nephron tube. As the filtered fluid passes through the first part of the nephron tube (labeled as the proximal arm in the figure below), useful sugars, amino acids, and ions (such as Ca++) are recovered from it by active transport, leaving the water and metabolic wastes dissolved in a fluid, called urine. Water and salts are reabsorbed later in the nephron.
Although the same basic design has been retained in all vertebrate kidneys, there have been a few modifications. Because the original glomerular filtrate is isotonic to blood, all vertebrates can produce a urine that is isotonic to (by reabsorbing ions) or hypotonic to (more dilute than) blood. Only birds and mammals can reabsorb water from their glomerular filtrate to produce a urine that is hypertonic to (more concentrated than) blood.
Kidneys are thought to have evolved first among the freshwater teleosts, or bony fish. Because the body fluids of a freshwater fish have a greater osmotic concentration than the surrounding water, these animals face two serious problems because of osmosis and diffusion: (1) Water tends to enter the body from the environment, and (2) solutes tend to leave the body and enter the environment. Freshwater fish address the first problem by not drinking water (water enters the mouth but passes out through the gills—it is not swallowed) and by excreting a large volume of dilute urine, which is hypotonic to their body fluids (as shown in the freshwater fish above). They address the second problem by reabsorbing ions (NaCl) across the nephron tubules, from the glomerular filtrate back into the blood. In addition, they actively transport ions (NaCl) across their gills from the surrounding water into the blood.
Marine Bony Fish
Although most groups of animals seem to have evolved first in the sea, marine bony fish (teleosts) probably evolved from freshwater ancestors. They faced significant new problems in making the transition to the sea because their body fluids are hypotonic to the surrounding seawater. Consequently, water tends to leave their bodies by osmosis across their gills, and they also lose water in their urine. To compensate for this continuous water loss, marine fish drink large amounts of seawater.
Many of the divalent cations in the seawater that a marine fish drinks (principally Ca++ and Mg++ in the form of MgSO4) remain in the digestive tract and are eliminated through the anus. Some, however, are absorbed into the blood, as are the monovalent ions K+, Na+, and Cl-. Most of the monovalent ions are actively transported out of the blood across the gills, while the divalent ions that enter the blood (represented by MgSO4 in figure) are secreted into the nephron tubules and excreted in the urine. In these two ways, marine bony fish eliminate the ions they get from the seawater they drink. The urine they excrete is isotonic to their body fluids. It is more concentrated than the urine of freshwater fish but not as concentrated as that of birds and mammals.
The elasmobranchs—sharks, skates, and rays like the one in the photo—are by far the most common subclass in the class Chondrichthyes (cartilaginous fish). Elasmobranchs have solved the osmotic problem posed by their seawater environment in a different way than have the bony fish. Instead of having body fluids that are hypotonic to seawater, so that they have to continuously drink seawater and actively pump out ions, the elasmobranchs reabsorb urea from the nephron tubules and maintain a blood urea concentration that is 100 times higher than that of mammals. This added urea makes their blood approximately isotonic to the surrounding sea. Because there is no net water movement between isotonic solutions, water loss is prevented. Hence, these fish do not need to drink seawater for osmotic balance, and their kidneys and gills do not have to remove large amounts of ions from their bodies. The enzymes and tissues of the cartilaginous fish have evolved to tolerate the high urea concentrations.
Amphibians and Reptiles
The first terrestrial vertebrates were the amphibians (pictured at the top of figure 26.7), and the amphibian kidney is identical to that of freshwater fish. This is not surprising because amphibians spend a significant portion of their time in freshwater, and when on land, they generally stay in wet places. Like their freshwater ancestors, amphibians produce a very dilute urine and they compensate for their loss of Na+ by actively transporting Na+ across their skin from the surrounding water.
Figure 26.7. Osmoregulation by some vertebrates.
Only birds and mammals can produce a hypertonic urine and thereby retain water efficiently, but marine reptiles and birds can drink seawater and excrete the excess salt through salt glands.
Reptiles, on the other hand, live in diverse habitats. Those living mainly in freshwater, like some of the crocodilians, occupy a habitat in many ways similar to that of the freshwater fish and amphibians, and thus have similar kidneys. Marine reptiles, which consist of other crocodilians, turtles (the second entry in figure 26.7), sea snakes, and one lizard, possess kidneys similar to those of their freshwater relatives but face opposite problems; they tend to lose water and take in salts. Like marine teleosts (bony fish), they drink the seawater and excrete an isotonic urine. Marine teleosts eliminate the excess salt by transport across their gills, while marine reptiles eliminate excess salt through salt glands near the nose or eye.
The kidneys of terrestrial reptiles also reabsorb much of the salt and water in the nephron tubules, helping somewhat to conserve blood volume in dry environments. Like fish and amphibians, they cannot produce urine that is more concentrated than the blood plasma. However, when their urine enters their cloaca (the common exit of the digestive and urinary tracts), additional water can be reabsorbed.
Mammals and Birds
Mammals and birds are the only vertebrates able to produce urine with a higher osmotic concentration than their body fluids. This allows these vertebrates to excrete their waste products in a small volume of water, so that more water can be retained in the body. Human kidneys can produce urine that is as much as 4.2 times as concentrated as blood plasma, but the kidneys of some other mammals are even more efficient at conserving water. For example, camels, gerbils, and pocket mice, Perogna- thus, can excrete urine 8, 14, and 22 times as concentrated as their blood plasma, respectively. The kidneys of the kangaroo rat shown in figure 26.8 are so efficient it never has to drink water; it can obtain all the water it needs from its food and from water produced in aerobic cellular respiration!
Figure 26.8. A desert mammal.
The kangaroo rat (Dipodomys panamintensis) has very efficient kidneys that can concentrate urine to a high degree by reabsorbing water, thereby minimizing water loss from the body. This feature is extremely important to the kangaroo rat's survival in dry or desert habitats.
The production of hypertonic urine is accomplished by the looped portion of the nephron, found only in mammals and birds. A nephron with a long loop, called the loop of Henle, extends deeper into the tissue of the kidney and can produce more concentrated urine. Most mammals have some nephrons with short loops and other nephrons with loops that are much longer. Birds, however, have relatively few or no nephrons with long loops, so they cannot produce urine that is as concentrated as that of mammals. At most, they can only reabsorb enough water to produce a urine that is about twice the concentration of their blood. Marine birds solve the problem of water loss by drinking seawater and then excreting the excess salt from salt glands near the eyes, which dribbles down the beak as shown in figure 26.9.
Figure 26.9. Marine birds drink seawater and then excrete the salt through salt glands.
The moderately hypertonic urine of a bird is delivered to its cloaca, along with the fecal material from its digestive tract. If needed, additional water can be absorbed across the wall of the cloaca to produce a semisolid white paste or pellet, which is excreted.
Key Learning Outcome 26.3. The kidneys of freshwater fish must excrete copious amounts of very dilute urine, whereas marine teleosts drink seawater and excrete an isotonic urine. The basic design and function of the nephron of freshwater fish have been retained in the terrestrial vertebrates. Modifications, particularly the loop of Henle, allow mammals and birds to reabsorb water and produce a hypertonic urine.