20.1.1 20.12 Animals must balance their water content within a narrow range.

Organisms, whether they live in the water or on land, require water. But they can’t take in too much or they will burst. And they can’t lose too much or they will shrivel up and die. Osmoregulation, an important component of homeostasis in animals, is the regulation of water content and of the concentrations of dissolved solutes that influence osmosis. The most abundant solutes in animal fluids are sodium, chloride, potassium, magnesium, and calcium ions.

Although a variety of mechanisms of water regulation have evolved in animals, there are just four ways that animals get water: by drinking it, by eating foods that contain it, by absorbing it through osmosis, and as a by-product of cellular respiration. Similarly, animals lose water in four ways: urination, defecation, evaporation (including panting, breathing, and sweating), and osmosis.

Osmoregulation revolves around controlling and regulating these mechanisms of water loss and gain. Through osmosis (see Section 3-9), water flows from areas of low solute concentration to areas of high solute concentration. Thus organisms often control their water content indirectly by regulating their solute content. They generally balance the total amount of solutes—which influences the direction and magnitude of osmosis—and the concentration of each solute individually. Imbalances in the concentrations of certain solutes (minerals and electrolytes) can cause serious health problems, from muscle spasms to confusion to paralysis, and even death.

Because animals live in such a wide range of habitats, they encounter a similarly wide range of osmoregulatory challenges. The challenges are very different for aquatic organisms than for terrestrial organisms, for example. And among the aquatic organisms, the challenges faced by organisms living in saltwater habitats are very different from—almost the opposite of—those faced by organisms living in fresh water.

Most invertebrates that live in salt water are osmoconformers, meaning that they let the solute concentration of their body fluids reflect that of their environment. Most vertebrates, on the other hand, are osmoregulators, maintaining their fluids and solute concentrations within narrow ranges that differ from those of their environment (FIGURE 20-23; see also Figure 20-16).

A variety of structures have evolved that enable animals to regulate their water balance. In insects, for example, very small tubes branch off the digestive tract, near its end at the rectum. These extensions from the gut, called Malpighian tubules, function as insects’ chief excretory organs. Potassium ions (K⁺) and cellular waste products move by active transport from the body cavity of the insect—where its blood is—into the Malpighian tubules. The increase in solutes in the tubules attracts water by osmosis. Together, K⁺ ions, water, and waste products move down the digestive tract until, near its end, the K⁺ ions and water are mostly reabsorbed into the circulatory system. This is a very effective system for conserving water, leaving mostly waste products to be excreted as feces.

Another osmoregulatory structure is the vertebrate kidney. The kidney, as we shall see, is a complex organ that filters blood, removing metabolic waste products—particularly excess nitrogen from the breakdown of proteins and nucleic acids—and other solutes while regulating the organism’s water balance. The specific details of kidney function vary across the different groups of vertebrates. Let’s consider, for example, the challenges faced by two groups of aquatic vertebrates: freshwater and saltwater fishes. A freshwater fish faces the problems of high concentrations of water entering its body and of solutes leaving its body. A saltwater fish, on the other hand, tends to lose water to its environment while taking in high concentrations of solutes from the environmental water. As a consequence, vertebrate kidneys—depending on the type of animal—may function either to conserve water or to remove it. (This explains why a freshwater fish does not drink water, but a saltwater fish drinks large amounts.)

In the next section, we examine the details of kidney functioning, focusing specifically on the human kidney and how it can produce urine that has a solute concentration more than four times that of blood, and how it can eliminate the many potentially harmful molecules contained in the food and drink that we consume.