Water balance in vertebrates

Water, which makes up about 60 percent of the body weight of vertebrates, may be the most neglected nutrient. Drinking and eating are the obvious ways of obtaining water, but metabolism, the processes of synthesis and breakdown within the body’s cells, also provides water for organisms. In fact, diet and metabolism provide 40 percent of the water necessary for human life.

Drinking water is limited by the environment. Areas such as deserts, which have little rainfall, have little potable groundwater available; even plants must develop some means of conserving the little water their roots can find or the dew that settles on exposed surfaces during the cool desert night. With no surface water and few plants as sources of water, some desert mammals, such as the kangaroo rat, get most of their water from metabolism. They often do not drink even when a supply of water is nearby.

Water balance varies both daily and seasonally as environmental factors such as temperature, humidity, and wind vary and as activity levels change. The body must maintain a nearly constant volume and composition of the extracellular fluid despite fluctuations caused by drinking, eating, metabolism, activity, and environment.

Water Loss from Kidneys and Lungs

There are four sites of water loss: kidneys, lungs, skin, and intestines. Control of water balance depends on the efficiency of water retention compared with the necessity of water loss in the normal functioning of both terrestrial and marine vertebrates. For freshwater vertebrates, for example, excretion of excess water without losing salts is vital.

The kidney can produce urine that can be highly concentrated or very dilute. For humans, urine can be four times more concentrated than body fluids and contains 1,200 milliosmoles of solute. Other animals, particularly some desert species, can produce urine five times more concentrated than that. The more concentrated the urine, the more water is retained during the excretion of waste materials.

Generally, increasing the length of the loop of Henle in the nephron is associated with increased concentration ability of the kidney in mammals. Although some of their nephrons contain loops of Henle, birds cannot match the mammals’ concentrating ability. The maximum urine-to-plasma concentration ratio in birds is only a little more than five. This is because mammals excrete osmotically active urea, whereas birds excrete precipitates of uric acid and uric acid salts that do not contribute to osmotic pressure. The osmotic pressure of birds’ urine primarily comes from sodium chloride. Birds also allow their plasma to become twice as concentrated as that of mammals during dehydration.

When water must be conserved, urine is concentrated to a greater degree than when water is plentiful. As water becomes scarce, the concentrations of solutes in the body fluids increase. Osmoreceptors in the hypothalamus sense the increase and stimulate the release of antidiuretic hormone (ADH). Antidiuretic hormone is a small peptide that varies somewhat in composition among vertebrate classes. It affects kidney cells, increasing the permeability of the distal tubule of the nephron and the collecting duct to water. More water is reabsorbed and, therefore, retained by the body. In some species, ADH also affects the number of nephrons filtering.

The earliest water-balance problem that vertebrates faced in their evolutionary history was an excess of fresh water and a scarcity of sodium. The hormone aldosterone evolved in fish to cope with this. Aldosterone is secreted by the adrenal gland and increases the reabsorption of sodium by the kidney. Some potassium is lost from the body in exchange for sodium, but since plants are rich in potassium, its loss could be made up in the diet.

Water is also lost or gained when respiring; all gas-exchange surfaces must be moist. Aquatic organisms take in water through their gills in freshwater and lose water through the gills in marine environments. Terrestrial organisms have lungs, and the moist exchange surface is inside the body. Even if the atmosphere is dry, air entering the lungs is moistened to nearly 100 percent relative humidity during its passage through the airways. Expired air loses water to those same airway walls as it leaves the body. Some moisture, however, remains in the air and is lost to the body. The cloud of vapor seen as animals exhale in cold weather is this lost moisture condensing in the cold air.

Breathing rate and volume influence the amount of water lost from the lungs. As breathing rate increases, air moves out of the body more quickly, and there is less time for moisture to condense on the cool airway walls before it is exhaled. The movement of greater volumes of air also increases loss of water vapor from the respiratory tract because less comes in contact with the airway walls. Since heavy exertion often involves breathing through the mouth rather than through the moisture-conserving nasal cavity, further water is lost. As much as 25 percent of the moisture present in expired air may be lost in a dry environment.

Kangaroo rats have adapted to their desert environment and manage their water balance in several ways. Their kidneys and bladders are more efficient than most other animals, which ensures they extract the maximum water from their food, excrete the remaining salt, and store the water in their bladder. To stay cool, they dust themselves in the dirt rather than panting or sweating.

Water Loss from Skin and Intestines

The skin is another site through which water is lost. The sweating which occurs in warm weather is an obvious example. Even during the winter, when the air is cool, “insensible perspiration” occurs as water diffuses through the skin. Insensible perspiration always occurs. The perspiration that can be sensed comes from eccrine glands. They secrete a watery fluid that cools the skin by using body heat to evaporate the liquid. The amount of salts and nitrogenous wastes in perspiration is not large; however, when one is working in a hot environment, the loss may be significant. Primarily, though, it is water that is lost and must be replaced.

The apocrine sweat glands are located in the armpits and groin. They become active during emotional stimulation. These are the glands associated with the musky odor that some animals exude. These glands are not important in regulating body temperature, and their evaporation of water is not a major component of water balance mechanisms. The loss of water from the respiratory tract and the skin is obligatory, usually amounting to about 850 milliliters a day.

The intestinal tract is a source of water gain, as it ingests both liquids and food (with its associated water). Some water, however, is also lost because the copious intestinal secretions contain water. In fact, one day’s intestinal secretions may amount to twice the body’s plasma volume (from which it is derived). Not all water is reabsorbed in the passage through the stomach, small intestine, and large intestine: About one hundred milliliters are lost in the feces each day.

In a normal human diet, ingested fluid tends to exceed the minimum required by about one liter. Whatever excess is not used in evaporative cooling and lost from expired air or in feces is excreted by the kidney. The minimum water uptake required for balance is defined as the requirement to provide minimum urine volume without weight loss. The stomach and the small intestine reabsorb most of the ingested and excreted water. Only 35 percent reaches the large intestine. The large intestine is specialized to absorb water and produce semisolid feces for excretion. The maximum rate of water absorption by the intestines lies well above what is normally required.

The body fluid compartments provide an excellent example of the steady-state system characteristic of living things. Intracellular fluid must maintain a composition that promotes chemical reactions and diffusion despite the changes that those reactions bring. Extracellular compartments must retain their individual characteristics even though they communicate with one another. Hormones and nerves coordinate the interactions of the digestive, respiratory, integumentary, and urinary systems, which contribute to the constant conditions of volume and composition of the body fluid compartments.

Many vertebrates, including some species of reptiles, birds, saltwater fish, and amphibians, lose water through their skin in a process called cutaneous evaporation. Even species that do not have sweat glands experience water loss through evaporation. Cows, sheep, goats, and other animals with high-fiber diets lose significant amounts of water in their feces. Usually, 60 to 85 percent of their feces is water.

Studying Water Balance

The study of water balance is often difficult because so many body systems and physical factors are involved. Gross methods include measuring moisture in respired air, feces, and urine, as well as in ingested foods. In other studies, the weight of water in bodies or organs may be obtained by drying: The amount of water is the difference between the wet and dry weights. These methods are crude and give rough estimates of the fluid volume or fluid balance in the body or an individual compartment or organ.

Dilution techniques are used to obtain more precise information. One method is the injection or infusion of an indicator or test substance. This substance must be distributed only in the fluid compartment being measured. It must be safe for the organism while not being metabolized or synthesized in the body. If the substance can be excreted, it must be measurable in the excreta. Radioactive tracers are also used to determine the dilution of an injected or infused sample. The isotope chosen must not influence the mechanisms governing the size and composition of the compartment being measured. Moreover, the body must not be able to distinguish between the radioactive molecule and the unlabeled isotope.

The concentration of the test substance in a sample of the blood, lymph, or cerebrospinal fluid gives an indication of the dilution caused by the volume of the fluid within the compartment. There is no perfect test substance; each is associated with problems affecting the accuracy of the measurement. Inulin is used to determine the volume of interstitial fluid, but inulin diffuses slowly through dense connective tissue. Radioactive sodium enters most compartments, but it binds to the crystalline structure of bone. The dye Evans blue, which binds to plasma proteins, and radio-iodinated serum albumin are used to measure plasma volume, but these substances move out of capillaries.

All the fluid compartments communicate, but some, such as bone and cartilage, communicate more slowly than others, such as lymph and plasma. The resistance between compartments is often supposed to be at the interface between compartments. There is evidence, however, that the rate-limiting factors may not only be the permeability of the cell membrane alone but also the physical state of some of the water within the compartment. For example, in red blood cells, some water is bound to protein and is not accessible to solutes. A portion of the water in mitochondria is not free to participate in osmotic processes. For these reasons, dilution techniques may underestimate the amount of water in the compartment being measured. Although dilution techniques use sophisticated technology, the measurements are often extrapolations and not exact. These techniques do provide a general picture of the distribution of body fluids, but since water balance is a continuing, dynamic process, the values are not stable.

Regulating Water Loss

Loss of water through the skin and respiratory tract is obligatory. All respiratory surfaces must be moist so that gases can pass through them. Amphibians are limited in their geographical distribution primarily because their skin is a respiratory surface and, therefore, water loss from it cannot be curtailed. The water losses vary with environmental factors such as temperature, humidity, and wind. Because of this, amphibians must remain near a source of water that their skin can imbibe.

Loss of body water through “insensible perspiration” is not controllable; it is obligatory. The sweating that helps regulate body temperature is facultative, and it varies with weather and exercise. If sweating is prevented when the ambient temperature is high, the body temperature can rise explosively. This will cause death as surely as dehydration, which was prevented by not sweating, would have.

Mammals and birds can minimize water loss by modifying the depth and rate of breathing. On exertion, the rate and depth increase, and correspondingly, the loss of water through the airways and across the skin surface increases. Unless the organism intends to quit breathing and allows its body temperature to rise, some water must be lost in this way.

Losses through the digestive tract are often involuntary. Diarrhea and vomiting accompany many illnesses, and since these uncontrolled losses are from the digestive system, which secretes a volume twice that of the plasma each day, their unreclaimed losses are massive. Dehydration and electrolyte imbalance follow quickly if these losses are not made up. This is particularly crucial for infants, for whom the daily diet makes up 25 percent of the total body water. With dehydration, the volume of the circulatory system decreases, and circulatory failure results. In addition, since the extracellular fluid compartments are continuous with the intracellular compartments, the fluid inside the cells becomes hyperosmotic, and metabolic reactions cannot take place.

Water retention depends on several conditions, but the most important is the available water sources. If fresh water is not available, a human will die after eleven to twenty days, depending on the rapidity of the onset of dehydration. By then, the person will have lost 15 to 20 percent of their initial body weight. The excretion of wastes, breathing, and insensible perspiration (even in moderate temperatures with shade available) are accompanied by obligatory water losses that cannot be reduced.

For other organisms, water is present in excess and becomes a problem. Freshwater fish must release significant quantities of urine without losing the salts necessary to maintain proper internal osmotic conditions. A process called osmoregulation helps them maintain salt balance. Fish take in water through their respiratory surface, the gills, which house chloride cells that produce enzymes to stabilize the fish's osmotic body pressure by controlling the salt entering their blood. The hormone aldosterone promotes salt absorption from the nephrons. Osmoregulation also involves specialized kidneys that work quickly but reabsorb salt from the fish's urine before it is released, ensuring that the fish ingests is not wasted.

For marine creatures, on the other hand, the entry of salt is a problem. Some species of marine animals drink seawater, like cetaceans, pinnipeds, and bony fish. They must eliminate the excess salt without losing too much body water. Because kidney function always involves water and ions loss and fish and reptiles do not concentrate urine efficiently, many of these vertebrates have evolved salt glands. The salt glands use metabolic energy to excrete sodium chloride with little water. Sharks have a waste product called urea in their muscles and blood that helps them retain water. The saltwater is usually equal to the salt content of sharks, so their bodies usually function well without much adaptation. However, some sharks live in both fresh and saltwater, which requires them to excrete more urine and reduce the amount of urea in their muscles and blood.

Each environment presents unique water-balance problems to an organism. Yet, even in the world’s harshest, driest conditions in the Antarctic, tiny mites and spiders, penguins, and predatory birds find ways to live and obtain all the water they need in a land where water is usually solid.

Principal Terms

Apocrine Gland: A type of sweat gland that becomes active at puberty and responds to emotional stress; the glands are found at the armpits, groin, and nipples

Eccrine Gland: A type of sweat gland that helps maintain body temperature; the glands are located on the palms and soles, forehead, neck, and back

Extracellular Fluid: The fluid outside cell membranes, including fluid in spaces between cells (interstitial), in blood vessels (plasma), in lymph vessels (lymph), and in the central nervous system (cerebrospinal)

Homeostasis: The dynamic balance between body functions, needs, and environmental factors that results in internal constancy

Hyperosmotic: A solution with a higher osmotic pressure and more osmotically active particles than the solution with which it is being compared

Hypoosmotic: A solution with a lower osmotic pressure and fewer osmotically active particles than another solution with which it is being compared

Isosmotic: A solution with the same osmotic pressure and the same number of osmotically active particles as the solution with which it is being compared

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