Osmoregulation (zoology)

The osmotic pressure of a solution is the measure of the tendency of water to enter a solution from pure water. Osmoregulation, the regulation of osmotic pressure, is vital to every organism. The phenomenon collectively is called “osmosis.” It is the difference in hydrostatic pressure that must be created between that solution and pure water to prevent any net osmotic movement of particles in the water when the solution and pure water are separated by a semipermeable membrane. Hydrostatic pressure is a measuring device—a means of assessing the tendency of a solution to take on water osmotically.

Only dissolved solutes contribute to osmotic pressure. The number of individual particles determines the strength of osmotic pressure. Each particle makes a roughly equal contribution to osmotic pressure. The same number of molecules of a substance such as sodium chloride (table salt), which ionizes in water to release two ions (one sodium ion and one chlorine ion), display twice as much osmotic pressure as the same number of molecules of glucose, which retains its molecular form in water. Cells and other suspended materials do not contribute to osmotic pressure.

Several characteristics of solutions depend upon the number of particles in the solution. These are called “colligative properties.” Increasing the number of particles in a solute impairs the ability of the solvent to change state. The colligative properties are the freezing point, the boiling point, the osmotic pressure, and the vapor pressure. Only freezing point depression and vapor pressure are used to determine osmotic pressure.

Osmoticity refers to the osmotic pressure of solutions. Isosmotic solutions have equal osmotic pressures. A hypoosmotic solution has an osmotic pressure lower than the solution to which it is being compared; a hyperosmotic solution is one with a greater osmotic pressure than the solution to which it is being compared. These solutions can be body fluids, environmental liquids, or laboratory solutions.

The terms used to describe the changes in the volume of cells exposed to solutions of differing concentrations are often confused with those comparing osmotic pressure. Changes of the cell volume are described by the term “tonicity.” Solutions are isotonic to a cell, causing no change in cell volume. Hypotonic solutions will cause the cell to swell as water diffuses into the cell, and the cell may even burst. Hypertonic solutions will cause a cell to shrink as water diffuses across the cell membrane into the solution.

The Challenges of Osmoregulation

This discussion reveals some of the problems that an organism encounters in the environment as concentrations of water and salts vary. Most organisms attempt to regulate both their volume and their ion content. If the volume is not regulated, the chemicals within the cell will become too dilute to react or too concentrated to interact. If ions are not regulated, chemical reactions will be affected by inappropriate levels of ions, which may change the electrochemical properties of the cellular solution. Thus, there are independent challenges to volume, ion, and osmotic regulation. The homeostatic physiological responses to all three types of challenges are interconnected but distinct.

Osmoregulation is the regulation of the ratio between all dissolved particles, regardless of their chemical nature as ions or molecules, and water. All organisms are exposed to osmotic stress. Any organism incurs obligatory water losses. These occur during respiration, urination, and defecation. The organs most often thought of as participating in osmoregulation are the kidneys. They are intimately concerned with the elimination or conservation of water. Some salts are also found in the urine. The proportions of the salts excreted in urine may be different from those in the body fluids because the kidney can retain required ions while eliminating less desirable ones.

Freshwater organisms are in danger of dilution, and they excrete great quantities of dilute urine. Saltwater organisms usually produce small quantities of isosmotic urine, which preferentially excretes divalent ions such as magnesium and sulfate. Other surfaces lose water, causing desiccation. The composition of the diet also influences the need for excretion of urine. Nitrogenous wastes from protein metabolism must be eliminated in urine, often as urea, which requires water for its excretion.

Carbon dioxide released during the metabolism of carbohydrates and fats is eliminated by the respiratory organs. In terrestrial organisms, air leaving the lungs is usually saturated, and some water is lost upon expiration. The respiratory organs of aquatic organisms are gills. Their surfaces must be permeable to water. Freshwater organisms gain water through them, and hypoosmotic marine organisms lose water through them.

The metabolism of carbohydrates produces what is known as metabolic water, which can be used to prevent desiccation. Metabolic water produced from the metabolism of fats is lost because of the higher rate of respiration required to supply the oxygen needed in fat oxidation.

Salt Loss

Preformed water is present in any food. Even the driest seeds contain a small amount of water. The nutrients also always include salts. The presence of great quantities of salts may require urinary loss of water in excess of the preformed water found in the food.

Although feces may appear to be solid, they contain some water that was not absorbed in the gut. The presence of salts and other solutes in the digesta may also draw water from the hypoosmotic body fluids into the gut. One of the reasons that humans cannot drink seawater, in fact, is that the magnesium ions in ocean water increase the permeability of the gut and increase water loss because the seawater is hyperosmotic to body fluids. More water is lost than can be gained.

Salts can be lost from the body by means other than urine formation and defecation. Marine reptiles and birds have salt glands located on the head. Since neither can produce hyperosmotic urine, these glands allow the elimination of salt with a minimum loss of water, as the secretion may be four to five times as concentrated as body fluids. The cloaca of birds and the rectal glands of sharks also have the capacity to excrete salts.

One of the most fascinating mechanisms of osmoregulation is found in elasmobranch fish—the sharks, skates, and rays. Their body fluids are hyperosmotic but hypotonic to seawater. Blood salt concentrations are below those of seawater. Excess osmotic pressure is supplied by two molecules: urea and trimethylamine oxide (TMAO). Urea is toxic to most organs, but some organs resist its deleterious effects. Others would be harmed but are apparently protected by the TMAO. Retention of both urea and TMAO minimizes enzymatic disturbances by urea and allows the elasmobranchs to avoid the salt gain associated with hypoosmotic body fluids.

Organisms exposed to environmental variations have two choices: they can maintain internal constancy or homeostasis at the expense of metabolic energy, or they can allow their internal conditions to follow that of the environment. Organisms that maintain their internal osmotic pressure despite changes in external osmotic pressure are called osmoregulators. These euryosmotic organisms are protected from environmental changes. Their metabolism can continue to function, but much of the energy will be used to maintain their body fluids at the appropriate osmotic pressure.

Organisms that allow their osmotic pressure to follow that of the environment are called osmoconformers. These poikilosmotic organisms often have a limited tolerance for such changes. They are stenohaline. They may be less vigorous at salinities other than their optimal levels. The adults of such groups (for example, mollusks such as oysters and mussels) may be found in salinity extremes that are not tolerated by their young. These populations must be maintained by the immigration of young spawned in more favorable salinity conditions.

Hormonal Regulation of Osmotic Pressure

Internal osmotic pressure is affected by the hormones present in the body's fluids. In invertebrates such as annelids, mollusks, and arthropods, neuroendocrine changes are seen upon changing the osmotic pressure of the environment. These changes indicate that nervous and endocrine systems are at work regulating the osmotic pressure of the organism. In most invertebrates, the biochemical nature of these hormones is unknown. Some freshwater pulmonate snails, however, produce an antidiuretic hormone and a neurosecretory factor associated with electrolyte balance. Depending upon the demands placed on them, insects such as grasshoppers and cockroaches can synthesize diuretic or antidiuretic hormones.

The best-known hormonal factors in ion regulation are studied in vertebrates. The pituitary gland produces antidiuretic hormone (ADH), which promotes water retention in terrestrial vertebrates. In fish and amphibians, ADH may induce urine formation and increase water loss through diuresis.

The adrenal gland also produces hormones that influence ion retention. In mammals, aldosterone increases the reabsorption of sodium in the kidney and promotes the excretion of potassium. In nonmammalian vertebrates, extrarenal glands maintain salt and water balance by affecting the gills and intestines of fishes, the urinary bladder and skin of amphibians, and the salt glands of elasmobranchs, reptiles, and birds.

Measuring Osmoregulation

Osmoregulation involves the balancing of water and solutes in the body so that the animal can continue to function. Because the presence of particles influences certain physical characteristics of the solution, these colligative properties can be used to determine the osmotic pressure of solutions. Colligative properties change with increasing numbers of particles in solution: The osmotic pressure increases, the boiling point increases, the freezing point decreases; the vapor pressure decreases.

The freezing point depression and vapor pressure can be used to measure a solution’s osmotic pressure. Freezing point is used most often. It works for the same reason that salt is spread on ice on sidewalks in winter. The salt lowers the freezing temperature of the water. Body fluids are much more dilute than the salt and water mixtures that melt ice, but the salinity of the ocean (approximately thirty-five parts salt for every thousand parts of solution) causes it to freeze as much as 1.6 degrees Celsius lower than pure water. Only marine organisms have body fluid osmotic pressures in that range. Terrestrial organisms have much less salt and, therefore, much lower osmotic pressures in their body fluids. Because most body fluids are so dilute, a large sample may be required to determine freezing point depression. When only small volumes of body fluid exist, the determination becomes more difficult.

Vapor pressure determinations are also used in osmometry. Usually, a small amount of the fluid being studied is tested in a capillary tube. In one ingenious method, the capillary tube is placed in a solution that is more concentrated than the experimental fluid. The higher osmotic pressure of the reference solution pushes a meniscus up the tube. The rate of movement of the meniscus depends upon the difference in concentrations between the experimental and the reference solutions.

Another ingenious method of using vapor pressure to determine the osmotic pressure of an experimental solution requires enough fluid to fill a depression. A glass plate with capillary tubes filled with reference solutions of known osmotic concentrations is mounted over the experimental fluid. The reference tube that exhibits no movement is at equilibrium with the experimental fluid. One of the rigors of this method is that all movement must stop.

Another method involves capturing a precise volume of experimental fluid in a capillary tube. The shape of drops of the same volume of reference solutions is compared to that of the experimental solution. Those of the same concentration will have the same shape because their vapor pressures are exerting equal force on the drop. Thermocouples are also used in vapor pressure determinations. This procedure is delicate and costly and is used infrequently. All these procedures are difficult and require patience. Now, electronic instruments analyze the constituents of solutions and allow easier calculation of the osmotic pressures of solutions than ever before.

Freshwater Versus Saltwater Environments

All organisms experience osmotic stress. There is no environment in which the osmotic pressure and the ion composition exactly match the requirements of all cells. Every organism must expend metabolic energy to maintain appropriate water and ion concentrations.

Freshwater organisms are hyperosmotic to their environment. They risk losing scarce ions through their permeable gills and in their urine and also tend to take up water through their gills or other surfaces and in their food. They face the problem of dilution of their body fluids by the environment.

Marine organisms are often isosmotic to the saltwater they inhabit. They must change the concentrations of some ions, however, in order to attain this state. Magnesium is present in greater amounts in seawater than is desirable in their body fluids and must be eliminated. These organisms' ion regulate, though they are not in danger of volume changes.

Marine organisms that evolved from freshwater or terrestrial ancestors are often hypoosmotic to seawater. They are in danger of desiccation as water from body fluids diffuses into the hyperosmotic ocean water. They also must regulate the types of ions that are retained and eliminated from their bodies.

Marine organisms may also be exposed to freshwater when they enter rivers, which dilute the salt content of the incoming tidal water. Under these conditions, the water is brackish—not as salty as the sea, but not as pure as freshwater. The criticality of this situation depends upon whether the organism is tolerant or intolerant of salinity changes. Organisms that can live in only a narrow range of salinities are called stenohaline. Organisms that are tolerant of wide ranges of salinities are called euryhaline.

Terrestrial organisms are always hyperosmotic to their environment, so they continually face desiccation in the air. They also must adjust the ion composition of their body fluids because the foods that they eat may not have inorganic ions in the desired ratios and because some ions are always lost in urine.

One example of the influence of these effects concerns the interaction of oysters and the protistan parasite known as MSX. (MSX stands for “multinucleate sphere unknown,” which refers to the Protista Haplosporidium nelsoni.) The MSX organism survives in osmotic pressures greater than 0.4 osmolar. Oysters are osmoconformers that grow in saline, brackish, and nearly fresh water. At osmotic concentrations less than 0.4 osmole, oysters can survive and are unaffected by MSX. When rainfall is abnormally low, however, the salinity of brackish water increases and oysters that were protected in low-salinity water are exposed to higher-salinity water, which allows the MSX organism to infect them.

Organisms exposed to tides or sudden fluctuations in salinity due to rainstorms or continental runoff may protect themselves from exposure to variations in osmotic pressure by sealing themselves off like snails and bivalve mollusks. The periwinkle snail Littorina littorea is particularly well-adapted to survive in these conditions by closing its shell to retain an inner hyperosmotic environment. After being exposed to fresh water for twelve days in laboratory studies, the osmolarity in these snails' mantle cavities was only reduced by 25 percent. Other organisms may move offshore to more saline waters or onshore, away from the increasing salinity. Worms that burrow in saltwater sediments are protected from transient changes in salinity because there is little exchange of solutes with the overlying saltwater. The arc shell clam Scapharca subcrenata uses antioxidant defense mechanisms to protect itself during salinity fluctuations. Their soluble protein, hemoglobin, taurine, and aminusesd levels increase as salinity decreases. The Green mud crab Scylla paramamosain is a popular test subject in investigating salinity adaptations because it is an osmoconformer and an osmoregulator. Studies of the green mud crab have revealed the importance of the transport protein ABCC1 in adapting to hypoosmotic stress.

The vertebrates adapted to their various environments by using hormones to regulate salt and water balance. Because of the differing demands of aquatic and terrestrial environments, in different groups, the same hormone may have opposite effects, but that effect is always to maintain the optimal osmotic pressure to ensure survival.

Principal Terms

Euryhaline: The ability of an organism to tolerate wide ranges of salinity

Hyperosmotic: Describes a solution with a higher osmotic pressure, one containing more osmotically active particles relative to the same volume, than the solution to which it is being compared

Hypoosmotic: A solution with a lower osmotic pressure, and fewer osmotically active particles relative to the same volume, than the solution to which it is being compared

Isosmotic: A solution having the same osmotic pressure, the same number of osmotically active particles relative to the same volume, as the solution to which it is being compared

Osmoconformer: An organism whose internal osmotic pressure approximates the osmotic pressure of its environment; such an organism is also referred to as “poikilosmotic”

Osmoregulator: An organism that maintains its internal osmotic pressure despite changes in environmental osmotic pressure; such an organism is also referred to as “euryosmotic”

Stenohaline: The inability of an organism to tolerate wide ranges of salinity

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