Chapter 28 Summary

Summary

Animals in Freshwater

  • All freshwater animals are hyperosmotic to the water in which they live. They tend to gain water by osmosis and lose ions by diffusion, especially across their permeable gill membranes. These passive fluxes of water and ions tend to dilute their body fluids.
  • To void their excess of water, freshwater animals produce a copious urine.
  • In nearly all freshwater animals, the urine is dilute compared with the blood plasma. The dilute condition of the urine helps to maintain not only the blood osmotic pressure but also blood concentrations of major ions at levels higher than those in the environment.
  • To replace ions lost by direct diffusion into the environment and excretion in urine, freshwater animals take up Na+, Cl, and some other ions by active transport. The gill epithelium is the principal site of active ion uptake in adult teleost fish and crayfish. Foods also help to replenish ions.

Animals in the Ocean

  • Most marine invertebrates are approximately isosmotic to seawater, but their blood differs from seawater in ionic composition. They exhibit ionic regulation but have little or no need for osmotic regulation. Hagfish display the same pattern.
  • Marine teleost fish are hyposmotic to seawater, apparently because they are descended from freshwater or coastal ancestors.
  • Because they are hyposmotic to seawater, marine teleosts tend to lose water by osmosis and gain ions by diffusion. To replace water, they drink; however, to absorb H2O from the seawater in their gut, they must actively take up NaCl, increasing their problem of salt loading. Their kidneys make urine that is approximately isosmotic to their blood plasma but rich in divalent ions, thereby assuming chief responsibility for divalent ion regulation. Monovalent ions are excreted across their gills; although Cl is secreted actively into the ambient water by mitochondria-rich (chloride) cells, Na+ secretion is often secondary to Cl secretion and passive.
  • Marine birds, turtles, and lizards have cranial salt glands that permit them to excrete ions at higher concentrations than possible in their urine.
  • Marine mammals lack salt glands but have kidneys that can produce more-concentrated urine than reptiles (including birds). Their urine-concentrating abilities are not exceptional compared with those of other mammals, however, and their water–salt balance is not entirely understood.
  • Marine elasmobranch fish, although they have blood ion concentrations far lower than those of seawater, are slightly hyperosmotic to seawater because of high concentrations of two counteracting organic solutes, urea and trimethylamine oxide (TMAO). Unlike teleosts, therefore, elasmobranchs need not drink and need not incur an extra NaCl load to gain H2O from ingested seawater.

Animals That Face Changes in Salinity

  • Some groups of marine invertebrates, such as molluscs, are uniformly osmoconformers. The euryhaline species in these groups are tolerant of wide ranges of blood osmotic pressure.
  • Other groups of marine invertebrates, such as crustaceans, include osmoconforming and osmoregulating species. In general in these groups, there is a correlation between osmoregulation and euryhalinity: The euryhaline species are osmoregulators.
  • Animals that are hyper-isosmotic regulators have mechanisms for hyperosmotic regulation but not hyposmotic regulation. Hyper-hyposmotic regulators have mechanisms for both types of regulation.
  • Euryhaline fish, such as species that migrate between seawater and freshwater, are excellent hyper-hyposmotic regulators. When they transition between freshwater and seawater, they undergo many changes in gill, kidney, and intestinal function—including molecular remodeling—under control of prolactin, cortisol, and other hormones.

Animals on Land: Fundamental Physiological Principles

  • Humidic terrestrial animals are restricted to humid, water-rich microenvironments. Xeric terrestrial animals are those that are capable of a fully exposed existence in the open air.
  • A low integumentary permeability to water—which reduces integumentary evaporative water loss—is required for animals to be xeric. All the major xeric groups—insects, arachnids, birds, nonavian reptiles, and mammals—have low permeabilities because of integumentary lipids.
  • Respiratory evaporative water loss depends directly on (1) an animal’s rate of O2 consumption (its metabolic rate) and (2) the amount of H2O lost per unit of O2 consumed. One way to reduce the latter in mammals and birds is countercurrent cooling of nasal exhalant air.
  • The animals with the lowest total rates of evaporative water loss (EWL) are those, such as lizards, that combine the advantages of low integumentary permeability to water, tightly controlled access of air to breathing organs, and low metabolic rates.
  • Water loss in urine can be reduced by producing concentrated urine (which reduces the amount of water needed to void soluble wastes) or by producing poorly soluble nitrogenous end products such as uric acid (which remove waste nitrogen from solution). Only three groups of animals can make urine hyperosmotic to their blood plasma: insects, birds, and mammals.
  • Within groups of related species, water dynamism tends to vary allometrically with body size. Weight-specific EWL and weight-specific total water turnover tend to decrease as size increases.

Animals on Land: Case Studies

  • Most terrestrial amphibians have meager physiological abilities to limit water loss because their skin is highly permeable to water and they cannot make urine that is hyperosmotic to their body fluids. Stringent behavioral control of water balance and seasonal dormancy are essential for their success in arid places. A few types of arboreal amphibians that live in arid areas have unusual adaptations such as cutaneous lipids that protect against rapid evaporative water loss.
  • Insects and lizards are among the animals that are most physiologically capable of living in the driest places on Earth. Their key traits for existence in extreme places include very low integumentary permeability to water, relatively low metabolic rates, excretion of poorly soluble nitrogenous wastes, and tolerance of profound changes in body-fluid composition. Insects can produce hyperosmotic urine and sometimes gain water from atmospheric water vapor, but the fact that they are small is in itself a physiological (although not behavioral) disadvantage.
  • Some small mammals that eat predominantly air-dried foods (e.g., seeds) live in deserts without needing to drink. In addition to having highly evolved physiological mechanisms of water conservation, they depend on behavioral selection of relatively benign microhabitats to maintain water balance. In the hottest places they live, they probably must supplement their diet with water-rich foods such as insects.
  • Although some desert birds seem to succeed because of general avian properties that are of advantage under desert conditions, others exhibit dramatic specializations for desert existence.

Control of Water and Salt Balance in Terrestrial Animals

  • The control of body-fluid volume, composition, and osmotic pressure is mediated mostly by hormones that are secreted under control of negative feedback systems. Stretch or pressure receptors provide information on blood volume, and osmoreceptors provide information on blood osmotic pressure.
  • In vertebrates, antidiuretic hormone (ADH) regulates the amount of pure, osmotically free water that is excreted by the kidneys; it does so by controlling whether a more-than-minimum amount of water is excreted with solutes.
  • Aldosterone and natriuretic hormones in vertebrates act to promote Na+ retention or Na+ excretion, respectively. The control of body Na+ content by these hormones helps to control extracellular-fluid volume because body Na+ is present mostly in the extracellular fluids.
Copyright 2016 Sinauer Associates
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