Terrestrial environments are characterized by limited water availability, so dehydration is a major threat. Evaporative water loss (EWL) across the skin and respiratory tract is a major avenue of water loss by terrestrial animals. Water is also lost in feces and urine. Water is gained in a terrestrial environment via drinking, as preformed water in food, and as metabolic water production. Water may also be absorbed across the body surface. Ions are gained from food and by drinking, and are excreted in urine and feces and sometimes by salt glands.
Many invertebrates (e.g., mollusks, crustaceans) and amphibians are restricted to moist terrestrial habitats, at least when active, but many are more successful terrestrial animals because they have adaptations to minimize EWL. Arthropods have a chitinous exoskeleton, covered in a waxy cuticle that forms a barrier to evaporation. Birds, mammals, and especially reptiles have a cornified epithelium that increases resistance to EWL (Figure 5). Insulating fur (mammals) or feathers (birds) is a further barrier. Nasal counter-current exchange of heat and water in the respiratory passages of reptiles, birds, and mammals reduces respiratory EWL. Arthropods, birds, and reptiles typically produce insoluble uric acid as their nitrogenous waste material, and the mixing of urine and feces in their hindgut (where water is reabsorbed) minimizes excretory water loss. Many desert reptiles and mammals survive without drinking, maintaining water balance with preformed and metabolic water alone. Most birds are able to travel long distances to obtain drinking water, although some can also survive without access to free water. Excess ions are lost by many reptiles and birds via cranial salt glands. Mammals do not have salt glands, and remove excess ions by producing urine that is hyperosmotic to
5000 Tenebrionid beetle
1500 Desert lizard
500 Cocooned frog
160 Ostrich skin
200 Gecko skin
Litoria tree frog 75 Terrestrial crab
30 Tsetse fly
2-3 Isopod, cane toad
Figure 5 Scale of resistance to evaporative water loss, from about 0 s cm"1 for a free-water surface to 5000 s cm"1 or more for animals that are very resistant to EWL.
blood (up to 9000 mOsm). Some birds are also able to produce hyperosmotic urine to excrete excess ions, but not to the same extent as mammals.
eliminating nitrogenous wastes and osmoconcentrating urine in some species.
Four major organ systems are responsible for excretion in animals. The respiratory system (lungs or gills) removes CO2, and gills also play a vital role in ammonia, carbonate, and ion excretion, by both diffusion and active transport. The digestive system, in addition to eliminating undigested food, is also a site of ion and water absorption and excretion, and the vertebrate liver excretes bilirubin (derived from the breakdown of red blood cells) into the gut. The integument and various glands of animals may have a primary or secondary excretory function, for example, water and ion uptake by the skin of amphibians, salt glands of reptiles and birds, rectal glands of elasmobranchs, and sweat glands of mammals. Renal organs, including protonephridia, nephridia, Malpighian tubules, and coelomoducts (e.g., the vertebrate kidney) consist of tubules that filter body fluids and then selectively secrete or reabsorb water, organic molecules, and ions. The major functions of these excretory tubules are initial formation of excretory fluid, typically by filtration, then reabsorption of fluid and 'useful' solutes and secretion of specific 'waste' solutes. Only a few terrestrial animals are able to excrete urine that is more osmotically concentrated than their blood; the vertebrate kidney can excrete hypoosmotic or isoosmotic urine but only mammals and birds can excrete hyperosmotic urine due to the counter-current multiplication role of the renal medulla.
Excretory organs are essential for maintaining iono- and osmohomeostatis as they balance the gains and losses of water and solutes. They regulate the concentrations of ions and water in the body and play a vital role in excreting waste products including inorganic and organic solutes derived from the animal's diet, metabolic processes or foreign materials, preventing these wastes from accumulating to toxic levels. Thus excretory organs must selectively retain or remove a range of solutes from the body.
Simple animals rely on diffusion and membrane transport systems to remove wastes. However, the evolution of larger and more complex animals necessitated specialized excretory organs. Although in most animals the integument is relatively impermeable to water and solutes, specific epithelial regions can be specialized for the regulation of particular solutes or water. Tubular excretory organs are more generalized than these epithelial organs, and occur in most multicellular animals. They evolved primarily for water and solute excretion, but in a terrestrial environment they also play a crucial role in
Most animals require oxygen to sustain their metabolic demands. Food is oxidized to produce adenosine triphosphate (ATP) and carbon dioxide is produced as a waste product, so animals must obtain oxygen from their environment and release carbon dioxide back into the environment. Gas exchange between the internal and external environment in all animals occurs through passive diffusion. For small, simple animals, diffusion across the body surface is sufficient to meet their metabolic demands. However, an evolutionary trend among animals for increased size and metabolic rate requires specialized surface regions for specific functions such as gas exchange (as well as locomotion, feeding, digestion, and sensory reception). So, a large body size and complexity necessitates specialized respiratory structures. Most respiratory structures require ventilation, the continual replacement of the external medium at the respiratory surface with fresh medium to maintain favorable concentration gradients for diffusion (Figure 6). Animals are classified as air and/or water breathers. The physical characteristics of these two media constrain the ventilatory mechanisms
Water a v
Figure 6 Schematic diagrams of respiratory gas exchange across skin, gills, and lungs, showing patterns of fluid flow and O2 exchange between the medium and blood, showing complete equilibration between the water (or air) and blood for skin exchange (left), typical counter-current arrangement of water and blood flow for gills (center), and a tidal pool of air for lungs (right). i, incurrent; e, excurrent; a, arterial; v, venous. Modified from Withers PC (1992) Comparative Animal Physiology. Philadelphia: Saunders College Publishing.
necessary to maintain gas exchange across the respiratory surface, and therefore the nature of the surface itself.
Aquatic animals have gills, evaginated and highly folded external surfaces, for gas exchange. Water is dense and viscous (compared to air) so unidirectional flow over the gill surface is preferable. This also means that gills can have a counter-current flow of external medium (water) and internal fluid (blood/hemolymph) for very efficient O2 extraction by counter-current exchange (Figure 6). The O2 concentration is also much lower for water (5-6 ml l-1) than for air (210 ml l-1) so a high efficiency of counter-current exchange is important. CO2, however, is extremely soluble in water so its loss to the aquatic environment is not so problematic as O2 uptake. Consequently, aquatic animals generally have low body fluid CO2 levels.
Terrestrial animals have internalized respiratory structures, lungs or trachea, because avoiding desiccation is a major challenge. Moist externalized respiratory structures such as gills can have an excessively high EWL, but internalized structures have a lower EWL. Air is much less dense and viscous than water, and has a higher oxygen concentration, so lung ventilation by a tidal pool or cross-current system is not too inefficient or energetically restrictive. Lungs may be ventilated by positive pressure 'buccal pumping', as in amphibians, or by negative pressure inspiration, as in reptiles, mammals, and birds. Unlike the one-way tidal ventila-tory pattern of most vertebrates, birds have a system of air sacs before and after the lung, which enables a oneway flow of air over the respiratory surface and allows a more efficient cross-current exchange system between the air and blood. The gas exchange system of arthropods consists of a series of air-filled tubes (tracheae) that infiltrate the body tissues and open to the external environment through spiracles at the body surface. Tracheal systems are generally not actively ventilated, relying on diffusion for gas exchange, a factor that limits the size of arthropods. The lungs of pulmonate snails are similarly diffusion driven.
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