Respiration | Research & Encyclopedia Articles

Organisms respire in order to obtain oxygen and get rid of carbon dioxide. Oxygen drives cellular metabolism, the process in which glucose from food is converted to usable energy resources in the form of adenosine triphosphate, (ATP). Carbon dioxide is a by-product of this reaction. Aquatic species obtain oxygen from water, while terrestrial species obtain it from air. These two media, water and air, are often associated with different respiratory strategies. This is partly because the amount of oxygen in air is far greater than that dissolved in water. In addition, air requires much less energy to pump than does water, which is considerably more dense.

Many organisms do not require special respiratory organs because they obtain an adequate supply of oxygen through diffusion across the body surface. This is known as cutaneous gas exchange. Cutaneous gas exchange is often employed by small animals, which have a high ratio of surface area to volume. Some larger animals, such as certain annelid worms, are also able to use cutaneous exchange because of their large surface areas and modest energy demands.

Larger organisms almost invariably possess special respiratory organs. Respiratory specializations can be grouped into three major categories: gills, lungs, and trachea. All three types of organs evolved to provide extensive surface areas for use in gas exchange.

Gills describe respiratory structures formed from external extensions of the body. They characterize diverse species, including some annelids, some arthropods such as horseshoe crabs and crustaceans, mollusks, certain echinoderms, and vertebrates such as fish and larval amphibians. Gills are typically found in aquatic environments, although some terrestrial species, such as terrestrial crustaceans, make use of them as well. Species with very active lifestyles tend to have more highly developed gills, with considerable surface areas for gas exchange. In the fishes, for example, a series of bony gill arches supports many primary gill filaments, each of which has numerous minute folds referred to as the secondary gill lamellae.

Oxygen is absorbed into the bloodstream as oxygenated water flows over the gills. Some aquatic species rely on natural currents to carry oxygenated water to them. Others expend energy to force water over the gills. Among fishes, ram ventilators swim rapidly with open mouths, forcing water to flow into the mouth, across the gills, and out through the spiracle or operculum. Active swimmers, such as sharks and tuna, often employ ram ventilation.

Other fish species use a complex series of mouth and opercular expansions and compressions to pump water over the gills. Many aquatic species, including fishes, are characterized by countercurrent exchange, a system in which oxygenated water and deoxygenated blood flow in opposite directions, maximizing the amount of oxygen absorption that occurs.

Lungs characterize many terrestrial species. Unlike gills, lungs are series of internal branchings that function in respiration. In humans, for example, air flows initially into the trachea. The trachea then splits into two bronchi, which split several more times into smaller and smaller bronchioles. These end at small sacs called alveoli, which are closely surrounded by capillary blood vessels.

It is in the alveoli that gas exchange actually occurs, with oxygen diffusing across the alveoli into the bloodstream. The alveoli are lined with substances called surfactants, which help to prevent them from collapsing. Humans have approximately 150 million alveoli. As with gills, the surface area available for gas exchange can be quite large.

Lungs characterize a diverse array of terrestrial species, including gastropod mollusks such as snails and slugs, spiders (whose respiratory organsare called book lungs because they contain a series of lamellae that resemble pages), and most terrestrial vertebrates.

Gas exchange using lungs depends on ventilation, the process through which air is brought from the external environment into the lungs. Ventilation mechanisms vary from taxon to taxon. In amphibian species, air is actively gulped, or forced into the lung by positive pressure. Reptiles, birds, and mammals use negative pressure to ventilate the lungs. These species expand the volume of the thoracic cavity where the lungs lie, causing air to be drawn into the lungs.

Creating negative pressure in the thoracic cavity is accomplished in a variety of ways. Lizards and snakes use special muscles to expand their rib cages. Turtles extend forelimbs and hindlimbs out of their shells to create negative pressure. In crocodiles, the liver is pulled posteriorly, towards the rear of the animal, in order to expand the thoracic cavity. Mammals employ a combination of contracting their diaphragm, a muscular sheet that lies at the base of the thoracic cavity, and expanding the rib cage.

Birds have evolved unusually efficient respiratory systems, most likely because of the tremendous energy required for flight. The bird respiratory system includes large, well-developed lungs and a series of air sacs connected to the lungs and trachea. Some of the air sacs occupy the hollow spaces in larger bones such as the humerus and femur. The air sacs are involved in ventilating the lungs, and allow for a one-directional flow of air through the respiratory system. This is unique among the terrestrial vertebrates.

The respiratory exchange system of birds is described as crosscurrent exchange. Crosscurrent exchange is less efficient than the countercurrent exchange system of fishes. However, because the concentration of oxygen in air is much greater than in water, this method is extremely effective.

Cutaneous respiration may supplement gas exchange through lungs or gills in many species. It is critical in most amphibians, which are characterized by moist skins consisting of live cells in which oxygen can dissolve and then be taken up and used by the organism. Some amphibians have evolved special adaptations that make gas exchange across the skin more effective. In some aquatic salamanders, for example, the skin has become highly folded, an adaptation that increases the surface area available for gas absorption. There is even a large family of salamanders, the family Plethodontidae, in which the lungs have been lost entirely. This group relies almost entirely on cutaneous exchange. Similar developments are seen in other groups, including some species of slugs.

The tracheal system is a respiratory system that is unique to air-breathing arthropods such as millipedes, centipedes, and insects. The tracheal system consists of air-filled tubes that extend into the body from pores on the body surface known as spiracles. These tubules provide oxygen to tissues directly. Centipedes have one spiracle per segment, and millipedes have two per segment. The number of spiracles in insects varies, but there can be as many as ten pairs.

The spiracle is merely an opening to the environment in primitive insect species. However, more advanced groups can close their spiracles, andsome are even outfitted with filtering devices. The ability to close the spiracle is advantageous because it allows for water conservation. Interestingly, the limits of the tracheal gas exchange system are believed to restrict insects to their generally small size. Insects, unlike animals such as vertebrates, do not use their circulatory systems to aid in the transport of oxygen.

Water is lost from all respiratory surfaces in air-breathing organisms. This is particularly serious for species that rely on cutaneous exchange, and explains why most species of amphibians are limited to fairly moist habitats.

Respiration is regulated either by the central nervous system or by more localized mechanisms. Vertebrates have a respiratory pacemaker in the medulla of the brain. In addition, respiratory rates are influenced by internal gas concentrations. Aquatic species usually regulate respiration based on internal oxygen levels, whereas terrestrial species tend to rely on internal carbon dioxide levels.

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