Wherever they live, animals need oxygen in order to survive. By breathing, or respiring, they extract oxygen from their surroundings and dispose of carbon dioxide waste (see Respiration).
Gallery animal: Both the lung structure of air-breathing organisms and the swim bladders of most modern fishes evolved from paired air sacs of primitive bony fishes. In the primitive fish, as in the modern bony fishes, these sacs served as a buoyancy device that inflated and deflated to alter the fish’s depth in the water. In other fish, they became primitive lung structures, repeatedly folding inward to maximize oxygen uptake in an oxygen-deprived environment. Both kinds of fishes improved upon a preexisting adaptation but in so doing evolved into very different groups of organisms.
Gallery animal: A fish breathes by absorbing oxygen from the water it drinks. Water flows into the mouth, through the gills, and out of the body through gill slits. As water flows through the gills, the oxygen it contains passes into blood circulating through gill structures called filaments and lamellae. At the same time, carbon dioxide in the fish’s bloodstream passes into the water and is carried out of the body.
Very small animals do not need any special adaptations for obtaining oxygen. Oxygen simply diffuses in through their body surface, with carbon dioxide traveling out the same way. Larger animals cannot rely on this system because they have a much bigger volume relative to their surface area. To obtain sufficient oxygen, large animals have to boost their oxygen intake by using special respiratory organs. In water, many animals breathe by using gills. A typical gill consists of a stack of thin flaps connected to the animal's blood supply. Water moves past the flaps in a one-way flow, either when the animal moves, or when it pumps water through its body. The flaps extract oxygen from the water and pass it into the blood, which transfers it to needed tissues. The blood releases carbon dioxide in exchange.
Gallery animal: The axolotl is actually the aquatic larval stage of the brown salamander. Axolotls are of interest to scientists because not all axolotls metamorphose, or change, into adult salamanders. More interestingly, those axolotls that do not transform may become sexually mature while in the larval stage. In captivity, axolotls can be induced to change into adult salamanders by the addition of thyroid extract to the surrounding water.
Gills do not work on land because their flaps collapse and stick together. Instead, land animals have evolved two different kinds of respiratory organs: tracheal systems and lungs. Tracheal systems are found in insects and many other arthropods. They consist of slender hollow tubes, called tracheae, that reach deep into the body, delivering oxygen from outside. Lungs are hollow cavities that have a large surface area. They are found in vertebrates and also in some invertebrates, such as terrestrial mollusks.
Gallery animal: A Pacific Sea whitesided dolphin breaks the surface of the water while swimming to breathe through a blowhole on the top of its head. Underwater, a dolphin communicates with whistles emitted in single-toned squeals to convey alarm, sexual excitement, and perhaps other emotional states. Dolphins inhabit all the world’s oceans, using their streamlined bodies to reach underwater speeds of 40 km/h (25 mph). This swimming ability coupled with sharp teeth enables dolphins to capture fish and squid, their principal prey.
In tracheae and most lungs, gases move in a two-way flow. Most vertebrates actively pump air in and out of their lungs to step up the rate of gas exchange. By stretching and squeezing their bodies, some arthropods behave in a similar way.