Respiratory Organs

Every living animal must take in oxygen and get rid of carbon dioxide, but the way this gas exchange is carried out is not the same across the animal kingdom. How an animal breathes depends largely on where it lives and how complex its body is.

Gas exchange scales with body plan and habitat

• Simple, direct diffusion over the body surface works for the lowest invertebrates. Sponges, coelenterates and flatworms have no special breathing organ at all; oxygen and carbon dioxide simply diffuse in and out across their entire outer surface.
• Moist skin does the job in earthworms, which rely on their damp cuticle for exchange.
• Tracheal tubes are used by insects, which possess a branching network of fine air tubes that carry atmospheric air directly to the tissues inside the body.
• Gills (branchial respiration) are special richly-vascularised outgrowths used by most aquatic arthropods and molluscs to draw dissolved gases from water.
• Lungs (pulmonary respiration) are vascularised, bag-like structures that terrestrial animals use for exchanging gases with air.

Among the vertebrates

Fishes breathe using gills, while the land-dwelling groups — amphibians, reptiles, birds and mammals — respire through lungs. Amphibians such as frogs have an additional route: they can also exchange gases through their moist skin, a mode called cutaneous respiration.

What this chapter covers …