Respiratory Organs in Animals

Every living cell needs a continuous supply of oxygen to carry out aerobic respiration, and must equally get rid of the carbon dioxide that this respiration produces. In single-celled organisms and in the simplest multicellular animals, this exchange happens without any dedicated organ at all: protozoans, sponges and coelenterates simply exchange gases by diffusion across the general body surface, a strategy that works only because these organisms are small, or thin, enough that no living cell lies very far from the surrounding water or moist environment. Flatworms (platyhelminthes) rely on essentially the same body-surface diffusion, their flattened, elongated body plan again keeping every cell within a short diffusion distance of the exterior.

As animal bodies became larger and more complex, this simple surface-diffusion strategy stopped being sufficient on its own, and different animal groups evolved their own specialised respiratory organs, suited to their particular habitat and level of organisation. Annelids such as the earthworm still exchange gases across the body surface, but theirs is a considerably more specialised version of it: a thin, permanently moist and richly vascularised skin, kept moist by mucous secretions, through which oxygen and carbon dioxide diffuse directly into and out of an underlying network of blood capillaries -- a mode of respiration usually called cutaneous respiration.

Insects and most other terrestrial arthropods instead rely on an entirely different solution: a tracheal system, a network of branching air-filled tubes (tracheae and their finer branches, tracheoles) that opens to the outside through small paired pores called spiracles along the sides of the body, and carries air directly to individual tissues and cells deep within the body, largely bypassing the need for a respiratory pigment or a blood-borne gas-transport system for this purpose.

Aquatic animals across several different groups -- arthropods such as prawns and crabs, molluscs such as snails and bivalves, and all fish -- instead rely on gills: thin, richly vascularised, feathery or plate-like outgrowths, ideally suited to extracting the comparatively small amount of dissolved oxygen present in water by exposing a very large surface area to a continuous flow of water passing over them.

Amphibians such as the frog occupy a genuinely intermediate position, reflecting their dual aquatic-and-terrestrial life history. Amphibian larvae (such as the tadpole) breathe using gills, exactly as fish do, appropriate to their fully aquatic early life; but as metamorphosis proceeds and the animal moves onto land, these larval gills are lost, and the adult instead relies on a combination of routes -- its moist, richly vascularised skin, the lining of its buccal (mouth) cavity, and a simple pair of lungs -- so that, unlike the animal groups considered so far, adult amphibians genuinely combine more than one respiratory organ at the same time.

Reptiles, birds and mammals, including humans, are the animal groups in which a pair of lungs finally becomes the sole, fully dedicated respiratory organ, reflecting their fully terrestrial life and, in birds and mammals, their comparatively high, warm-blooded metabolic demand for oxygen. It is this human respiratory system, built around a pair of lungs, that the remainder of this chapter examines in detail -- its structure, the mechanism by which air is drawn in and expelled, how the gases it carries are exchanged and transported within the body, how the whole process is regulated, and the disorders that can affect it.