Types of Respiratory System

Why gases must be exchanged

Every living cell releases carbon dioxide and needs oxygen for respiration. The structure or surface across which these gases pass is the respiratory surface. In small organisms the whole body surface is enough, but larger, more active animals need specialised organs. The rate of gas exchange must keep pace with the rate of respiration, so a very active animal such as a bird needs a far more efficient system than a slow-moving worm.

Plants have no special respiratory organs of their own: gases diffuse in and out through stomata in the leaves and through lenticels in woody stems.

Respiratory structures in different organisms

• Body surface — Amoeba, Paramecium, Hydra, planarians and earthworms exchange gases by diffusion across the moist body surface or thin skin.
• Tracheal system — insects such as grasshoppers take in air through openings called spiracles that lead into a network of tracheae and fine tracheoles delivering air directly to tissues.
• Gills — fish pass water over feathery gill filaments; a countercurrent flow of water and blood keeps a diffusion gradient along the gills.
• Lungs — mammals, birds, reptiles and adult amphibians have internal lungs. Frogs also use their moist skin and the lining of the mouth.

Features of an efficient respiratory surface

Whatever the organism, a good respiratory surface shares the same features:

• Large surface area — more room for diffusion.
• Thin — usually one cell thick, giving a short diffusion distance.
• Moist — gases dissolve before diffusing across the membrane.
• Permeable — allows gases to pass freely.
• Good transport supply — a rich network of blood capillaries (or air tubes in insects) carries gases quickly to and from the surface, maintaining a steep concentration gradient.