Biology · Gaseous exchange. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Gaseous exchange is the movement of oxygen and carbon dioxide between an organism and its surroundings by diffusion. A good respiratory surface is large, thin, moist and, in larger animals, well supplied with blood and ventilated.
Air is a better respiratory medium than water. It holds far more oxygen, oxygen diffuses through it much faster, and it is lighter and easier to move. So fish must pass a large volume of water over their gills and need the efficient countercurrent flow.
Small or thin animals exchange gases through the body surface. Insects carry air directly to the cells through tracheae. Plants have no special organs: gases pass through stomata and lenticels and diffuse through air spaces.
Students say gas exchange in birds happens in the air sacs. The air sacs only store and move air. Exchange with the blood takes place in the parabronchi of the lungs.
Answer: The tracheal system delivers air directly to the cells, so no blood pigment is needed.
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