Concept of gaseous exchange

Gaseous exchange is the movement of oxygen and carbon dioxide across a respiratory surface. Unicellular organisms carry out gaseous exchange by diffusion across the cell membrane.

Respiratory surfaces in various organisms

Organism	Respiratory surface
Amoeba	Cell membrane
Insects	Tracheal system
Spider	Book lung
Fish	Gills
Plants	Leaves, stems, roots
Amphibians	Skin, gills and lungs
Mammals	Lungs
Birds	Lungs
Reptiles	Lungs

Features of respiratory surface

1. They are thin to reduce diffusion distance
2. They are moist to dissolve gases so that they dissolve and diffuse in solution form
3. They are highly branched, folded or flattened in order to increase the surface area for gaseous exchange
4. They are close to efficient transport and exchange easily by the cells
5. They are well ventilated so that gases pass easily