In single-celled organisms, e.g. Amoeba, the surface area is large enough to meet the needs of the organism and therefore materials can be exchanged directly across its thin and permeable cell surface membrane
One consequence of maintaining a moist respiratory surface in terrestrial animals is water loss: this is minimised by having internal gas exchange surfaces, called lungs
The alveoli produce a surfactant, which lowers the surface tension preventing the alveoli from collapsing and sticking together, and allows gases to dissolve