How are the alveoli adapted to allow efficient gas exchange within the lungs?

The efficiency of gas exchange across a surface is determined by Fick's Law. Fick's Law states that, for the diffusion of a gas to be efficient, three conditions should be met. Firstly, the surface area of the diffusion pathway should be large. Secondly, there should be large difference in concentration between the area the gas is diffusion to, and the area it is diffusing from. Finally, the diffusion pathway should be short.

The alveoli allow all three of these conditions to be met, allowing efficient gas exhange within the lungs. There are around 350 million alveoli on each lung, creating a very large surface area for diffusion to take place across. Alveolar cells are specially adapted to create a short diffusion pathway to the blood capillary network. They are flattened, single-celled epithelia, meaning there is only a small area of cytoplasm for gases to diffuse through before they reached the blood capillary epithelia. Finally, a large concentration gradient is created between the alveoli and the capillaries through the removal of oxygenated blood, which is replaced with oxygenated blood. This creates a well-ventilated gas exchange surface.