The epithelial cells in the lungs are arranged into alveoli. Explain how the alveoli create a surface for efficient gaseous exchange.

Alveoli are made of two main types of cells. The alveoli walls are made of a single layer of squamous alveolar cells (aka Type I cells), which are flat and thin, further shortening the diffusion distance. Alveoli also contain great alveolar cells (Type II cells) which secrete a fluid called surfactant. Surfactant reduces the surface tension of water, thus keeping alveoli dry as well as helping to stabilise alveolar size, which in turn helps in the exchange of dissolved gasses. Within the alveolar sacs, there are collagen and elastin fibres, which help them to recoil, pushing air out to expel waste products on expiration.
A typical pair of human lungs contain around 700 million alveoli, each with an average diameter of around 200μm which increases when inflated. The large number of alveoli, as well as their large SA:V ratio provides a massive surface area for gas exchange - around 75m². Wrapped around the alveoli are capillaries, which cover about 70% of the SA of each alveolus. This helps to maintain a high concentration gradient of "useful" and waste products on either side of the lung, allowing for efficient gas exchange.