The alveoli is the functional unit of the lungs responsible for gas exchange. Water is not exchanged in the lungs therefore water pressure difference are not significant in this case. Oxygen on the other hand, there will be a significant difference from the partial pressure of oxygen in the environment compared to the partial pressure of oxygen in the alveoli cells and the lung capillaries. The partial pressure of oxygen in the alveoli cells and the lung capillaries are lower than the partial pressure of oxygen in the environment. This pressure gradient predicts the direction of diffusion from environmental oxygen to the alveoli then to the lung capillaries, and this process is called oxygenation.