Gaseous exchange in lungs
In respiration, oxygen (O2) is required to enter cells whilst waste carbon dioxide (CO2) must be removed – the opposite is true for photosynthesis, where CO2 enters plants and O2 is released. The exchange of gases essentially occurs as a result of diffusion down a concentration gradient– gas molecules moving from an area of high concentration 
low concentration.
Diffusion is the chemical process when molecules from a material move from an area of high concentration (where there are lots of molecules) to an area of low concentration (where there are fewer molecules).
This happens through otherwise random movement. Diffusion usually happens in a gas although it can happen in a liquid. It is possible to see diffusion happening when two liquids are mixed in a transparent container. It describes the constant movement of particles in all liquids and gases. These particles move in all directions bumping into each other. Diffusion can only work with gases and liquids. Here are some examples of diffusion.
Diffusion is one of the several transport phenomena that occur in nature. Diffusion occurs throughout the human body and without it, cells and body tissues could not get important nutrients for survival, the eyes would dry out, and many medicines could not be absorbed into the body. One of the most important examples of diffusion is gaseous exchange in lungs.