Now, what's the point of calculating this value? Well, it was basically to show you that our lungs are capable of exchanging a great number of different molecules and although most of these molecules are oxygen and nitrogen molecules, a good portion of these are actually harmful particles such as pollutants, contaminants, allergens. They can also be viruses and bacterial cells and this can cause a problem because our lungs creates a barrier, a boundary between the outside and the internal environment. And so, in order to ensure that none of these harmful things actually end up inside our body and cause harm to our body, the lungs have to have some type of protective capability. And this is what we're going to focus on in this lecture. So our lungs have several important methods by which they can actually protect themselves from these different types of agents that enter our body when we breathe in air.