What are "alveoli"?
The alveoli are the little sacs out on the end, and you need to visualize this paper thin, very fine little sac that has capillaries around it that the red blood cells flow around in a constant monolayer around the alveolar sac so that these red blood cells are maximally exposed to the oxygen that's available in the alveolus, and are capable of, with the blood flow and the red blood cells, getting rid of the CO2 and then come back together again on the other side, transporting it back into another system that carries the blood back to the left side of the heart and then out to the rest of the body. There are probably about three hundred million alveolar sacs in the lungs and if you visualize a football field, that package inside your chest in these alveoli are enough alveoli to give you a surface area that would extend from the goal line out to about thirty yards when you lay that out as a monolayer. And it's an amazing bit of packaging that goes on inside our lungs that allow not only the blood to flow through but also for that much surface area to be available for these little alveoli.
