Oxygen and Respiration — Page 37, Lesson 64

I want to walk you through the start of the Oxygen and Respiration chapter. We're going to look at how air gets from outside your body down to the deepest parts of your lungs, and then I'll define the key lung volumes you need to know as a professional pilot. Let's begin with the airway. Air enters through the nose, where it gets warmed, moistened, and filtered, or through the mouth. From there it passes into the trachea — that's your windpipe. The trachea is a tube that's reinforced with cartilage rings, so it stays open even when you move your neck or when pressure changes around you. The trachea then divides into the left and right bronchi, and those take the air into the two lungs. Once inside the lungs, the airways keep branching into smaller and smaller passages. They eventually end in tiny sacs called the alveoli. Each alveolus is very small, but a normal lung contains thousands of them. When you add up all their surface area, you get some hundreds of square metres — that's a huge area for gas exchange. Now, the walls of the alveoli are very thin, and they're covered by fine capillaries, which themselves have only a thin wall. This is critical: oxygen from the alveoli diffuses into the blood, and carbon dioxide and water pass from the blood into the lungs to be exhaled — that's what we call expiration. But effective gas exchange only happens between the alveoli and the capillaries. The walls of the larger passages in the lung are too thick to allow diffusion, so the alveoli are where the real work gets done. That figure shows the main divisions of the respiratory system. Now let's move to pulmonary volumes and capacities. 'Pulmonary' simply means 'of the lungs'. You need to be familiar with these definitions as a pilot, because they relate to how much air you can move and what's left in your lungs at different stages of breathing. First is Tidal Volume. That's the volume of air inhaled and exhaled with each normal breath. In a normal male adult, it amounts to about 500 millilitres. Next is Inspiratory Reserve Volume. This is the extra volume of air that can be inhaled over and beyond the normal tidal volume. So if you take a normal breath in, then force yourself to take in even more air on top of that, that extra amount is your inspiratory reserve volume. It amounts to about 3000 millilitres in a normal male adult. Then we have Expiratory Reserve Volume. This is the amount of air that can still be exhaled by forceful expiration after the end of the normal tidal expiration. So after you breathe out normally, if you then force out as much air as you can, that extra air you push out is the expiratory reserve volume. It amounts to about 1100 millilitres in a normal male adult. Finally, there's Residual Volume. This is the volume of air remaining in the lungs even after the most forceful expiration. No matter how hard you try to empty your lungs, you cannot get this air out — it stays there. It amounts to about 1200 millilitres in a normal male adult. One important note: all pulmonary volumes and capacities are about 20 to 25 percent less in the female. So if you're calculating or comparing these values, you need to account for that difference. That covers the basic airway anatomy and the four key lung volumes you'll need to understand as we go further into oxygen and respiration.