 We'll move on and discuss long volumes in the physiology of the respiratory tract. It's important to be able to understand this respiratory flow chart for the USMLE. We're going to discuss several different measurements that you must know and be able to calculate. First of which is title volume. The title volume is what air moves into the lung during an inspiration and expiration. The title volume is your normal inspiration-expiration curve. Beyond that, the expiratory reserve volume is air that can still be breathed out after you finish your normal expiration. That is here. The inspiratory reserve volume is the air that you can still breathe in after a normal inspiration, also seen here. And then your residual volume is the air that's in your lung after you have done your maximum expiration and got all the air out that you can. That is seen here. We cannot measure all of the residual volume because there is air that cannot be measured by spirometry. These are the four volumes that we can measure with spirometry. And then beyond that, we can calculate capacities based upon at least two or more of these individual volumes. So one of those capacities is the functional residual capacity. The functional residual capacity is the residual volume plus the expiratory reserve volume. So that is seen here. Next is the inspiratory capacity. The inspiratory capacity is the inspiratory reserve volume as well as the title volume. It is measured from the bottom of our title volume all the way up to the top of our inspiratory reserve volume and is our inspiratory capacity. The next capacity that we can measure is the vital capacity. The vital capacity is calculated by adding together the title volume, the inspiratory reserve volume, and the expiratory reserve volume. The vital capacity is the maximum amount of gas that can be expired during a maximal inspiration. So we calculate this from the top of the inspiration curve to the very bottom of the expiration curve, which is our vital capacity. The final calculation that we can do is the total lung capacity. The total lung capacity is calculated by adding together the inspiratory reserve volume, the expiratory reserve volume, the title volume, and the residual volume. And it is the entire curve from top to bottom, which is our total lung capacity. As we mentioned, the lungs do have physiologic dead space. Dead space is portions of the airway that do not participate in gas exchange, and the air in that dead space cannot be expired from the respiratory tract. So our physiologic dead space is dead space of conducting airways plus our alveolar dead space. As you see here in this picture, our alveolar or our physiologic dead space is everything that you can see that does not participate in gas exchange. We can calculate our physiologic dead space using this formula. Our dead space volume is equal to our title volume multiplied by the arterial partial pressure of carbon dioxide, which we subtract from the expired air partial pressure of carbon dioxide, and divide that once again by the arterial partial pressure of carbon dioxide. The apex of the healthy lung is the largest contributor to alveolar dead space, which leads us to the conclusion that everything is better at the bottom of the lung. We'll talk about more things that are better at the bottom of the lung, but in regards to gas exchange is better at the bottom of the lung because there is less physiologic dead space. Next, let's talk about elastic recoil. Elastic recoil is the tendency for lungs to collapse inward and the chest wall to spring outward. Our chest wall is used to inhale air, and then the elastic recoil of the lung is used to exhale air. We also use the term compliance in regard to the lung. Compliance is the change in lung volume for a particular change in pressure. So if we increase compliance, it makes our lungs easier to fill. If we decrease compliance, it makes our lung harder to fill. Examples of increasing in compliance is emphysema and aging. Examples of diseases that decrease compliance in the lung is pulmonary fibrosis, pneumonia, acute respiratory distress syndrome, and pulmonary edema. Let's talk a little bit more about what aging does to the respiratory tract. As you see in this chart, aging increases compliance as we just spoke about. It also increases the residual volume. We have an increase in the VQ mismatch, and we also have an increase in our AA gradient. Aging decreases compliance of the chest wall specifically, because this causes an increase in the chest wall stiffness. We also see a decrease in the functional vital capacity and the FEV1. Aging also decreases the respiratory muscle strength, and we also see a decrease in our body's natural response to hypoxia.