Let’s redo the lungs, shall we? They are toxicologist’s nightmare. First, let’s point out what’s great about our airway systems: the filtration provided by our nose, nasopharynx, pharynx and larynx; the clearance of foreign substances by dual layer of mucous in the airways (a thin layer in underneath the thick so the cilia can beat in the thin and move the thick layer up and out); and the generally competent immune function of the lungs. Let’s see how things go wrong.
Gravity isn’t much help here, when you breathe particles that are around 5 īm in diameter as it helps to keep them down in the lung (smaller than that and the major effector is Brownian motion, not gravity); consequently, there’s a lot of stuff that never gets out. And for the stuff that gets trapped it’s much harder to push it up than over or down. So that’s my first fix – turn ’em upside down and give the cilia a break. Maybe they wouldn’t get so tired by the time we get old. (Interesting note: most mammals in the evolutionary tree aren’t working against their lungs since they aren’t upright for a lot of the time).
The one ended design of the lungs also leaves a lot to be desired. The lungs can hold 6 liters or so if you really wanted them to, but during a normal breath you only breathe in and out about 0.5 liters. Even if you exhaled as hard as you could, you would still be left with about 1.2 liters (this is a good thing in and of itself, you wouldn’t want the lung to self-collapse and not be able to open it again). During a normal breath you are left with a whopping 2.4 liters that never get exchanged. This means that to get adequate exchange of gases in the lungs you need a huge surface area, which increases your exposure to any toxicants that come wandering in but since most of the dead space air has already equilibrated the gas exchange you get no benefit. Furthermore, acute exposures to your face become semi-chronic, albeit diluted too, to your lungs because the air isn’t exchanged enough. So that’s the second fix, by getting some air sacs like birds, we could have a unidirectional flow of air through the lungs, which means that we could have smaller lungs with less surface area exposed to toxicants but enough to diffuse gasses more efficiently with the blood (you’ll note that this wouldn’t be a good change for toxicants that diffuse through the lungs and into the blood as easily as oxygen and carbon dioxide, but those are few and far between the rest of the toxics).
The last thing that could be changed for the better is getting the airway out from right beside the food intake. Pneumonia in the elderly is largely caused by aspirating food into the lungs that doesn’t get adequately moved up and out (there’s that darn gravity again!). This could be solved by moving the lungs over the food intake as in suggestion 1 and the epiglottis would move to the upper back of the throat.
Granted this would take some rearranging of the body to accommodate the upright lungs (which for various reasons should stay attached to the same opening for food). It sure wouldn’t be sexy but it would be quite efficient and less vulnerable.