7. At equilibrium, deflated 760 mm Hg, atmospheric pressure Inflation forces = deflation forces Inflation p. = intrapleural p. + elastic recoil 760 mm = 756 mm + 4 mm 760 4 756

8. What is Elastic Recoil? Inflation forces = deflation forces Inflation p. = intrapleural p. + elastic recoil 760 mm = 756 mm + 4 mm 764 4 mm Elastic Recoil It is the force generated on the lung that causes it to contract, compressing the air inside. If the lung was removed, partly filled with air, and its bronchus was sealed, there would be about 764 mm Hg pressure (~ 4 mm above ambient pressure) inside the lung due to this squeezing. 760 mm Hg, atmospheric pressure 764 mm = 760 mm + 4 mm

9. Starting to inhale …. 760 mm Hg, atmospheric pressure Inflation forces = deflation forces Inflation p. = intrapleural p. + elastic recoil e.g ., it drops ~6 mm, to 750 mm …. 760 mm >> 750 mm + 4 mm 758 4 750 Pleural cavity’s volume increases * , and therefore its pressure decreases! Actually, as air flows into the lung, pressure in the lung (Alveolar p.) drops to 758 because of airway resistance , but it’s still greater than 754, still enough pressure gradient . * The action of muscles (diaphragm, ext. intercostals, etc.) do this.

10. Inhaling …. 760 mm Hg, atmospheric pressure Inflation forces = deflation forces Inflation p. = intrapleural p. + elastic recoil 758 4 750 No longer in equilibrium, the lung’s now greater inflation pressure allows air to fill it and it expands As it expands, it now encroaches on and reduces the intrapleural volume that had initially increased ….

11. 760 mm Hg, atmospheric pressure 760 4 756 At equilibrium, inflated Inflation forces = deflation forces Inflation p. = intrapleural p. + elastic recoil The intrapleural volume is now reduced (by the inflated lung) to the same as it was, and therefore pressure returns to 756 again. Air no longer flows into the lung, and alveolar pressure is again 760 – equilibrium returns !

12. 760 mm Hg, atmospheric pressure 762 4 758 Beginning to exhale …. * Either passive (elastic recoil), or with muscles (int. intercostals, abdominals, etc. Inflation forces = deflation forces Inflation p. = intrapleural p. + elastic recoil Pleural cavity’s volume decreases * , and therefore its pressure increases! e.g ., it rises ~2 mm, to 758 mm …. 760 mm << 758 mm + 4 mm Actually, as air flows out of the lung, pressure in the airway follows a gradient from 762 to 760. But this could be higher if muscles are used !

13. Ending exhalation …. 760 mm Hg, atmospheric pressure Inflation forces = deflation forces Inflation p. = intrapleural p. + elastic recoil 762 4 758 The lung deflates as air flows out, and now taking up less of the pleural cavity’s volume, the volume increases and the pressure drops. When exhaling ends, the pleural pressure drops from 758 to 756 mm 760 mm = 756 mm + 4 mm And we have a new equililbrium

14. At equilibrium, deflated 760 mm Hg, atmospheric pressure Inflation forces = deflation forces Inflation p. = intrapleural p. + elastic recoil 760 mm = 756 mm + 4 mm 760 4 756

21. An Endotracheal Tube looks like this: Inflatable cuff Valve

22. And is inserted like this: The inflated cuff seals the airway around the ET tube, so that air can’t leak past.

23. Mammalian Respiration, because of their unique characteristic of having a diaphragm, uses Negative Pressure Breathing . This means that the driving force to inhale air is by creating a negative pressure around the lungs, generated by a decreased (relative to ambient) intrapleural pressure around the lungs from the action of the diaphragm, intercostal muscles, etc.

24. All other vertebrates (amphibians, reptiles, and birds) are Positive Pressure breathers . This means that they force air into their lungs by generating a positive pressure – i.e. , one that is greater than the ambient pressure – that forces air into the lungs and pushes outward against the chest wall to inflate the lungs (overcoming the ambient pressure which otherwise compresses the chest and keeps the lungs deflated).

26. The Endotracheal Tube, when attached to a respirator – a machine that cyclicly pumps air through the tube into the lungs to inflate them – essentially turns a patient into a Positive Pressure breather (the machine = the frog’s mouth’s muscles). Thus, a patient such as the Governor doesn’t have to rely on an intact chest wall that is required of Negative Pressure breathers. No doubt, after the ribs and sternum are repaired, he’ll no longer need the tube.