The opposing forces during inspiration and expiration

[Lothric]

Hey,

From what I can gather, during a forced expiration, the intrapleural pressure becomes positive and the force will be pointed inward in synergism with the elastic recoil of the lung. This is why it gets more and more difficult as you expire.

But if we flip the steak, how does it work if I inspire more than a tidal volume? I.e., will the force generated during a inspiratory reserve volume also lead to the elastic recoil force from the lungs to change direction to the opposite? This way both the outward-oriented force from the chest wall and now the lungs will contribute to the increased lung volume. Or is this wrong?

I'm really struggling with the respiratory physiology so I would really appreciate help!

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[dnnash]

I'm not sure I understand the question, but the elastic recoil of the lung will tend to make it want to collapse always. The expansion of the chest caused by inspiratory muscles causes a transient decrease in intrapleural pressure which pulls the lung open, against its elastic recoil.

 

[Lothric]

Thanks for your reply,

Are the opposing forces always equal or is the elastic recoil strength of the lung greater than the outward force from chest wall during expiration? And vice versa, would the expanding force be greater than the inward force during inspiration?

 

[dnnash]

Yes the net force determines what the lung will do. When breathing out, the recoil is stronger than any outward chest wall force. When no air is moving through an open airway, that means the opposing forces are equal and the net is zero. If you have a closed airway, you can introduce a third force by having pressure of the air inside the lung being greater than atmospheric pressure. For example, just before a cough, your muscles work to compress your chest in addition to the elastic recoil, but the airway is closed so no air flows. The reason your lung doesn't collapse is the third force of the elevated pressure inside the lung, which equals to the sum of the other two forces. With COPD, you can have collapse of small airways when air is flowing through, which increases the work of breathing because the body must produce a force that is stronger than the pressure built up behind the block. Hope that makes sense. Not sure how much you'd really need to know for step though