Why does forceful “expiration” while not letting the air out cause your face turn red and potentially make you faint? What is the vascular/muscular mechanism behind the pressure buildup?
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Blood returning to your heart in your thorax is at very low pressure, basically atmospheric to negative. When you do that (it’s called a Valsalva maneuver BTW) you increase your intrathoracic pressure so much that blood basically cannot return to your heart. That causes it to pool elsewhere in your body, such as face. That causes redness in your face.
As for fainting, heart need blood to go in to increase pressure, dilate, and then do a strong contraction to pump blood in your system. Empty heart can neither pump adequately, nor does it have blood to pump. That causes your blood pressure to drop, and consequently you lose consciousness.
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