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?
When u inspire, ur lungs are expanding. That causes a negative pressure inside ur chest which helps pull blood from ur legs up into ur heart. Kinda like sucking water thru a straw.. when ur sucking, ur lungs are expanding and water is drawn up the straw..
Now, if u blow hard without letting the air out, ur doing the reverse.. ur increasing the pressure inside of ur chest which means the blood in ur legs are getting pushed away from the heart rather than getting pulled into it.
Once ur heart dosent have is blood supply, it can’t pump it to it’s most precious organ, the brain.. hence the brain switches off and u faint. Once u fall, the breath holding stops so the pressure is back to neutral and there’s no gravity to keep the blood in the legs and so it casually flows into the heart and ur brain gets what it wants and ur back to reality again! Ta daaaa
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.