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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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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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

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