On TV shows, when someone gets nearly hysterical they may be instructed to breath into a paper bag.
What, if anything, does this do? Does it affect BP, heartbeat, blood pH? All the above? None of the above?
When someone hyperventilates (breathes too fast), they blow off too much carbon dioxide and it can cause some physical problems. You can get light-headed, dizzy, and panic even more. In the most severe cases of hyperventilation I've seen, they actually had tetany, which is when I think your calcium actually locks up your muscles. It causes people's hands to be doubled up.
When you breath into a paper bag, you re-breathe that carbon dioxide and it helps with the imbalance. Its a very real thing.
I used to hyperventilate as part of my migraine, and it would help alot to breath into a paper bag. Some hyperventilation is a bio-chemical imbalance that can happen out of the blue, and isn't necessarily associated with being upset or hysterical.
Hyperventilation can cause tetany. However, there is no such phenomenon as 'calcium locking up your muscles'. This is incorrect. Hyperventilation causes decreased carbon dioxide in the blood. This leads to an acid base imbalance making the pH of the blood more alkaline. This affects the binding of free calcium within the blood causing it to become bound to various binding proteins. The decreased calcium leads to increased excitability of neurones since they are not blocking sodium channels. Hence, neurones depolarise and lead to widespread muscle contraction, hence the tetany phenomenon