Breathing is impossible to override, it's also very easily disturbed. Every time you get upset, you hold your breath - not a lot, but just enough to decrease the intake of oxygen. Think about those times when you were really sad and felt like crying, but because you were in your boss's office or in the third pew of the church at your cousin's wedding, you were too embarrassed to let it all hang out. By holding your breath, you managed to hold back the tears. Whenever you're scared, your breathing tends to speed up and become shallow.
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