"Butterflies in the stomach" are due to the outflow of blood and are a reaction to stress Bashny.Net

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Physical sensations, romantically called "butterflies in the stomach" appear not only as a result of love: You can experience this feeling when much frightened or excited.

Falling in love is fear, change "chemistry" in your brain and body, causing stress. Stress, in turn, triggers the reaction of "fight or flight", accompanied by a number of physiological changes: increased levels of the hormones cortisol and adrenaline, which "give the command" increase blood pressure, enhance blood flow to the muscles and lungs, using all the resources of the body. The sharp outflow of blood from the abdomen and gives the most "ёkayuschee" feeling, accompanied by mild nausea, which many well known.

Slightly less than a romance, a little more knowledge.

via factroom.ru