What is happening in that “wave” of nausea that overcomes you right before you vomit?
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The vomiting reflex also includes strong intestinal contractions moving in a reverse direction, bottom to top. The initial contractions are generally not strong enough to push intestinal contents through the thoracic cavity into the mouth because the upper esophageal sphincter is skeletal muscle and its contraction is quite strong. Thus the vomitus comes up then goes back down in response to an esophageal contraction moving down. The esophagus responds to stretch of its walls by contracting to push material down. This cycle may be repeated several times until sufficient pressure builds to overcome the contraction of the upper sphincter. When the upper sphincter releases, vomitus enters the mouth. Depending on the pressure generated the vomiting may or may not be projectile – blown out vs dribbles. The yellow color of vomitus is bile staining, the recognizable smell is pancreatic fluid (a mixture of enzymes in a sodium bicarbonate solution).
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