The body takes a deep breath to avoid aspirating vomit. Retroperistalsis starts from the middle of the small intestine and sweeps up digestive tract contents into the stomach, through the relaxed pyloric sphincter. Intrathoracic pressure lowers (by inspiration against a closed glottis), coupled with an increase in abdominal pressure as the abdominal muscles contract, propels stomach contents into the esophagus as the lower esophageal sphincter relaxes. The stomach itself does not contract in the process of vomiting except for at the angular notch, nor is there any retroperistalsis in the esophagus. Vomiting is ordinarily preceded by retching. Vomiting also initiates an SNS response causing both sweating and increased heart rate. Phases
The vomiting act has two phases. In the retching phase, the abdominal muscles undergo a few rounds of coordinated contractions together with the diaphragm and the muscles used in respiratory inspiration. For this reason, an individual may confuse this phase with an episode of violent hiccups. In this retching phase, nothing has yet been expelled. In the next phase, also termed the expulsive phase, intense pressure is formed in the stomach brought about by enormous shifts in both the diaphragm and the abdomen. These shifts are, in essence, vigorous contractions of these muscles that last for extended periods of time—much longer than a normal period of muscular contraction. The pressure is then suddenly released when the upper esophageal sphincter relaxes resulting in the expulsion of gastric contents. Individuals who do not regularly exercise their abdominal muscles may experience pain in those muscles for a few days.
Professor Trevor Cox of the University of Salfords Acoustic Research Centre said, "We are pre-programmed to be repulsed by horrible things such as vomiting, as it is fundamental to staying alive to avoid nasty stuff." It is thought that disgust is triggered by the sound of vomiting to protect those nearby from possibly diseased food. Psychology
Emetophilia is sexual arousal from vomiting, or watching others vomit. Emetophobia is a phobia that causes overwhelming, intense anxiety pertaining to vomiting. See also
Bulimia nervosa
Emetophilia
Cancer and nausea
Emetophobia
Vasodilation
Diarrhea
Nose-blowing
Belching
Chyme

Notes
References
External links

Cyclical Vomiting Syndrome
"Emetics" . The American Cyclopædia. 1879.