When you vomit, your stomach barely squeezes; your diaphragm and belly do the work
Throwing up feels like the stomach heaving, but it hardly contracts at all. Instead the brain coordinates a deep breath, a wave running backwards from the small intestine, and crushing pressure from the diaphragm and abdominal muscles. A burst of endorphins afterwards explains the odd relief that follows.
Vomiting, known medically as emesis, can come from food poisoning, gastroenteritis, pregnancy, motion sickness or a hangover, and also from brain tumours, raised pressure inside the skull or heavy doses of ionising radiation. Nausea usually comes first but does not always lead to it. The control hub sits on the floor of the brain's fourth ventricle in a small region called the area postrema. Unusually, it lies outside the blood-brain barrier, so drugs circulating in the blood can reach it directly, either to provoke vomiting or to block it. It carries receptors for dopamine, serotonin, opioids and substance P, which seems to be the final shared signal.
Other inputs feed the same centre. The balance organs, reporting through the eighth cranial nerve, drive motion sickness. The vagus nerve fires the gag reflex when the throat is irritated, and gut nerves send alarms when chemotherapy, radiation or infection inflames the lining. Stress and psychiatric conditions act through higher brain centres, and the medulla triggers the act itself.
The sequence runs in stages. Saliva increases, partly shielding the teeth from acid. A deep breath helps guard the airway. A reverse wave pushes contents from mid-intestine back into the stomach. During retching, the diaphragm and abdominal muscles contract together in rounds that can feel like violent hiccups, with nothing yet coming up. In the expulsive phase those muscles clamp down for unusually long, and when the upper oesophageal sphincter opens the pressure escapes through the mouth and sometimes the nose. Sweating and a faster heartbeat come from the sympathetic nervous system.
The dangers lie mostly in repetition and in lost reflexes. People sedated by alcohol or anaesthesia may inhale vomit and choke or develop aspiration pneumonia. Prolonged bouts drain water, acid and chloride, which can push blood chemistry towards alkalosis and low potassium. Repeated vomiting can tear the oesophageal lining, known as a Mallory–Weiss tear, and in bulimia it can wear away tooth enamel.
Source: Vomiting
