Vomiting, which people also call throwing up, is the stomach forcing what it holds back up and out through the mouth. A person often feels sick first, and nausea is the name for that urge to vomit. The medical word for vomiting is emesis, and it means what the stomach holds is pushed out of the mouth with force. Retching belongs to the process as well. In retching, muscles in the belly, in the chest wall and in the diaphragm squeeze again and again while the mouth and glottis stay shut, so what it holds moves to and fro in the esophagus. A baby who spits up a little milk is doing something different, because real throwing up needs strong squeezes of muscle, and those squeezes are what make it happen.

The gut's role in vomiting has been studied for close to a century, and a 1925 paper by W. C. Alvarez, titled 'Reverse peristalsis in the bowel, a precursor of vomiting', is one of the early reports. Researchers worked out the timing of vomiting by recording muscle contractions and electrical activity at the same sites along the digestive tract. Slow electrical waves normally make gut muscles contract in a regular rhythm, and that rhythm pushes food forward, in peristalsis, and stops squeezes travelling backwards. For a squeeze to move backwards along the small intestine, the normal electrical rhythm there has to be disrupted first. That recording work is how the timing became known.

The stomach is an organ inside the belly, and the food you eat gets churned and mixed by its muscles. Each bout of throwing up goes through three phases. The early part, called pre-ejection, brings nausea, extra saliva, swallowing, pale skin and a quicker heartbeat. Next is ejection, made up of the retching and the throwing up itself, and the third is post-ejection, as the body settles back down. You may also look pale and feel sweaty when you feel sick. Nausea is felt in the back of the throat and upper belly, and it comes with loss of stomach tone, small-bowel contractions and reflux of intestinal contents into the stomach. Extra saliva, or spit, can flood the mouth in the moments before you throw up.

Part of the brain, the brainstem, also helps choose the moment of vomiting. The brainstem's final signal leaves through the dorsal motor nucleus of the vagus, which sends instructions to the upper digestive tract. Running from the brain down through the neck to many of the body's organs is the vagus, a long nerve, and heaves and throwing up can be triggered by messages it carries. At the last moment before vomit rises, your windpipe's opening, the glottis, closes off, and your voice box, the larynx, lifts. At that point the diaphragm pushes down and the strong muscles in the belly wall press in on it. What comes up tends to resemble the latest meal, sometimes with food chunks in it, and any bile in it is yellow-green and bitter-tasting. Relief often follows, lasting a little while, since the thing that was upsetting it has been emptied out.

One job of stomach acid is to break food into smaller bits, which it does inside the stomach. A slippery coat called mucus, produced in one section of the stomach, protects the tough lining from its own digestive juices. That acid also softens the surface of tooth enamel. When acid touches teeth, it strips out minerals and softens the enamel, and repeated contact can let the softened surface wear away. Loss of mineral reaches a few micrometres below the surface, and if it continues, whole layers of enamel can dissolve and expose the dentin beneath. Enamel stays softened for about 20 to 30 minutes after acid contact, until saliva washes the acid away. Drinking water or rinsing with it afterward helps protect your teeth, and dental advice is to wait at least 30 minutes before brushing.

The steps in the body are the same for adults and children. Yet children react more strongly, and the reflex may fire over things that an adult would hardly notice. Stomach bugs, which come from germs, are the leading cause in kids, and norovirus is one of the viruses behind many of them and passes easily from person to person. Spoiled food, known as food poisoning, is another cause, and overeating is one more, since it stretches the stomach too full. Dizziness and a sick stomach can come from a car or boat ride. Feelings such as fear, disgust and worry, and even great excitement, make some people throw up. Another guard against choking is the gag reflex, which an object reaching too deep into the throat can trigger.

Germs that cause stomach bugs pass from one person to the next, and hand washing with soap and water slows that spread. Childhood throwing up is common and tends to be brief; with a stomach bug it normally ends in a day or two. It may also be the body protecting you by clearing out something that could hurt it. Throwing up a lot may leave you dehydrated, meaning low on water, because vomit carries off water along with small minerals named electrolytes. If you feel sick, tell a grown-up, like your mom or dad. A grown-up can check on you and get a doctor if you need one.

Words to know

• peristalsis: gut squeezing waves
• nausea: sick feeling
• retching: empty heaving
• enamel: tooth coating
• glottis: windpipe opening