The brainstem is one of the parts of the brain that help decide when throwing up begins. Think of the area postrema, a brain region, as a poison alarm: chemicals in the blood set it off, and it can begin the urge to throw up. The brainstem's trigger zone likewise detects chemicals carried in the blood, which is how certain chemicals can bring on the urge. The chemoreceptor trigger zone can be switched on by blood chemicals, but sending electricity straight into the zone does not make an animal vomit. That makes the zone only a piece of the whole circuit. Serotonin is a chemical that is key to the gut's movement, to nausea and to vomiting, and the cells lining the gut produce more than nine tenths of it.

The throwing-up reflex is controlled by the dorsal vagal complex, which is a cluster of brainstem structures. Most of it is the area postrema together with the nucleus tractus solitarius. Starting in the brain, the vagus nerve is long enough to pass through the neck and reach the body's organs. The vomiting center, found in the brainstem, takes in messages from the gut, from the throat, from the inner ear and from the brain's thinking regions. Within the digestive tract, emesis divides into three parts that can be told apart. Number one is a set-up that involves the stomach and throat; number two is a backward giant contraction; number three covers retching and expulsion. Each part is handled by a brainstem network, and a single central pattern generator keeps all three in step. Strong feelings, including fear, disgust, worry and big excitement, can lead some people to throw up.

In experiments, lesions of the area postrema blocked throwing up triggered through the vagus nerve. One idea is that the lesion damaged a nerve pathway that runs from the nucleus tractus solitarius to the pattern generator. When the area postrema is activated, normal stomach movement slows and the esophageal reflexes that would block upward flow switch off; vomit needs both changes to travel the wrong way. When it was lesioned, a belch stimulus that normally switches off the esophageal block now activated it, which shows that it helps open the way for vomit. The lower esophageal sphincter and nearby diaphragm fibers relax during throwing up in a way that looks a lot like belching, and the same brainstem motor nucleus may produce all of these actions.

Digestion begins in the mouth, where spit breaks down some food, and spit, or saliva, can also pool there right before you throw up. Muscles in the stomach, an organ of the belly, churn the food you eat and mix it. Acid from the stomach helps reduce food to smaller pieces, while a slippery mucus coat, made by part of the stomach, shields the tough lining from digestive juices. Like a door, a ring of muscle guards the lower end of the esophagus. Food passes down through the ring into the stomach, after which the ring closes tightly so that neither food nor acid can come back. In this process the diaphragm bears down while strong muscles in the wall of the belly squeeze the stomach.

Ahead of the throw-up, a single strong wave of squeezing moves backwards through the upper small intestine, heading for the stomach. Scientists name it the retrograde giant contraction. In dogs, a cut across the muscular wall of the small intestine turned one backward wave into two separate waves, each moving on its own. The intestine's usual electrical rhythm halts about five seconds before the wave begins, and its motor activity is held back. This wave empties the upper small intestine ahead of the vomit, and it occurs in every form of throwing up, even when nothing that causes vomiting was swallowed. You may turn pale and feel your heart speed up shortly before you throw up. Fluids from the intestine are carried into the stomach by this wave too, and there they can neutralize its acid before it is expelled, protecting the esophagus.

Projectile vomiting is the name for severe cases in which vomit leaves the mouth with enough force to travel over a metre. Kids go through the same basic steps as adults, yet a child's body is touchier about triggers an adult would not mind, which makes the reflex fire sooner. The most common cause in kids is a germ-caused stomach bug. A meal of spoiled food leads to food poisoning, which can bring it on, as can eating too much. Most kids have a bout now and then, and it generally passes fast. With a stomach bug behind it, the vomiting normally stops inside a day or two. When babies spit up a little milk, that is something else, because throwing up takes strong squeezes of muscle.

Among non-emetic animals, which cannot throw up, are rats and mice; dogs, cats, ferrets and shrews belong to the group that can. Rats and other animals that cannot throw up still show other signs of nausea, such as eating non-food clay or making gaping movements after something that makes other species throw up. Learning is part of nausea as well: people and animals come to steer clear of whatever they linked with feeling sick, for example a meal that came before the sick feeling began. Sometimes throwing up is the body's way of getting rid of a possible threat. Nausea and vomiting are described as protective defence processes as of November 2026. In humans and in other animals able to throw up, they keep poisons from being taken in or digested. When 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

• postrema: brain alarm spot
• brainstem: base of brain
• sphincter: ring muscle
• serotonin: body chemical
• lesion: damaged area