The gag reflex works by muscles contracting in the pharynx to force out what is stuck there, the opposite of swallowing. The vagus and glossopharyngeal nerves control it. Touching the soft palate can also set off gagging, but those signals travel on the trigeminal nerve, not the glossopharyngeal nerve. The gag reflex depends on context. Everyday eating does not trigger it, even with large mouthfuls of bread, because the throat reads the touch as expected. A gag reflex guards you against choking, and it can be set off by something that goes too far down your throat.

Food goes down a tube called the esophagus, or food pipe, to your stomach, and throwing up sends food back up that same tube. In humans the muscle part of the esophagus is short, so the reverse wave that carries vomit upward may matter less than in animals with a much longer segment. At the bottom of the esophagus sits a ring of muscle that works like a door. The ring lets food into the stomach and then squeezes shut, so food and acid do not flow back up. During retching, pressure in the stomach rises, but the diaphragm's hiatal fibers tighten around the lower esophageal sphincter and block unwanted reflux.

Part of your brain, the brainstem, helps decide when you throw up. Vomiting seems to run as a hierarchy. Researchers think a central pattern generator in the brainstem times three sets of events: the lower esophagus and upper stomach relax while throat muscles tighten, then a backward wave runs through the intestine, then the vomit is pushed out. The gut part of vomiting can occur without the throat and voice-box part. In tests, low doses of every vomit-triggering agent tried set off only the gut events, so the gut part can happen on its own. Some brainstem cells that help switch on retching also help with the airway's defensive reflexes that protect the lungs. So the same cells may do more than one protective job, which makes the vomiting circuit harder to untangle.

The first reported responses to a vomiting trigger are relaxation of the upper stomach and the lower esophageal sphincter, and tighter muscle at the top of the esophagus. These changes last through the whole episode. The pre-retch step closes and shortens the top of the esophagus. That helps keep vomit from flowing up into the throat during retching. During retching the diaphragm, chest wall and belly muscles squeeze rhythmically, and the mouth and glottis stay closed. Throat muscles and the diaphragm pull the stomach up and down during retching, which mixes the stomach contents and gives them momentum.

Your stomach makes acid that helps break food into smaller pieces. A classic experiment showed that stomach acid injected into the intestines of animals did no harm in the duodenum, which has Brunner's glands, but ulcerated the jejunum, which does not. Ahead of a vomit, one strong wave of squeezing runs backwards along the upper small intestine toward the stomach. The name for it is the retrograde giant contraction. Just before the backward wave arrives, the gall bladder squeezes and pumps bile into the duodenum. Bile is usually basic, at a pH of about 7.5 to 8. That wave also carries intestinal fluids into the stomach, where they can neutralize the stomach acid before it is expelled, which helps protect the esophagus.

Experiments in animals show how these steps are tied together. Stimulating the caudal subnucleus of the dorsal vagal nucleus relaxes the lower esophageal sphincter and the top of the stomach, and cutting the vagus nerve blocked that relaxation. In dog experiments, cutting through the muscle wall of the small intestine split one backward wave into two that travelled independently of each other. About five seconds before the backward giant contraction begins, the intestine's normal electrical rhythm stops and its motor activity is inhibited. The backward giant contraction clears the upper small intestine in advance of the vomit, and it happens in all forms of vomiting, even when no vomit-causing substance was swallowed. Switching on the area postrema slows normal stomach movement and turns off esophageal reflexes that would stop upward flow, and both changes are needed for vomit to move the wrong way. In lesion experiments, damage to the area postrema blocked vomiting triggered through the vagus nerve.

Most of the force that expels vomit comes from the dome of the diaphragm, which contracts very strongly and compresses the stomach while the fibers around the esophagus relax. When you throw up, the strong muscles of your belly wall squeeze your stomach too. In expulsion the esophagus is stretched lengthwise, so its wall tightens and is less likely to balloon out and more likely to move the vomit upward. Once the vomit has left the mouth, the body swallows, which cleans the mouth and clears leftover bits from the esophagus.

Nausea and vomiting do not always go together. Severe nausea can happen without vomiting, and vomiting can sometimes happen without nausea, so nausea is not just a step before vomiting. Some signals from the brainstem's dorsal vagal complex and from balance and position sensors may reach the cortex, where the feeling of nausea is thought to be produced. To study nausea in animals, researchers ideally would scan the brain, as they do in people. But fMRI in animals is limited, because the animal must be awake and still, so scientists lean on other methods. Mice and rats have no emesis reflex, so researchers study nausea in them by giving nausea-causing chemicals and watching for strong avoidance of cues linked to them.

Nausea teaches avoidance: humans and animals learn to stay away from cues linked to feeling sick, such as a food eaten before the nausea started. It is common for kids to throw up, and it usually does not last long. Children and adults share the same basic body steps for throwing up, but kids trigger the reflex more easily. 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

• pharynx: back of throat
• diaphragm: breathing muscle
• esophagus: food pipe
• bile: digestive fluid
• retching: empty heaving