Nausea is not the same event as vomiting: it is the abstract feeling of internal sickness that often, but not always, leads on to vomiting. Emesis, another word for vomiting, is the forceful push that sends the contents of the stomach out through the mouth. When people say they felt sick and then threw up, they are describing two things that came one after the other. Most often, kids throw up because of a stomach bug that germs have caused. For most kids, a bout of throwing up comes now and then and usually ends soon. Bad food can bring on food poisoning, which can also lead to throwing up. Adults and children go through the same steps, and feeling sick can make a person pale and sweaty. Let's follow the second thing, step by step, from the first signal to the last swallow.

First, how do researchers learn about nausea? In one study, a spinning striped drum, called an optokinetic drum, made volunteers motion sick. While fMRI brain scans were recorded, the volunteers reported their nausea. The results suggest many brain regions are involved in nausea. Motion sickness can happen when the eyes and inner ears send the brain messages that do not agree about how you are moving. Balance parts in the inner ear are tied to the brain part that controls vomiting. A look at the road ahead or the horizon can help with car sickness, because then the eyes see the same motion the inner ears feel. Riding in a boat, car or plane can make some people feel sick.

The vomiting itself starts with a signal in the brainstem. In the brain, the area postrema works as a poison alarm by sensing chemicals in the blood. When it is activated, it slows normal stomach movement and turns off esophageal reflexes that would stop upward flow. Both changes are needed for vomit to move the wrong way, and neither one can do the job alone. Running from the brain through the neck to the body's organs, the vagus nerve can carry signals that set off heaves and throwing up. Even big excitement, as well as fear, disgust and worry, can make some people throw up. Something that goes too far down your throat can set off the gag reflex, a guard against choking.

Once the signal is sent, the first body responses are relaxation of the top of the stomach, called the fundus, and of the lower esophageal sphincter, plus tighter muscles at the top of the esophagus. That sphincter is the ring of muscle at the bottom of the esophagus. It works like a door that lets food into the stomach, then squeezes shut so that food and acid cannot flow back up. Food goes down the esophagus, the food pipe, to the stomach, and throwing up sends food back up that same tube. The stomach, an organ in the belly, uses its muscles to churn and mix the food you eat. Food normally goes one way, down to the stomach.

Next comes the backward wave. Before a vomit, a strong single wave of squeezing runs backwards along the upper small intestine toward the stomach. Scientists call it the retrograde giant contraction. It 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. The same wave also brings intestinal fluids into the stomach, where they can neutralize the stomach acid before it is expelled. This helps protect the esophagus. The gall bladder squeezes just before the backward wave arrives, pumping bile into the duodenum. Bile is usually basic, with a pH of about 7.5 to 8.

Then comes the pre-retch step, which closes and shortens the top of the esophagus. This helps stop vomit from flowing up into the throat during retching. Retching pulls the stomach up and down using throat muscles and the diaphragm, which mixes the stomach contents and gives them momentum. Next the glottis shuts, which is the opening to your windpipe, and the larynx rises, just before vomit comes up. Now the main push arrives. The main force for expelling vomit comes from the dome of the diaphragm contracting very strongly and compressing the stomach, while the fibers around the esophagus relax. The muscles of the belly wall squeeze the stomach as well, and the diaphragm presses down hard. During expulsion the esophagus is stretched lengthwise, which tightens its wall so it is less likely to balloon out and more likely to move the vomit upward.

Last, when the big push is over and everything has settled, the body cleans up. After the vomit leaves the mouth, the body swallows. This cleans the mouth and clears leftover bits from the esophagus. Enamel, the outer layer of the teeth, can be softened by stomach acid, so drinking or rinsing with water afterward helps protect it. A lot of fluid can leave the body in vomit, which can leave it short of water, a condition called dehydration. After vomiting, the stomach no longer holds what was bothering it, so many people feel better for a time. In the end, 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

• nausea: sick feeling
• sphincter: ring of muscle
• contraction: muscle squeeze
• bile: digestive fluid
• expulsion: pushing out