How does the stomach digest food?

Digestion starts in the mouth, where teeth and the tongue turn food into a soft ball (see TongueClear). Then it goes down the oesophagus, a muscular tube about 25 cm long that runs behind the windpipe and the heart and passes through the diaphragm, the sheet of muscle you breathe with.

At the bottom a ring of muscle, the lower oesophageal sphincter, opens to let food into the stomach and closes to keep acid out of the oesophagus. We are looking at the patient from the front, so the patient's left is on your right. The stomach sits in the upper left of the belly, tucked under the left ribs, with the big liver on its right and the pancreas behind it.

The stomach is shaped like a J. Food enters at the cardia. The dome that bulges up is the fundus, the big middle part is the body, and the narrower lower part is the antrum, the grinding end. It finishes at the pylorus, a strong ring of muscle (the pyloric sphincter) that lets food out into the duodenum, the start of the small intestine. What happens next is in IntestineClear.

The wall has layers. Inside is the mucosa, folded into ridges called rugae and dotted with millions of tiny gastric pits that make gastric juice. Under it is the submucosa, with blood vessels and nerves. Then come three layers of muscle running in different directions, oblique, circular and longitudinal, so the stomach can squeeze every which way. Most of the gut has only two.

Digestion starts before you swallow. Teeth grind the food and the tongue mixes it with saliva, about 1 to 1.5 litres a day. Saliva makes food slippery and carries an enzyme, salivary amylase, that starts breaking starch into sugar. Chew a piece of plain rice or roti for a minute and it starts to taste sweet. The tongue then rolls the food into a soft ball, a bolus. (How the tongue tastes is in TongueClear.)

This is a side view, cut down the middle: the face is on the left. Pushing the bolus to the back of the mouth is the only part you choose. After that a reflex takes over. The soft palate lifts to close off the nose, the voice box rises and a flap, the epiglottis, tips over the windpipe. For about a second you cannot breathe, so food goes down the right pipe.

In the oesophagus, rings of muscle squeeze one after another just behind the bolus: a peristaltic wave. It takes about 8 to 10 seconds to cross the 25 cm, or about 5 to 8 when you are sitting up and gravity helps. Because it is a squeeze, not a slide, it works upside down too, which is how astronauts eat. At the bottom, the lower oesophageal sphincter relaxes just in time, lets the bolus into the stomach and shuts again.

The stomach lining is covered in millions of tiny gastric pits. Each leads down into a gland lined with different workers. Parietal cells make hydrochloric acid. Each has millions of proton pumps (the H⁺/K⁺-ATPase), tiny machines that use energy from ATP to push hydrogen ions out in exchange for potassium. The juice they make is about pH 0.8, and the stomach’s contents are usually about pH 1.5 to 3.5, as sour as lemon juice or stronger.

Chief cells make pepsinogen. Acid snips it into pepsin, an enzyme that cuts proteins into shorter pieces. Pepsin works best at about pH 2 and stops above about pH 5. A little gastric lipase starts on fats. Altogether the stomach makes about 1.5 litres of gastric juice a day. The acid also kills many germs swallowed with food.

So why doesn’t the stomach digest itself? Its surface cells coat it in a thick mucus gel and pump bicarbonate, an antacid, into it, so the cells sit in almost neutral conditions while acid swirls just above. The surface cells are also replaced every few days. If that shield is weakened, most often by the germ H. pylori or by some painkillers, acid and pepsin can eat a sore into the lining: an ulcer. That needs a doctor.

The stomach isn’t just a bag: it is a mill. About three times a minute, a ring of squeezing muscle starts in the middle of the stomach and sweeps down towards the exit, getting stronger in the antrum. This peristalsis mixes food with acid and pepsin into a thick soup called chyme.

At the end sits the pylorus. As a wave arrives, it opens just a crack and lets a small squirt of chyme into the duodenum. Then it shuts, and the wave has nowhere to go, so it jets the lumps back into the stomach to be ground again. Only pieces smaller than about 1 to 2 mm get through. A mixer grinder does the same job with blades at about 20,000 turns a minute; your stomach does it with slow, strong squeezes and chemistry.

How long it takes depends on the food. Water starts leaving within minutes. A meal usually takes about 2 to 4 hours: carbohydrates like plain rice leave fastest, protein-rich dal takes longer, and fat slows things most, because fat reaching the duodenum sends signals that hold the stomach back. The duodenum then gets bile from the liver and juices from the pancreas, and the rest of digestion happens in the intestines (IntestineClear).

An empty stomach holds only about 75 mL, and its lining lies in deep folds. As you eat, the top of the stomach relaxes to make room, so it can take about 1 to 1.5 litres comfortably without the pressure inside rising much. At the most it stretches to about 2 to 4 litres, which is very uncomfortable.

The wall is full of stretch receptors. As it fills, they send signals up the vagus nerve to the brain: that’s enough. More “full” signals follow from hormones made in the intestine once food arrives there, which takes a while. That is one reason eating slowly helps you notice when you are full.

The stomach makes hormones too. Ghrelin, the hunger hormone, comes mostly from the top of the stomach. Its level climbs before your usual meal times and falls within about an hour after eating. Gastrin, from the antrum, is released when food arrives and tells the parietal cells to make more acid.

And the growling? Its proper name is borborygmi: gas and liquid being squeezed along. Between meals a strong “housekeeping” wave sweeps through about every 1.5 to 2 hours to clear leftovers, and with little food to muffle it, an empty stomach sounds loudest.

Stomach trouble is very common, so it is worth understanding. This chapter shares general facts only: for anything about your own health, talk to a doctor.

Acidity and heartburn. If the lower oesophageal sphincter is weak or relaxes at the wrong time, acid splashes up into the oesophagus, whose lining has no mucus shield. That burning feeling behind the breastbone is heartburn; when it keeps happening, doctors call it GERD. A big or fatty meal and lying down soon after eating make reflux more likely. It is common: about 1 in 5 people in the US, and a survey in India found weekly symptoms in about 1 in 13.

H. pylori and ulcers. For a century doctors blamed ulcers on stress and spicy food. In 1982 two Australians, Barry Marshall and Robin Warren, grew a spiral bacterium, Helicobacter pylori, from stomach samples. Nobody believed it could live in acid, so in 1984 Marshall drank a culture and made himself ill. They won the 2005 Nobel Prize. H. pylori hides in the mucus and makes ammonia to neutralise acid around itself. About half the world carries it and most never notice, but it causes most ulcers. Doctors can test for it and treat it. Spicy food does not cause ulcers, though it can make an existing one hurt more.

Food poisoning and vomiting. About 600 million people a year fall ill from contaminated food (WHO). Stomach acid kills many germs, but not all. Vomiting is a protective reflex run by the brainstem: a deep breath, the airway shuts, and the diaphragm and belly muscles squeeze the stomach until its contents are forced out. See a doctor for vomiting that won’t stop, signs of dehydration, blood in vomit or black stools, trouble swallowing, or heartburn that keeps coming back.