Where your food goes
Follow a bite of food from your mouth through the long, winding tube inside you.
Meet your gut
Your food travels through one long tube that runs from your mouth all the way through your body. In a grown-up it is several metres long, and much of it is folded up inside your tummy. Three helpers sit next to it: the liver, the gallbladder and the pancreas. Watch each part light up along the way.
The gastrointestinal tract is a continuous tube: mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum) and anus. Its wall has muscle layers that squeeze in waves (peristalsis) to push contents along. The liver, gallbladder and pancreas are accessory organs: food never passes through them, but their secretions enter the tube. This model lumps the jejunum and ileum together and stops at the sigmoid colon.
Chew and swallow
Your teeth mash the food and your spit makes it soft and slippery. Then you swallow. A little flap closes over your windpipe so the food goes down the right tube. The food tube squeezes in waves, like a hand squeezing a tube of toothpaste, and pushes the food down to your stomach. You could even swallow upside down!
Digestion starts in the mouth: chewing breaks food into small pieces and saliva moistens it and begins digesting starch with the enzyme amylase. Swallowing moves the bolus into the pharynx; the epiglottis covers the larynx so it enters the oesophagus, a muscular tube about 25 cm long. Peristaltic waves carry it to the stomach in a few seconds, and the lower oesophageal sphincter opens to let it in, then closes to stop acid coming back up (when it fails, that is reflux).
The mixing bag
The stomach is a stretchy bag with strong muscle walls. It squeezes and churns the food and mixes it with strong acid and special juices until it turns into a thick soup. That takes a few hours. Then a small door at the bottom lets the soup out a little at a time.
The stomach stores food, churns it with its three muscle layers and mixes it with gastric juice: hydrochloric acid (pH about 1.5 to 3.5), which kills microbes and unfolds proteins, and pepsin, which begins breaking proteins down. A thick mucus lining protects the stomach from its own acid. The result is a semi-liquid called chyme. The pyloric sphincter releases it into the duodenum in small squirts; a meal takes about 2 to 4 hours to leave the stomach, fat-rich meals longer.
The helpers join in
Right after the stomach, two helpers squirt their juices onto the food. The liver makes a green juice called bile, which is stored in the gallbladder and helps break up fat. The pancreas makes a juice full of tiny tools called enzymes that chop up the food. The food never goes inside these helpers, they just add their juices to it.
In the duodenum, chyme meets bile and pancreatic juice. Bile is made continuously by the liver, concentrated and stored in the gallbladder, and released through the bile duct after a meal; its bile salts emulsify fats into tiny droplets so enzymes can reach them. The pancreas secretes bicarbonate to neutralise the stomach acid, plus enzymes for each food group: amylase (carbohydrates), lipase (fats), and the protein-digesting enzymes trypsin and chymotrypsin, which are released as inactive precursors (trypsinogen, chymotrypsinogen) and only switched on inside the small intestine, so the pancreas does not digest itself. Both ducts usually open together at the major duodenal papilla.
The long, winding road
The small intestine is a very long, thin tube, folded up so it fits. Its inside is covered in millions of tiny fingers that soak up the good stuff from the food once it has been chopped up small. Sugars, the building blocks of protein and some vitamins go into your blood and straight to the liver, which checks them and sends them around your body. Most fat, and the vitamins that dissolve in fat, take a different road, through tiny tubes called lymph vessels, before they join the blood.
The small intestine is about 6 metres long: duodenum, jejunum and ileum. Its lining is folded into villi and microvilli, giving a huge absorptive surface. Enzymes on the villi finish digestion. Sugars, amino acids, water-soluble vitamins and minerals are absorbed into the blood, and that blood does not go straight to the heart: the hepatic portal vein carries it to the liver first, which stores and processes the nutrients and detoxifies what it can. Most dietary fat takes a different route: fatty acids are repackaged in the lining cells into chylomicrons, which enter the lymph vessels and reach the bloodstream later. Food spends roughly 3 to 5 hours here.
Water out, waste out
What is left goes into the large intestine, a wider tube that goes up one side, across, and down the other. Here your body takes back the water, and friendly germs that live there eat some of the leftovers. What remains turns into poo, which waits at the end until you go to the toilet. The whole trip takes one to three days.
The large intestine (about 1.5 metres: caecum, ascending, transverse, descending and sigmoid colon, rectum) absorbs water and salts and turns the remaining chyme into stool. Trillions of gut bacteria ferment undigested fibre here, producing some vitamins (K, some B vitamins) and gases. The appendix hangs off the caecum. Stool is stored in the rectum until the defecation reflex. Total transit time from mouth to anus is typically 24 to 72 hours, most of it in the colon.