The digestive system

Follow a meal

Scroll and the view travels with one bite. It follows the tube the food takes, opens at each stop so you can see inside, and pulls back at the end so the whole route is on screen again.

The tube food actually enters is the alimentary canal. In an adult it runs about 9 meters, or 30 feet, from the mouth to the anus. A meal usually takes one to three days to finish that trip. Three organs on this tour are not part of the tube: the liver, the gallbladder, and the pancreas. Food never goes inside them. They pour bile and enzymes in from the side.

This view faces you. The liver is on the left of the picture because it sits on the right side of the body. The stomach is on the right of the picture, the body's left. Click any organ to jump there.

Food path
Liver
Gallbladder
Pancreas

Stop 01 · Mouth

Where a meal becomes a bolus

Teeth tear and grind the bite. The tongue packs it into a soft ball, the bolus, and saliva soaks it. Taste, chewing, and the decision to swallow all happen here. After this, the trip is mostly automatic.

Breaks down: Teeth cut every kind of food into smaller pieces. Salivary amylase starts clipping starch into shorter sugars.
Location: The opening of the canal, in the head.
Shape and regions: An irregular chamber with two spaces. The vestibule sits between the lips and the teeth. The oral cavity proper is the space inside the teeth. The roof is the hard palate in front and the soft palate behind.
Alimentary canal: Yes. Food passes through the mouth.
Time for one bite: Chewing a bite usually takes well under a minute. The swallow itself is about a second.
Extra fact: Glands under the tongue release lingual lipase, a fat-cutting enzyme. It does little in the mouth. It switches on later, in the acid of the stomach.

Stop 02 · Esophagus

A swallow that doesn't need gravity

The epiglottis folds over the windpipe so the bolus enters the esophagus. Rings of muscle then fire in a wave called peristalsis and squeeze it toward the stomach. The same wave works upside down. Gravity is optional. At the bottom, the lower esophageal sphincter opens to let the bolus through, then closes so stomach acid stays in the stomach.

Breaks down: The esophagus adds no enzyme of its own. Starch digestion that started in the mouth can continue for these few seconds, until stomach acid stops it.
Location: In the neck and chest, behind the windpipe, then through an opening in the diaphragm into the upper abdomen.
Shape and regions: A straight muscular tube about 25 centimeters long, flattened until a bolus opens it. Three regions follow its path: cervical in the neck, thoracic in the chest, and a short abdominal piece just before the stomach. A sphincter guards each end.
Alimentary canal: Yes. Food passes through the esophagus.
Time for one bite: About 5 to 10 seconds for a solid bite. A drink can be quicker.
Extra fact: The stomach and intestine wear an outer serosa, a slippery coat. The esophagus does not. A fibrous adventitia ties it directly to the structures beside it in the chest.

Stop 03 · Stomach

Acid, enzyme, and a churn

Three muscle layers, lengthwise, circular, and slanted, knead the meal into a milky liquid called chyme. Glands in the wall release hydrochloric acid, strong enough to push the pH toward 2, and pepsinogen, which the acid switches on into pepsin. The same glands release intrinsic factor, needed later to absorb vitamin B12. The pyloric sphincter lets chyme out a little at a time. Carbohydrate leaves first. Fat stays the longest.

Breaks down: Pepsin cuts proteins. Gastric lipase starts on some fats. The churning breaks the meal apart mechanically. Acid stops the starch digestion that began in the mouth.
Location: Left side of the upper abdomen, under the diaphragm. That is the right side of this picture.
Shape and regions: C-shaped. The four regions are the fundus, the cardiac region, the body, and the pylorus.
Alimentary canal: Yes. Food passes through the stomach.
Time for one bite: A mixed meal stays about 2 to 4 hours. A fatty meal can stay longer, toward 6 hours.
Extra fact: The surface lining replaces itself about every 3 to 4 days. Acid and pepsin wear it down constantly, and mucus plus bicarbonate keep the wall from being digested all at once.

Stop 04 · Liver

Bile, made off to the side

The liver makes bile, a green-gold liquid, and it is the first stop for nutrient-rich blood leaving the intestine. The meal itself never enters the liver. Fat and protein in the duodenum release the hormone CCK, which calls for bile. Acid releases secretin.

Breaks down: Nothing is digested inside the liver. Bile made here reaches the duodenum and breaks large fat droplets into tiny ones, so enzymes can reach them. Bile is not an enzyme.
Location: Right upper abdomen, tucked under the diaphragm. That is the left side of this picture.
Shape and regions: A large wedge, the biggest gland in the body. On the front, a ligament divides a large right lobe from a smaller left lobe. Tucked underneath are two more, the caudate lobe and the quadrate lobe.
Alimentary canal: No. The liver is an accessory organ. Food does not pass through it.
Time for one bite: Food does not travel through the liver. Bile can reach the meal in the duodenum within minutes of fat arriving.
Extra fact: Kupffer cells, macrophages that live in the liver's blood channels, clear bacteria that slipped into the blood from the gut.

Stop 05 · Gallbladder

A stored dose of bile

The gallbladder holds bile made by the liver and concentrates it. When fat arrives in the duodenum, CCK makes the gallbladder squeeze a dose down the bile duct. The meal never goes inside.

Breaks down: Nothing is digested inside the gallbladder. The bile it releases emulsifies fat in the duodenum.
Location: Tucked under the right lobe of the liver. In this picture it is the green sac on the underside of the liver.
Shape and regions: Pear-shaped, about 8 to 10 centimeters long, holding roughly 30 to 50 milliliters. The regions are the fundus, the rounded end, the body, and the neck, which narrows into the cystic duct.
Alimentary canal: No. The gallbladder is an accessory organ. Food does not pass through it.
Time for one bite: Food never enters. After a fatty meal the gallbladder empties over about 30 to 60 minutes.
Extra fact: It concentrates bile about tenfold by absorbing water and salts, so a small squirt still carries a full dose of bile salts.

Stop 06 · Pancreas

Enzymes delivered to the tube

The pancreas sits behind the stomach and sends its juice into the duodenum, the first curve of the small intestine. The marker stays in that tube. Food never enters the pancreas. Bicarbonate in the juice neutralizes stomach acid. CCK calls for enzymes, and secretin calls for the bicarbonate.

Breaks down: Food is not digested inside the pancreas. Juice delivered to the duodenum supplies amylase for starch, lipase for fat, and proteases for protein.
Location: Behind the stomach, stretching across the upper abdomen. The head sits in the C of the duodenum. The tail points toward the spleen, on the body's left.
Shape and regions: A long flattened gland. The regions are the head, the neck, the body, and the tail. A small uncinate process hooks off the head.
Alimentary canal: No. The pancreas is an accessory organ. Food does not pass through it.
Time for one bite: Food never enters. Juice begins arriving in the duodenum within minutes and keeps flowing while the meal is in the upper small intestine.
Extra fact: The hormone-making islets of Langerhans are only about 1 to 2 percent of the pancreas. Almost all of the organ is the exocrine tissue that makes digestive juice.

Stop 07 · Small intestine

Where almost everything is absorbed

Chemical digestion finishes in this coiled tube, and almost all of the useful molecules cross the lining here: sugars, amino acids, fatty acids, vitamins, minerals, and water. Circular folds carry finger-like villi, and each villus wears a brush of microvilli. Careful measurement puts that surface near 30 square meters. Capillaries take sugars and amino acids into the blood. A lymph vessel, the lacteal, carries rebuilt fats. The terminal ileum absorbs vitamin B12 with the intrinsic factor made back in the stomach. The ileocecal valve then meters whatever is left into the large intestine.

Close view of intestinal villi. One finger is cut open around a red capillary loop. The open villus beside it has a pale core, the lacteal. — Capillary — sugars and amino acids Lacteal — fats into lymph

Breaks down: Carbohydrates become simple sugars, proteins become amino acids, and fats become fatty acids and monoglycerides. Enzymes on the brush border make the last cuts.
Location: Coiled in the center of the abdomen, framed by the large intestine.
Shape and regions: A coiled tube with three regions in order. The duodenum is a short C around the head of the pancreas, about 25 centimeters. Then the jejunum, then the ileum. In the anatomy lab the whole tube measures about 6 meters. Muscle tone in a living person holds it shorter.
Alimentary canal: Yes. Food passes through the small intestine.
Time for one bite: About 3 to 6 hours.
Extra fact: The ileum carries patches of immune tissue called Peyer's patches. They sample bacteria in the gut and help the immune system decide what to tolerate.

Stop 08 · Large intestine

Water, bacteria, and a slower clock

Leftover material enters the cecum, climbs the ascending colon, crosses on the transverse colon, and comes down the descending colon. The main job is reclaiming water and salts, so a thin leftover becomes stool. Bacteria ferment fiber that human enzymes cannot cut, release short-chain fatty acids the lining can use, and make some vitamin K and B vitamins. The appendix, the narrow pouch off the cecum, may be a shelter for those bacteria.

Breaks down: Human enzymes do not digest what arrives here. Gut bacteria ferment some fiber into short-chain fatty acids and gas. The lining takes back water and salts.
Location: A frame around the small intestine. On this front view the ascending colon is on the left of the picture, the body's right, and the descending colon is on the right of the picture, the body's left.
Shape and regions: Wider and shorter than the small intestine, about 1.5 meters, like a picture frame. In order: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum. The wall is puckered into pouches called haustra.
Alimentary canal: Yes. Food, by now mostly residue, passes through the large intestine.
Time for one bite: Often about a day. The usual range is about 12 to 48 hours.
Extra fact: Three bands of longitudinal muscle, the teniae coli, are shorter than the tube itself. That mismatch is what puckers the wall into haustra.

Stop 09 · Anal canal

The last few centimeters

The sigmoid colon curves into the rectum, a short chamber that stores stool. Stretch there is the signal that it is time. The anal canal is the passage below the rectum, and it is where you can notice the urge and still wait.

Breaks down: Nothing. Digestion is finished. What leaves is mostly water, bacteria, undigested fiber, and cells the lining has shed.
Location: The last few centimeters of the canal, in the pelvis, from the rectum down to the anus.
Shape and regions: A short tube about 3 to 4 centimeters long. The upper part has vertical folds called anal columns. The pectinate line divides that upper part from the lower part, which is lined more like skin. Two sphincters wrap it: an internal one of smooth muscle, which opens on its own, and an external one of skeletal muscle, which answers to you.
Alimentary canal: Yes. The anal canal is the end of the alimentary canal.
Time for one bite: Once emptying starts, passage through the anal canal takes only seconds.
Extra fact: The pectinate line is also a change in the nerves. A problem above that line can be nearly painless. A problem below it hurts, because the lower part is supplied like skin.

The whole route

One path, with help from the side

The alimentary canal is the mouth, esophagus, stomach, small intestine, large intestine, and anal canal. Laid out, that tube is about 9 meters, or 30 feet. A meal often takes one to three days to travel it. Most of what was useful crossed the villi in the small intestine, about 3 to 6 hours in. The colon took back the spare water over about a day. The anal canal is only seconds at the end.

The liver, gallbladder, and pancreas are accessory organs. They never hold the meal. They deliver bile and enzymes at the duodenum, and the liver processes the blood that leaves full of nutrients.

About 9 meters from mouth to anus
Often 1 to 3 days for a meal
Three organs sit off the tube