A map of the digestive system

In a living person the digestive tract is a single tube about 7.6 metres long, running from the mouth to the anus.

One long tube and its helpers

A sandwich eaten at lunch is still being worked on the next day. OpenStax notes that digestion from start to finish typically takes 24 to 72 hours. During that time the food passes through a tube that is open to the outside at both ends. Strictly speaking, food inside the tube is still outside your body. Only when nutrients are absorbed through the wall of the tube do they enter the body's inner space.

The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) divides the digestive system into two groups. The gastrointestinal (GI) tract is a series of hollow organs joined into a long, twisting tube: the mouth, esophagus, stomach, small intestine, large intestine and anus. The liver, pancreas and gallbladder are accessory digestive organs outside the GI tract. Food never passes through them, but they send juices into the tube through small ducts. OpenStax adds the teeth, tongue and salivary glands to the accessory organs, and points out that you could not live without them despite the modest name.

What happens where

Each part of the tract moves food along, breaks it into smaller pieces, or both. Breaking food down happens in two ways. Mechanical digestion is physical: chewing, churning and squeezing. Chemical digestion uses enzymes to split large molecules into small ones, with stomach acid helping create suitable conditions. Bile emulsifies fats into small droplets, helping enzymes reach them. Once the pieces are small enough, the wall of the small intestine absorbs them into the blood or the lymph. The table follows the NIDDK summary.

The same four layers all the way

From the esophagus to the anus the wall of the tube is built from the same four layers. The innermost is the mucosa, which touches the food and makes mucus. Its surface cells live only from a couple of days in the mouth to about a week in the gut, so the lining is constantly renewed. Next comes the submucosa, with blood vessels, lymph vessels and nerves. Then the muscularis, layers of muscle that squeeze and mix. On the outside, in the abdomen, is the serosa, a smooth slippery covering.

Food moves by peristalsis. The NIDDK describes it simply: the muscle behind the food contracts and squeezes it forward while the muscle in front relaxes to let it through. The gut also has its own network of nerve cells, the enteric nervous system, which runs from the esophagus to the anus. OpenStax puts it at about 100 million neurons. Hormones made in the stomach and small intestine add more control and tell the brain when you are hungry or full. Digestion also takes a lot of blood: while you rest after a meal, about a quarter of the blood pumped with each heartbeat goes to the intestines.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.