The human digestive system is made up of a long, continuous tube called the gastrointestinal (GI) tract plus several accessory organs that supply the chemicals needed to break food down. The GI tract runs from your mouth to your anus, and in an average adult the small and large intestines alone stretch roughly 16 feet. Each part has a specific job, and food passes through them in a fixed sequence.
The Two Groups of Digestive Organs
Your digestive organs fall into two categories. The first is the GI tract itself: a series of hollow organs that food physically moves through. These are the mouth, esophagus, stomach, small intestine, large intestine, and anus. The second category is the accessory organs, which never touch food directly but deliver enzymes, bile, and other substances into the tract. The accessory organs are the salivary glands, liver, gallbladder, and pancreas.
Mouth, Throat, and Esophagus
Digestion starts the moment you take a bite. Your teeth break food into smaller pieces while your tongue mixes it with saliva from three pairs of salivary glands. Saliva does more than moisten food. It contains an enzyme that begins breaking down starches into simpler sugars, and it dissolves molecules so your taste buds can detect flavor.
Once you swallow, food passes through the upper esophageal sphincter into the esophagus, a muscular tube about 10 inches long. The esophagus doesn’t digest anything. Its job is transport: rhythmic muscle contractions called peristalsis push the food ball downward. At the bottom, the lower esophageal sphincter opens to let food into the stomach and then closes to keep stomach acid from splashing back up.
Stomach
The stomach is a muscular, J-shaped sac that serves as both a holding tank and a chemical processing plant. Its glands produce roughly 3 liters of gastric juice per day. That juice is intensely acidic, with a mean pH around 2.0, though it can rise to about 6.0 right after a large meal. The acid kills most bacteria and activates pepsin, an enzyme that breaks proteins into smaller fragments. A second enzyme, gastric lipase, starts working on fats.
Strong muscular walls churn food back and forth, mixing it with gastric juice until it becomes a thick, soupy liquid. The pyloric sphincter at the stomach’s exit controls how quickly this mixture is released into the small intestine, letting only small amounts through at a time.
Small Intestine
The small intestine is where most digestion and nutrient absorption happen. Despite the name, it’s the longest section of the GI tract, averaging about 291 cm (roughly 9.5 feet) with a diameter of only 2.5 cm. It has three distinct sections, each with its own role.
Duodenum
The duodenum is the first and shortest segment, only about 10 inches long. It receives the acidic food mixture from the stomach and immediately mixes it with digestive juices from the pancreas and bile from the liver and gallbladder. The sphincter of Oddi controls the flow of both bile and pancreatic juice into the duodenum. This is where the heaviest chemical digestion takes place: fats, proteins, and carbohydrates are all broken down rapidly.
Jejunum
The middle section is the jejunum. Its inner walls are lined with finger-like projections that dramatically increase the surface area available for absorbing nutrients. Most sugars, amino acids, and fatty acids pass through the jejunum’s walls into the bloodstream.
Ileum
The ileum is the longest section and the place where food spends the most time in the small intestine. It absorbs whatever nutrients remain, including vitamins, minerals, and bile salts that get recycled back to the liver. At its end, the ileocecal sphincter controls the passage of leftover material into the large intestine and prevents backflow.
Large Intestine
The large intestine is shorter than the small intestine (about 190 cm, or roughly 6 feet) but wider, with an average diameter of 4.8 cm. By the time food residue arrives here, most usable nutrients have already been absorbed. The large intestine’s primary job is to reclaim water and electrolytes from the remaining waste, compacting it into stool.
The large intestine is also home to a dense community of bacteria known as the gut microbiome. These bacteria perform tasks your own cells cannot. They break down complex carbohydrates and dietary fibers that passed through the small intestine untouched, producing short-chain fatty acids as byproducts that nourish the cells lining your colon. Gut bacteria also help metabolize leftover bile, breaking it down so bile acids can be reabsorbed and recycled by the liver.
The major segments of the large intestine are the cecum (a small pouch where the small intestine connects), the ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. Two sphincters at the anus, one involuntary and one under your conscious control, regulate when stool is released.
Liver, Gallbladder, and Pancreas
These three accessory organs sit outside the GI tract but are essential to digestion.
The liver is the largest internal organ and one of the busiest. For digestion, its key contribution is bile, a yellow-green fluid made from bile salts, cholesterol, and waste pigments. Bile acts as an emulsifier, breaking large fat globules into tiny droplets so enzymes can reach them more easily. Beyond digestion, the liver also processes nutrients absorbed from the small intestine, detoxifies harmful substances, stores vitamins and minerals, and manufactures blood proteins.
The gallbladder is a small, pear-shaped sac tucked beneath the liver. It stores and concentrates bile between meals. When fatty food enters the duodenum, the gallbladder contracts and squeezes bile through the sphincter of Oddi into the small intestine. You can live without a gallbladder; the liver simply sends bile directly to the intestine instead.
The pancreas plays a dual role. The majority of the organ produces digestive enzymes that break down starches, proteins, peptides, and fats. These enzymes flow into the duodenum alongside bile. The pancreas also contains small clusters of cells that release the hormones insulin and glucagon into the bloodstream to regulate blood sugar, though that function is metabolic rather than digestive.
Sphincters That Control the Flow
Scattered along the GI tract are ring-shaped muscles called sphincters that act like one-way valves. They open to let material pass and close to prevent it from flowing backward. The digestive tract has seven major sphincters:
- Upper esophageal sphincter: separates the throat from the esophagus
- Lower esophageal sphincter: separates the esophagus from the stomach
- Pyloric sphincter: controls flow from the stomach into the small intestine
- Sphincter of Oddi: regulates bile and pancreatic juice entering the duodenum
- Ileocecal sphincter: controls flow from the small intestine into the large intestine
- Internal anal sphincter: automatically prevents leakage from the rectum
- External anal sphincter: gives you voluntary control over bowel movements
When any of these sphincters malfunction, it can cause familiar problems. A weak lower esophageal sphincter, for instance, is the most common cause of acid reflux.
The Mesentery
One structure that often gets left off diagrams is the mesentery, a continuous sheet of tissue that anchors your intestines to the back wall of your abdomen. Some researchers now classify it as its own organ. The mesentery keeps your intestines secured in place so they don’t collapse or twist, and it carries the blood vessels and nerves that supply them. Different sections of the mesentery branch out to connect with various digestive organs, making it a central support structure for the entire system.