The digestive system is made up of a long, continuous tube called the gastrointestinal (GI) tract plus three accessory organs that supply the chemicals needed to break food down. From mouth to anus, the GI tract runs about 30 feet in an adult. Each segment has a specific job, and together they turn a meal into usable nutrients in roughly 6 hours, with waste taking another 36 to 48 hours to finish its journey through the large intestine.
Mouth, Tongue, and Salivary Glands
Digestion starts the moment you take a bite. Your teeth tear and grind food into smaller pieces while your tongue moves it around and eventually pushes it toward your throat for swallowing. Three pairs of salivary glands release saliva, which does two things at once: it moistens the food so it slides easily down the esophagus, and it contains an enzyme that starts breaking down starches into simpler sugars. By the time you swallow, your food is already a soft, partially digested ball.
Esophagus
The esophagus is a muscular tube connecting your throat to your stomach. Once you swallow, your brain signals the esophagus muscles to begin peristalsis, a wave-like contraction that pushes food downward. You don’t have to think about this; it’s automatic. At the bottom of the esophagus sits a ring-shaped muscle called the lower esophageal sphincter. It opens to let food into the stomach, then closes again to prevent stomach acid from splashing back up. When this sphincter doesn’t close properly, the result is acid reflux.
Stomach
Your stomach is a muscular, J-shaped pouch that acts as both a mixer and a chemical reactor. Glands in the stomach lining produce hydrochloric acid, bringing the environment to a pH around 2.0, acidic enough to kill most bacteria and activate the protein-digesting enzyme pepsin. Stomach muscles churn food together with these digestive juices until everything becomes a thick, semi-liquid mixture called chyme.
The stomach doesn’t dump everything into the small intestine at once. It releases chyme in small, controlled amounts. On average, food spends a combined six hours moving through the stomach and small intestine, though fatty meals take longer because fat is slower to break down.
Small Intestine
The small intestine is where most digestion and nearly all nutrient absorption happen. Despite its name, it’s the longest part of the GI tract. It has three distinct sections, each with a slightly different role.
Duodenum
The duodenum is the first and shortest segment, about 10 inches long. This is the mixing chamber. Bile from the gallbladder and digestive enzymes from the pancreas enter here through small ducts. The duodenum’s own lining also produces digestive juices. Together, these chemicals go to work on proteins, fats, and carbohydrates all at once.
Jejunum
The jejunum is the middle section, roughly 8 feet long. Its muscular walls churn food back and forth, mixing it thoroughly with digestive juices. This is where the bulk of carbohydrate and protein absorption begins, as the intestinal lining is densely packed with tiny, finger-like projections called villi that increase its surface area enormously.
Ileum
The ileum is the final and longest section of the small intestine. It absorbs whatever nutrients the jejunum didn’t catch, including vitamins, minerals, fats, and remaining proteins. It also reabsorbs bile salts so the body can recycle them. At its end, the ileum connects to the large intestine through another valve that prevents waste from flowing backward.
Large Intestine (Colon)
By the time food reaches the large intestine, most nutrients have already been extracted. The colon’s primary job is pulling water and electrolytes back into the bloodstream, gradually turning liquid waste into solid stool. Peristalsis moves this waste along, but much more slowly than in the small intestine. Transit through the large intestine typically takes 36 to 48 hours.
The large intestine also hosts trillions of bacteria that perform functions your own cells cannot. These microbes break down remaining fiber and other undigested material, and they synthesize vitamin K, which plays a key role in blood clotting. The colon itself is divided into the ascending, transverse, descending, and sigmoid sections, forming a frame around the small intestine in your abdomen.
Rectum and Anus
The rectum is the final six inches or so of the large intestine. It serves as a holding area for stool. Stretch receptors in the rectal wall signal your brain when it’s full, creating the urge to have a bowel movement. The anus, at the very end, has two rings of muscle (sphincters): an internal one you can’t control and an external one you can. Together, they allow you to decide when to release stool.
Liver
The liver is the largest accessory organ in the digestive system and one of the busiest organs in the body. Its digestive role centers on producing bile, a greenish-yellow fluid made from water, bile salts, cholesterol, and pigments from old red blood cells. Bile salts act as emulsifiers: they break large fat droplets into tiny ones, giving fat-digesting enzymes much more surface area to work with. Without bile, your body would struggle to absorb dietary fat and the fat-soluble vitamins A, D, E, and K.
Beyond bile production, the liver processes nutrients absorbed from the small intestine. It metabolizes carbohydrates, proteins, and fats, stores certain vitamins and minerals, manufactures blood proteins, and detoxifies harmful substances before they reach the rest of your body. Every drop of nutrient-rich blood from the intestines passes through the liver first.
Gallbladder
The gallbladder is a small, pear-shaped sac tucked beneath the liver. It stores and concentrates bile between meals. When you eat something containing fat, the gallbladder contracts and squeezes bile through a duct into the duodenum. People who have their gallbladder removed can still digest fat because the liver continues making bile; it just drips into the intestine continuously rather than being released in a concentrated burst.
Pancreas
The pancreas sits behind the stomach and serves a dual purpose. Its exocrine cells, which make up the bulk of the organ, produce a powerful cocktail of digestive enzymes that drain into the duodenum. These enzymes target every major nutrient: amylase breaks down starches, lipase breaks down fats (after bile has emulsified them), and proteases like trypsin break proteins into small peptides and amino acids.
The pancreas also releases bicarbonate, which neutralizes the acidic chyme coming from the stomach. This is critical because the enzymes in the small intestine work best in a near-neutral environment, unlike pepsin in the stomach, which needs extreme acidity. The pancreas’s endocrine cells, scattered in clusters called islets of Langerhans, handle blood sugar regulation by releasing insulin and glucagon, a function more metabolic than digestive but still essential to how your body uses the nutrients it absorbs.
How These Enzymes Work Together
Digestion isn’t one chemical reaction. It’s a relay. Starch digestion begins in the mouth with salivary amylase, pauses in the acidic stomach, then resumes in the small intestine with pancreatic amylase. The end products are simple sugars like glucose and fructose, which the intestinal lining absorbs directly. Protein digestion starts in the stomach, where pepsin (activated only in highly acidic conditions) clips proteins into large fragments. In the duodenum, pancreatic proteases break those fragments into smaller peptides, and intestinal enzymes called peptidases finish the job by releasing individual amino acids. Fat digestion depends on teamwork between bile and pancreatic lipase: bile emulsifies the fat, lipase splits it into fatty acids and glycerol, and the intestinal lining absorbs both.
The Gut’s Own Nervous System
Running through the entire wall of the GI tract is a network of nerve cells so extensive it’s sometimes called the “second brain.” This enteric nervous system contains two major layers of nerve clusters. One layer, nestled between the muscle layers, controls the strength and rhythm of peristalsis. The other sits closer to the inner lining and regulates enzyme secretion and blood flow. These nerve networks can coordinate digestion on their own, without direct instructions from the brain, though the brain can influence them (which is why stress sometimes causes nausea or changes in bowel habits). This local control is what keeps food moving steadily from one segment to the next, at the right pace, with the right mix of digestive juices at each stage.