How food is broken down and absorbed, from mouth to colon

The digestive tract, also called the alimentary canal or GI tract, is one long tube that runs from the mouth to the anus. Food only moves one way through it:

1. Mouth: teeth and tongue chew food and shape it into a soft ball called a bolus.
2. Pharynx and esophagus: swallowing pushes the bolus down, and waves of muscle contraction called peristalsis carry it to the stomach. The lower esophageal sphincter then closes to keep stomach acid out of the esophagus.
3. Stomach: holds the meal, mixes it with gastric juice, and turns it into a soupy liquid called chyme. The pyloric sphincter lets only small amounts out at a time.
4. Small intestine: the duodenum (shortest), jejunum, and ileum (longest). This is where most digestion and almost all absorption happen.
5. Large intestine: the cecum (with the appendix attached), colon, rectum, and anal canal.

Accessory organs help digestion, but food never travels through the liver, gallbladder, or pancreas. They send their secretions into the duodenum through ducts.

• Salivary glands release saliva, which moistens food and contains salivary amylase.
• The liver makes bile. Bile salts emulsify fat: they break big fat globules into tiny droplets, which gives fat-digesting enzymes much more surface to attack.
• The gallbladder stores bile between meals, concentrates it, and squeezes it out when fatty chyme arrives.
• The pancreas makes pancreatic juice: enzymes for every major nutrient plus bicarbonate, which neutralizes the acidic chyme coming from the stomach.

Two hormones from the duodenum coordinate this. Secretin triggers bicarbonate release, and cholecystokinin (CCK) triggers pancreatic enzyme release and the release of bile from the gallbladder.

Mechanical digestion makes food physically smaller without changing its molecules. Chewing, the churning of the stomach, and segmentation (back-and-forth squeezing in the small intestine) all count.

Chemical digestion uses enzymes to split large molecules into small building blocks the body can absorb. In the stomach, parietal cells release hydrochloric acid (HCl), which makes the stomach very acidic and converts inactive pepsinogen from chief cells into pepsin. Parietal cells also make intrinsic factor, which the body needs to absorb vitamin B12.

Some enzymes sit right on the intestinal lining, the brush border. If a person makes too little lactase, lactose stays undigested and gut bacteria ferment it, which is the basis of lactose intolerance.

Three layers of folding give the small intestine an enormous surface. Circular folds are deep ridges in the lining. Covering them are villi, tiny fingerlike projections. Each absorptive cell on a villus has microvilli, which form the brush border.

Inside each villus is a capillary bed and a lymphatic capillary called a lacteal. Glucose, amino acids, and other water-soluble nutrients enter the capillaries and go first to the liver through the hepatic portal vein. Long-chain fat products are packed into chylomicrons, which are too large for blood capillaries, so they enter the lacteals and reach the blood later through the lymphatic system.

The small intestine also absorbs about 90 percent of the water that passes through the tract. Bile salts and vitamin B12 are absorbed at the end of the ileum.

By the time material passes the ileocecal valve into the cecum, it is mostly fiber, water, and bacteria. The large intestine absorbs much of the remaining water and salts, forms and stores feces, and pushes them toward the rectum. Its huge population of bacteria, the bacterial flora, makes certain vitamins, mainly vitamin K, biotin, and pantothenic acid, which the colon absorbs.