The digestive system is one long tube with accessory organs feeding into it, and it tends to look chaotic on a diagram because every label is competing for attention. The trick is to follow a bite of food in order, naming the two jobs each organ does — mechanical digestion (breaking food apart physically) and chemical digestion (using enzymes to break the molecules) — plus what each part absorbs. Walk through the tube organ by organ and the digestive system stops being a list and becomes a sequence.

Mouth, Pharynx, and Esophagus: Where Digestion Begins

In the mouth, teeth handle mechanical digestion by tearing and grinding food into smaller pieces. Three pairs of salivary glands — parotid, submandibular, and sublingual — release saliva containing salivary amylase, the first digestive enzyme to act. Amylase starts breaking starch into shorter sugars; it gets minutes at most before stomach acid stops it. Saliva also contains mucus, which binds the chewed food into a swallowable mass called a bolus.

The bolus enters the pharynx (throat) and is pushed into the esophagus, a roughly 25-cm muscular tube. The esophagus does no chemical digestion. Its job is transport — waves of peristalsis (coordinated smooth-muscle contractions) push the bolus down to the stomach, even if you swallow upside down. A ring of muscle at the bottom, the lower esophageal sphincter, relaxes to let food enter the stomach and contracts to keep acid from splashing back up.

Stomach: Acid, Pepsin, and a Brutal pH

The stomach is a J-shaped sac that does heavy mechanical and chemical work. Its inner wall has glands holding several specialized cells.

  • Parietal cells secrete hydrochloric acid (HCl), dropping the stomach's pH to roughly 1.5–3.5. The acid denatures proteins (unfolds their structure) and kills most microbes that arrived with the food.
  • Chief cells secrete pepsinogen, an inactive form that is converted to the active enzyme pepsin by the acid. Pepsin begins protein digestion, snipping long proteins into shorter peptides.
  • Mucous cells coat the lining with thick mucus, which is why the stomach does not digest itself.

Smooth muscle in three layers churns the contents, turning the bolus into a soupy mix called chyme. The pyloric sphincter at the stomach's exit opens periodically and lets small amounts of chyme through into the small intestine — slowly, so the duodenum is not overwhelmed.

An overhead still life of a glass of water, a piece of bread, and an apple slice on a wooden table
An overhead still life of a glass of water, a piece of bread, and an apple slice on a wooden table

Small Intestine: Where Most Digestion and Almost All Absorption Happen

The small intestine is about 6 meters long and has three parts: duodenum, jejunum, ileum. This is where the heavy chemical digestion finishes and where ~90% of absorption occurs.

Two accessory organs dump their secretions into the duodenum.

  • The pancreas sends in pancreatic juice, a cocktail of enzymes plus bicarbonate. Bicarbonate neutralizes the stomach acid (the chyme arrives at pH ~2 and is buffered to ~7–8). The enzymes finish digestion across every food class: pancreatic amylase (carbohydrates), trypsin and chymotrypsin (proteins), pancreatic lipase (fats), and nucleases (nucleic acids).
  • The liver produces bile, which is stored and concentrated in the gallbladder and released into the duodenum. Bile contains no enzymes. Instead, bile salts emulsify fats — they coat large fat droplets and break them into many tiny ones, hugely increasing the surface area for lipase to act on. Without bile, lipase can barely touch fat.

The wall of the small intestine itself is built for absorption. It is folded into circular folds, covered in finger-like villi, and each villus is covered in tiny microvilli (the brush border). Together they multiply surface area roughly 600-fold — the small intestine has an absorptive surface of about 250 square meters. Brush-border enzymes finish the last steps: maltase, sucrase, and lactase break disaccharides into monosaccharides; peptidases finish protein digestion to amino acids.

Absorbed nutrients enter capillaries inside each villus and travel via the hepatic portal vein to the liver for first-pass processing — except for most fats, which enter lacteals (lymphatic vessels in the villi) and bypass the liver initially.

Large Intestine: Water, Electrolytes, and the Microbiome

By the time chyme reaches the large intestine (about 1.5 m long), almost all nutrients have been absorbed. The large intestine handles three jobs:

  1. Absorbs water and electrolytes — about 1.5 L of fluid per day is reclaimed here, turning the watery slurry into formed feces.
  2. Houses gut bacteria that ferment indigestible fiber. These bacteria also produce vitamin K and several B vitamins, which the colon absorbs.
  3. Compacts and stores waste in the rectum until defecation.

The parts in order are the cecum (which holds the appendix), ascending colon, transverse colon, descending colon, sigmoid colon, and finally the rectum and anal canal.

Getting Help

The body's regulation of all this — including how feedback loops control digestive hormones and acid release — builds on the rules in negative vs. positive feedback. For more organ-by-organ walkthroughs, browse the full set of Anatomy & Physiology study guides.

Conclusion

The digestive system step by step is just one tube with two jobs at every organ. Mouth: chew, start starch with amylase. Stomach: acid plus pepsin, makes chyme. Small intestine: pancreatic enzymes plus bile finish digestion; villi and microvilli absorb nutrients. Large intestine: water, electrolytes, and waste. Name the organ, name the mechanical and chemical jobs, name what is absorbed, and the whole sequence holds together.