The Function of Digestive Organs

Core Concepts

The mouth, teeth, and salivary glands

The mouth starts the line. Mastication (chewing) grinds food and mixes it with saliva, which moistens the food into a bolus and begins starch digestion via salivary amylase; the tongue positions food, enables taste, and initiates swallowing. The mouth also matters beyond digestion: poor dentition, oral pain, or dry mouth can silently reduce how much a person eats.

The pharynx and esophagus

Swallowing moves the bolus through the pharynx into the esophagus; the epiglottis closes over the airway so food travels down the right tube. The esophagus delivers the bolus to the stomach by peristalsis; at its base, the lower esophageal sphincter (LES) opens to admit food and closes to keep stomach contents from refluxing — a weakened LES is behind "heartburn."

The stomach: reservoir, mixer, and protein starter

The stomach is a muscular reservoir that holds a meal, churns it into chyme, and begins protein digestion. Its glands release hydrochloric acid — which kills microbes, denatures proteins, and activates pepsinogen into pepsin — and intrinsic factor, essential for later vitamin B12 absorption. Little absorption occurs here, and fat slows gastric emptying — one reason fatty meals feel heavy.

The small intestine: completing digestion and absorbing

The small intestine — duodenum, jejunum, ileum — completes digestion and does nearly all absorption. The duodenum receives bile (from the liver, via the gallbladder) and pancreatic juice, and its walls host brush-border enzymes that finish splitting carbohydrates and proteins. The jejunum and ileum absorb the end products; the ileum also absorbs vitamin B12 (using intrinsic factor) and recycles bile salts. Villi and microvilli create an enormous absorptive surface area; the small intestine also contains gut-associated lymphoid tissue — part of the body's immune defenses.

The liver: the chemical factory

The liver is the body's largest internal organ and chemical factory. For digestion, it produces bile, which emulsifies fat into tiny droplets so enzymes can attack it. Beyond digestion, it metabolizes nutrients, stores glycogen and some vitamins and minerals, and detoxifies drugs and metabolic waste; liver disease can affect appetite and nutrient metabolism, and nurses defer specifics to the provider and RD.

The gallbladder: bile storage and release

The gallbladder is a small pouch beneath the liver that stores and concentrates bile and releases it into the duodenum when fat arrives. It is convenient, not essential: if the gallbladder is removed, the liver still makes bile, but it drips continuously instead of releasing a burst. Some people find fatty meals harder to handle afterward; many do not, and individualized guidance after surgery comes from the provider and RD.

The pancreas: enzymes and neutralizer

The pancreas has dual roles: it produces hormones (insulin and glucagon) for blood sugar regulation, and digestive pancreatic juice — enzymes for starch (amylase), protein (proteases), and fat (lipase), plus bicarbonate, which neutralizes acidic chyme so intestinal enzymes can work. When the pancreas cannot deliver these, digestion of all three macronutrients suffers — a reminder of how much the tract depends on outside helpers.

The large intestine: water recovery and waste

By the time contents reach the large intestine (cecum, colon, rectum), nutrients are largely absorbed; what remains is fiber, water, electrolytes, and undigested matter. The colon reabsorbs water and electrolytes, forming more solid stool, and hosts the gut microbiota — trillions of microorganisms that ferment fiber, produce some vitamin K and B vitamins, and influence gut health. Slow peristalsis moves contents toward the rectum; distention triggers the defecation reflex, and the external sphincter's voluntary control lets the person decide when. Fiber and fluids support this final station — a general teaching point, with individualized plans left to the RD.

Sphincters, the enteric nervous system, and coordination

Sphincters — muscular rings at key junctions — keep the line one-way: the LES, the pyloric sphincter, and the ileocecal valve. The tract has its own nervous system, the enteric nervous system ("second brain"), which coordinates motility and secretions locally, while the brain and gut communicate through the gut-brain axis — one reason stress can change appetite and digestion.

Common Confusions