Digestive Organs

The organ system that breaks down, absorbs, and excretes food, composed of the digestive tract and accessory digestive glands such as the liver and pancreas.

Digestive Organs

Overview

The digestive organs (消化器, digestive organs) are the organ system that ingests food, breaks it down mechanically and chemically, absorbs nutrients and water, and then excretes the remaining waste. The digestive tract (gastrointestinal tract) — a single tube running from the oral cavity to the anus — works together with accessory digestive glands such as the liver, gallbladder, pancreas, and salivary glands to carry out digestion, absorption, and excretion. The digestive organs are not a mere passageway but a complex physiological system regulated jointly by the autonomic nervous system, some twenty kinds of gastrointestinal hormones, and the gut microbiota.

Main Content

Structure of the Digestive Tract

The digestive tract runs in the order of oral cavity → pharynx → esophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (cecum, colon, rectum) → anus, with a total length of about 9 m. Its wall consists of four layers: mucosa, submucosa, muscular layer, and serosa (or adventitia). The folds, villi, and microvilli of the mucosa maximize surface area, so that the absorptive area of the small intestine reaches about 30–40 m².

Accessory Digestive Organs

Salivary glands: Secrete amylase (ptyalin), which breaks down starch into maltose.
Liver: Produces bile to emulsify fats, and carries out detoxification, nutrient metabolism, protein synthesis, and vitamin storage.
Gallbladder: Stores and concentrates bile, releasing it into the duodenum when fat is ingested.
Pancreas: Secretes digestive enzymes such as amylase, lipase, and trypsin along with bicarbonate, and regulates blood glucose through insulin and glucagon.

The Digestive Process

1. Oral cavity: Chewing breaks food into small pieces, and amylase begins the breakdown of starch.

2. Esophagus: Peristaltic movement propels food to the stomach.

3. Stomach: Gastric acid (HCl) denatures proteins and activates pepsinogen into pepsin, which breaks down proteins. Gastric movements turn the contents into chyme, which passes into the duodenum.

4. Small intestine: Through the action of pancreatic juice, bile, and intestinal juice, carbohydrates, proteins, and fats are finally broken down and absorbed by the villi.

5. Large intestine: Absorbs water and electrolytes and, through fermentation by gut bacteria, forms feces.

Absorption and Metabolism

Monosaccharides and amino acids are absorbed by the villous epithelium and carried to the liver via the portal vein. Fatty acids and glycerol are re-synthesized, absorbed into the lacteals (lymphatic vessels), and then enter the blood through the thoracic duct. In the large intestine, vitamin K, biotin, and other substances synthesized by gut bacteria are absorbed.

Neural and Hormonal Regulation

Digestive movement and secretion are regulated by the vagus nerve (parasympathetic), the sympathetic nerves, and the enteric nervous system — the gut's own neural network, known as the “second brain.” Various hormones, including gastrin, secretin, cholecystokinin (CCK), ghrelin, and PYY, finely coordinate appetite, digestive juice secretion, and the rate of gastric emptying.

Major Diseases

Representative conditions include gastroesophageal reflux disease (GERD), gastritis, gastric ulcer, Helicobacter pylori infection, irritable bowel syndrome (IBS), inflammatory bowel disease (Crohn's disease and ulcerative colitis), cirrhosis, gallstones, acute and chronic pancreatitis, and gastric and colorectal cancer. Diet, alcohol consumption, smoking, stress, and obesity are cited as major risk factors.

Recent Trends

Gut microbiome research: In 2024–2025, the links between specific bacterial strains and immune, metabolic, and nervous system diseases are being actively elucidated, and the clinical application of personalized probiotics and fecal microbiota transplantation (FMT) is expanding.
AI endoscopic diagnosis: Deep learning-based automatic polyp detection (CADe) systems are raising adenoma detection rates and are being reflected in colorectal endoscopy guidelines in various countries.
Minimally invasive diagnosis and treatment: Capsule endoscopy, single-port robotic surgery, and holographic image-guided endoscopy are becoming widespread, reducing the burden of examinations.
Metabolic disease and the digestive organs: Research is active on the effects of GLP-1-class obesity and diabetes treatments on digestive physiology, including delayed gastric emptying and gallbladder disease.
Precision nutrition and gut health trends: Diets centered on dietary fiber and fermented foods, along with precision nutrition, have become key keywords in gut health management.
Regenerative medicine: Research is under way on disease modeling and drug screening using intestinal organoids, and on regenerating damaged intestinal epithelium.