Digestion breaks down large food molecules into smaller, absorbable units that the body can use for energy, growth, and cellular repair. The human digestive tract is a continuous muscular tube spanning from the mouth to the anus, flanked by several accessory digestive organs. Two primary processes drive this system:
Nutrient absorption occurs primarily across the mucosal lining of the small intestine into the bloodstream and lymphatic vessels. Waste products not absorbed are compacted and eliminated through the large intestine. Proper coordination between neural networks and hormonal signals ensures that secretions and motility match the ingested meal's composition.
Ingestion begins the digestive journey inside the oral cavity, where teeth and tongue collaborate to masticate food. Mastication reduces particle size and mixes food with saliva, forming a softened mass called a bolus. Saliva contains water, electrolytes, mucus, and specific enzymes:
Swallowing, or deglutition, moves a food bolus from the mouth through the pharynx and esophagus into the stomach. This coordinated act features voluntary and involuntary phases. During the pharyngeal phase, the soft palate elevates to close the nasopharynx, and the epiglottis folds over the laryngeal opening to prevent aspiration.
Once in the esophagus, the bolus is propelled downward by peristalsis, a series of wave-like muscular contractions. The upper esophageal sphincter relaxes to let the bolus enter, and the lower esophageal sphincter relaxes briefly to allow entry into the stomach while preventing gastric reflux.
The stomach acts as a muscular mixing basin and temporary storage reservoir. Specialized cells within the gastric glands secrete gastric juice, transforming the bolus into a semi-liquid paste called chyme.
Digestion in the small intestine relies heavily on secretions delivered from three key accessory organs via ducts into the duodenum:
| Organ | Primary Secretion | Main Function |
|---|---|---|
| Liver | Bile | Emulsifies large fat droplets into smaller micelles |
| Gallbladder | Concentrated bile | Stores and releases bile in response to fatty meals |
| Pancreas | Pancreatic juice | Contains bicarbonate and digestive enzymes |
Pancreatic juice contains sodium bicarbonate, which neutralizes acidic chyme arriving from the stomach, and powerful enzymes like pancreatic amylase, trypsin, and lipase that digest carbohydrates, proteins, and fats.
The small intestine is the main site for chemical digestion and nutrient absorption. Its inner wall features circular folds, villi, and microvilli that maximize surface area. Brush border enzymes anchored to the microvilli complete the breakdown of carbohydrates and peptides.
Material entering the large intestine consists of indigestible food residues, unabsorbed substances, and bacteria. The large intestine reabsorbs large amounts of water and electrolytes, converting liquid chyme into semi-solid feces. Resident gut bacteria ferment remaining carbohydrates, producing gases and synthesizing essential vitamins like vitamin K.
Peristaltic movements are slower here, featuring mass movements that push fecal matter toward the rectum. As the rectum fills, rectal distension triggers the defecation reflex, relaxing the internal anal sphincter while voluntary control of the external anal sphincter allows elimination.
Gastrointestinal activity is strictly regulated by the enteric nervous system, the autonomic nervous system, and gastrointestinal hormones. The enteric nervous system acts as the gut's local brain, controlling motility and secretion independently or alongside central nervous system inputs.