Digestion is the physiological process of breaking down complex food molecules into simple, absorbable units that the body can use for energy, growth, and cellular repair. The human digestive system is organized around the gastrointestinal tract (GI tract), a continuous muscular tube extending from the mouth to the anus, accompanied by several vital accessory digestive organs.
Digestion begins in the oral cavity through both mechanical and chemical processes. Teeth perform mastication, tearing and grinding food into smaller pieces to increase its surface area for enzymatic attack. Simultaneously, the three pairs of salivary glands—parotid, submandibular, and sublingual—secrete saliva to moisten the food mass.
Saliva contains salivary amylase (ptyalin), an enzyme that initiates the chemical breakdown of complex carbohydrates into maltose. It also contains lingual lipase, which begins lipid digestion, and mucus, which lubricates the food into a soft, cohesive sphere called a bolus ready for swallowing.
Swallowing, or deglutition, transitions food from the mouth through the pharynx and into the esophagus. This complex action has a voluntary initial phase followed by involuntary pharyngeal and esophageal phases. As the bolus is pushed backward, the soft palate elevates to seal the nasopharynx, and the larynx moves upward causing the epiglottis to fold down over the glottis, preventing aspiration into the trachea.
once inside the esophagus, the bolus moves downward via peristalsis, a series of wave-like smooth muscle contractions. The upper esophageal sphincter relaxes to let food enter, and the lower esophageal sphincter relaxes briefly to allow passage into the stomach while preventing the reflux of acidic gastric contents.
The stomach acts as a muscular storage reservoir that mixes food with powerful gastric juices to create chyme. Gastric glands within the mucosa contain specialized cells that perform distinct secretory functions:
The small intestine is the primary site for both chemical digestion and nutrient absorption. Divided into the duodenum, jejunum, and ileum, its internal surface area is massively magnified by circular folds, villi, and microscopic microvilli forming the brush border.
Pancreatic enzymes and brush border enzymes complete the breakdown of macronutrients into monomers: carbohydrates become monosaccharides, proteins become amino acids, and lipids become fatty acids and monoglycerides. Water-soluble nutrients cross the mucosal epithelium directly into blood capillaries within the villi, heading to the liver via the hepatic portal system. Conversely, broken-down lipids are packaged into water-soluble lipoproteins called chylomicrons and absorbed into specialized lymphatic capillaries known as lacteals.
Although food never passes directly through them, the liver, gallbladder, and pancreas are critical contributors to the digestive process through their secretions into the duodenum.
The liver produces bile, a greenish-yellow fluid containing bile salts that emulsify large fat globules into tiny droplets, vastly increasing the surface area for lipase action. Bile is stored and concentrated in the gallbladder until hormonal signals prompt its release into the biliary duct system.
The pancreas has an exocrine function that produces pancreatic juice rich in bicarbonate—to neutralize acidic chyme coming from the stomach—and a suite of powerful digestive enzymes including pancreatic amylase, trypsin, and pancreatic lipase.
Remaining indigestible food residues, bacteria, and unabsorbed fluids pass into the large intestine, which comprises the cecum, colon, rectum, and anal canal. The primary functions here are the absorption of remaining water and electrolytes, converting liquid chyme into semi-solid feces.
Resident gut flora, or gut microbiota, ferment remaining carbohydrates and synthesize essential nutrients like vitamin K and certain B vitamins. Peristaltic movements are slower here, featuring periodic mass movements that propel feces toward the rectum, triggering the defecation reflex.
Digestion is meticulously coordinated by the autonomic nervous system and local gastrointestinal hormones. The gut possesses its own extensive neural network known as the enteric nervous system, often referred to as the 'second brain,' which regulates motility and secretion locally.
Key gastrointestinal hormones fine-tune this process across different segments of the tract: