The alimentary canal (gastrointestinal or GI tract) consists of the oral cavity, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum and appendix, colon, rectum) and anal canal. Muscle tone keeps it at roughly 5 m in life; at autopsy it relaxes to about 7–9 m. The accessory organs are the teeth and tongue, which handle food mechanically, and three glandular groups that secrete into the lumen: the salivary glands, the liver with its biliary tree and gallbladder, and the exocrine pancreas.
Digestion is usually broken into six processes. Ingestion is taking food in. Propulsion is swallowing followed by peristalsis. Mechanical digestion covers chewing, gastric churning and segmentation. Chemical digestion is enzymatic hydrolysis of polymers into monomers. Absorption moves nutrients across the epithelium into blood or lymph. Defecation eliminates indigestible residue. Most chemical digestion and nearly all nutrient absorption happen in the small intestine. Its mucosa is greatly enlarged by circular folds, villi and microvilli. The large intestine finishes the absorption of water and electrolytes, hosts the dense microbiota that ferment dietary fibre, and stores faeces.
From the oesophagus to the anal canal the wall has the same four layers: mucosa, submucosa, muscularis externa and serosa or adventitia. Two intrinsic nerve networks lie inside the wall, the submucosal (Meissner) plexus and the myenteric (Auerbach) plexus. Together they form the enteric nervous system, which can run motility, secretion and local blood flow even when its extrinsic nerves are cut. Each region modifies this basic plan to suit its job. Examples are the stratified squamous epithelium of the oesophagus, the extra oblique muscle layer and acid-secreting glands of the stomach, the villi of the small intestine and the taeniae coli of the colon.
GI function is controlled at three levels. Neural control comes from short enteric reflexes and long autonomic reflexes. Parasympathetic fibres in the vagus and the pelvic splanchnic nerves (S2–S4) generally stimulate the gut. Sympathetic fibres from T5–L2, running through the splanchnic nerves and prevertebral ganglia, inhibit motility and secretion and constrict the sphincters. Endocrine control comes from enteroendocrine cells scattered through the mucosa, which release gastrin, cholecystokinin, secretin, GIP, motilin, GLP-1 and others. Paracrine control is exerted by histamine and somatostatin. Together these match acid, enzyme and bile secretion and gastric emptying to the meal being processed.
Blood supply follows the embryological divisions of the gut. The coeliac trunk supplies the foregut, the superior mesenteric artery the midgut and the inferior mesenteric artery the hindgut. Venous blood from the abdominal gut, spleen and pancreas does not go straight back to the heart. It collects in the hepatic portal vein, so absorbed nutrients, drugs and toxins pass through the liver first (first-pass metabolism). Most dietary lipid is the exception: it leaves the enterocytes as chylomicrons, enters the lacteals and reaches the blood through the thoracic duct.
The gut handles a large volume of fluid. About 2 L is drunk or eaten each day, and roughly 7 L more is secreted as saliva, gastric juice, bile, pancreatic juice and intestinal juice, so about 9 L enters the lumen. The small intestine reabsorbs about 7.5 L of this and the colon most of the remaining 1.5 L, leaving only 100–200 mL in the faeces. This is why vomiting, diarrhoea or fistulae can quickly produce dehydration, electrolyte loss and acid–base disturbance.
Development
The primitive gut forms in week 4, when cephalocaudal and lateral folding of the embryo takes the dorsal part of the yolk sac inside. Its endoderm becomes the epithelium and glands, including the parenchyma of the liver and pancreas. The surrounding splanchnic mesoderm becomes the lamina propria, muscle, connective tissue and serosa. Neural crest cells migrate along the gut in a craniocaudal direction and form the enteric nervous system; failure of this migration causes Hirschsprung disease. The gut is open to the amniotic cavity only at the stomodeum (buccopharyngeal membrane) and the proctodeum (cloacal membrane). Both are lined by ectoderm, which is why the oral cavity and the lower anal canal (below the pectinate line) have a different origin from the rest of the tube.
The gut has three divisions, each with its own artery, nerve supply and pattern of referred visceral pain. The foregut (to the major duodenal papilla) is supplied by the coeliac trunk and refers pain to the epigastrium. The midgut (to the proximal two-thirds of the transverse colon) is supplied by the superior mesenteric artery and refers pain to the umbilical region. The hindgut (to the pectinate line) is supplied by the inferior mesenteric artery and refers pain to the hypogastrium. The stomach rotates 90° clockwise about its longitudinal axis, which is why the left vagus becomes the anterior vagal trunk. The midgut herniates physiologically into the umbilical cord in week 6, rotates 270° anticlockwise about the axis of the superior mesenteric artery, and returns by week 10. Errors in this process cause malrotation with midgut volvulus, omphalocele and a Meckel diverticulum, which is a persistent vitelline duct.
The liver bud (hepatic diverticulum) grows from the distal foregut into the septum transversum in week 4. Its caudal part forms the gallbladder and cystic duct. Dorsal and ventral pancreatic buds arise in week 5. The ventral bud swings behind the duodenum to fuse with the dorsal bud and forms the uncinate process and the lower head. Failure of the ducts to fuse gives pancreas divisum, and a ventral bud that encircles the duodenum gives an annular pancreas. In weeks 5–8 the duodenal lumen is temporarily occluded and then recanalises; failure causes duodenal atresia ("double bubble"). The urorectal septum divides the cloaca into the rectum and the urogenital sinus.