The digestive system is the group of organs that receives food, breaks it down, absorbs nutrients and water, and eliminates undigested material. It consists of the alimentary canal, a continuous passage through the body, and accessory organs that prepare food or deliver secretions into that passage.
What is the order of the alimentary canal?
The alimentary canal runs from the oral opening to the anus. Its organs lie in the head, neck, chest, abdomen and pelvis, so the digestive system is not confined to the abdominal cavity.
Food passes through the following sequence:
- The mouth, followed by the pharynx, the muscular passage behind the nasal and oral cavities.
- The esophagus, followed by the stomach.
- The small intestine, comprising the duodenum, jejunum and ileum, in that order.
- The large intestine, beginning with the cecum and continuing through the colon: ascending colon, transverse colon, descending colon and sigmoid colon.
- The rectum, followed by the anal canal, which opens at the anus.
The appendix is a narrow, blind-ended tube attached to the cecum. It contains lymphoid tissue, which participates in immune defence, but its blind-ended lumen is not part of the main route along which intestinal contents pass.
How do the mouth, pharynx and esophagus connect?
The mouth contains the teeth and tongue. The teeth cut and grind food, while the tongue moves it between the teeth and gathers it into a bolus, a mass of chewed food ready to swallow. Saliva moistens food and begins the digestion of some food components.
The hard palate forms the bony anterior roof of the mouth; the soft palate forms its flexible posterior roof. During swallowing, the soft palate rises to help close the passage towards the nasal cavity. Food travels through the oropharynx, the part of the pharynx behind the mouth, and then the laryngopharynx, its lower part behind the larynx or voice box.
The esophagus descends through the neck and chest, passing behind the trachea, or windpipe. It crosses the diaphragm, the muscular partition between the chest and abdomen, through an opening called the esophageal hiatus. Peristalsis, coordinated waves of contraction in a tubular organ, carries the bolus towards the stomach.
Where are the stomach and small intestine?
The stomach lies mainly in the upper left abdomen, between the esophagus and duodenum. Its regions include the cardia around the entry, the fundus above that entry, the body and the pyloric part leading towards the outlet. The pyloric sphincter, a ring of thickened circular muscle, regulates passage into the duodenum.
The duodenum curves around the head of the pancreas. It receives stomach contents together with bile and pancreatic secretions, which enter through ducts, or conducting tubes. Most of the duodenum is fixed against the posterior abdominal wall.
The jejunum follows the duodenum, and the ileum continues to the large intestine. There is no sharp external boundary between them. Jejunal loops tend to occupy the upper left abdomen, while ileal loops tend to occupy the lower right abdomen and pelvis.
The inner lining of the small intestine has folds and villi, small finger-like projections that increase its absorptive surface. The ileum enters the cecum at the ileocecal junction, where tissue folds help limit backward movement of large-intestinal contents.
What are the parts of the large intestine?
The cecum is a pouch in the lower right abdomen that receives contents from the ileum. The ascending colon runs upwards on the right and turns at the right colic flexure, a bend near the liver. The transverse colon crosses the abdomen and turns downwards at the left colic flexure near the spleen.
The descending colon runs down the left side. The sigmoid colon forms a curved, mobile segment that continues into the rectum in the pelvis. The rectum follows the curve of the sacrum, the bone at the back of the pelvis, and leads into the anal canal.
The colon absorbs water and electrolytes, dissolved substances such as sodium and chloride, while conveying and forming faeces. The rectum temporarily holds faeces before elimination. The anal canal is surrounded by an internal anal sphincter of involuntary smooth muscle and an external anal sphincter of skeletal muscle under voluntary control.
What are the accessory digestive organs and their functions?
Accessory organs assist digestion but do not form the continuous alimentary canal. They include the teeth, tongue, salivary glands, liver, gallbladder and pancreas. The major paired salivary glands are the parotid, submandibular and sublingual glands; their ducts deliver saliva into the mouth.
The liver lies mainly beneath the diaphragm on the right and produces bile, a fluid that helps disperse dietary fats. The gallbladder lies against the liver's inferior surface and stores and concentrates bile. Bile travels through a branching system of ducts towards the duodenum.
The pancreas lies behind the stomach, with its head within the duodenal curve and its tail extending towards the spleen. Its digestive secretions pass through pancreatic ducts to the duodenum.
| Part or group | Main digestive role |
|---|---|
| Mouth, teeth, tongue and salivary glands | Receive food, break it mechanically, mix it with saliva and prepare a bolus. |
| Pharynx and esophagus | Convey swallowed food from the mouth to the stomach. |
| Stomach | Stores and mixes food with gastric secretions, beginning protein digestion. |
| Duodenum | Receives stomach contents, bile and pancreatic secretions for continued digestion. |
| Jejunum and ileum | Continue digestion and absorb nutrients; the ileum also absorbs bile salts and vitamin B12. |
| Large intestine, rectum and anal canal | The cecum and colon absorb water and form faeces; the rectum stores them and the anal canal controls their passage. |
| Liver and gallbladder | The liver produces bile; the gallbladder stores, concentrates and releases it. |
| Pancreas | Supplies digestive enzymes and bicarbonate, which helps neutralise acidic stomach contents. |
What are the peritoneum and mesenteries?
The peritoneum is a thin membrane that lines the abdominal cavity and covers many abdominal organs. Its parietal layer lines the body wall, while its visceral layer covers organ surfaces. Between them is the peritoneal cavity, a potential space containing a thin film of lubricating fluid.
A mesentery is a double layer of peritoneum that connects an organ to the abdominal wall and carries blood vessels, lymphatic vessels and nerves. The mesentery of the small intestine supports the jejunum and ileum; the transverse and sigmoid mesocolons support their corresponding segments of colon.
Retroperitoneal structures lie behind the parietal peritoneum rather than being suspended within a mesentery. Most of the duodenum and pancreas occupy this position, as do the ascending and descending colon in most people; either of these colonic segments may retain a mesocolon. In the adjacent 3D model, rotate and zoom to compare organ positions, then hide or fade a selected structure to examine neighbouring relationships.
Which arteries supply the digestive system?
The principal arterial supply to the abdominal digestive organs comes from three unpaired branches of the abdominal aorta, the main artery descending through the abdomen:
- Coeliac trunk: supplies the abdominal esophagus, stomach, proximal duodenum, liver, gallbladder and parts of the pancreas through its branches.
- Superior mesenteric artery: supplies the distal duodenum, parts of the pancreas, jejunum, ileum, cecum, appendix, ascending colon and proximal transverse colon.
- Inferior mesenteric artery: supplies the distal transverse colon, descending colon, sigmoid colon and upper rectum.
These territories overlap through arterial connections called anastomoses. The lower rectum and anal canal also receive branches from arteries of the pelvis, so the three aortic branches do not supply the entire alimentary canal.
Most venous blood from the abdominal digestive organs reaches the liver through the hepatic portal vein before returning to the general circulation. This arrangement carries most water-soluble nutrients absorbed from the intestines directly to the liver. Most absorbed long-chain dietary lipids instead enter intestinal lymphatic vessels and reach the general circulation through the thoracic duct.