Small intestine (Intestinum tenue) is the part of the gastrointestinal tract, the continuous digestive passage, between the stomach and large intestine. It lies mainly in the central and lower abdomen, with its first part in the upper abdomen. Its main roles are completing digestion and absorbing nutrients.
Where is the small intestine located?
The small intestine begins at the pylorus, the outlet of the stomach, and ends at the caecum, the first part of the large intestine. Its mobile loops occupy much of the abdominal cavity and may extend into the pelvis. The large intestine generally frames these loops around the sides and above.
The duodenum curves around the head of the pancreas, a gland behind the stomach. Most of the duodenum is retroperitoneal: it lies behind the peritoneum, the membrane lining the abdominal cavity and covering many abdominal organs. This position makes it relatively fixed compared with the rest of the small intestine.
The jejunum and ileum are suspended by the mesentery, a double layer of peritoneum that attaches them to the posterior abdominal wall. The mesentery carries their blood vessels, lymphatic vessels and nerves. Jejunal loops usually lie more towards the upper left abdomen, while ileal loops tend to occupy the lower right abdomen and pelvis; their positions overlap.
What are the parts of the small intestine?
The regions occur in sequence, but their boundaries differ in how clearly they can be identified. The duodenum ends at the duodenojejunal flexure, the bend where it becomes the jejunum. The transition from jejunum to ileum is gradual, without a sharp external boundary.
| Region | Position and attachment | Distinguishing anatomy |
|---|---|---|
| Duodenum | Upper abdomen; mostly fixed behind the peritoneum | Curves around the pancreatic head and receives bile and pancreatic secretions |
| Jejunum | Mainly upper left abdomen; suspended by the mesentery | Generally has a thicker wall and more prominent internal folds |
| Ileum | Mainly lower right abdomen and pelvis; suspended by the mesentery | Generally has fewer prominent folds and more conspicuous lymphoid tissue |
The duodenum has superior, descending, horizontal and ascending parts. Bile from the liver and digestive secretions from the pancreas enter its descending part. The horizontal part crosses in front of the vertebral column, and the ascending part rises towards the duodenojejunal flexure.
The distal part of ileum, meaning its end region, reaches the ileocecal junction, where the ileum joins the caecum. Folds at this opening form the ileocecal valve, which helps regulate passage into the large intestine.
How is the wall of the small intestine organised?
The wall of small intestine surrounds the lumen, the hollow space through which intestinal contents pass. From inside out, its principal layers are:
- Mucosa: the inner lining, containing the absorptive surface, glands and supporting tissue.
- Submucosa: a supporting connective tissue layer containing blood vessels, lymphatic vessels and nerves.
- Muscular layer: an inner circular layer and an outer longitudinal layer of smooth muscle, muscle that contracts without conscious control.
- Outer covering: serosa, a smooth peritoneal covering, or adventitia, connective tissue attaching the wall to neighbouring structures where it lacks that covering.
Circular folds project into the lumen and are particularly prominent in the jejunum. Villi are small projections of the mucosa that increase the absorptive surface. Microvilli are much smaller projections on the lining cells themselves; together, these features provide an extensive surface for absorption.
The ileum contains prominent groups of lymphoid tissue, tissue involved in immune defence, called Peyer's patches. These lie within the mucosa and may extend into the submucosa.
What supplies its blood and drains its lymph?
The superior mesenteric artery supplies the jejunum, ileum and lower portion of the duodenum. The upper duodenum also receives blood through branches of the coeliac trunk, another major artery arising from the aorta. Pancreaticoduodenal arteries connect these arterial territories around the pancreatic head and duodenum.
Within the mesentery, jejunal and ileal arteries form arcades, interconnected arterial loops. Straight vessels called vasa recta run from these loops to the intestinal wall. Compared with the ileum, the jejunum generally has fewer tiers of arcades and longer vasa recta.
Venous blood drains mainly through the superior mesenteric vein into the hepatic portal vein, which carries blood to the liver. Lymph, fluid collected from tissues by lymphatic vessels, passes through regional lymph nodes, small filtering structures along those vessels. Drainage from intestinal nodes continues towards the intestinal lymph trunk.
Which nerves supply the small intestine?
The vagus nerve (X), the tenth cranial nerve, provides parasympathetic supply, part of the involuntary nervous system associated with digestive activity. Sympathetic fibres, another part of this system, reach the intestine through nerve networks around its supplying arteries.
Within the wall, enteric nerve plexuses are interconnected networks of nerve cells that coordinate local activity. The myenteric plexus lies between the muscle layers and helps control movement. The submucosal plexus lies in the submucosa and helps regulate secretion and local blood flow.
What does the small intestine do?
The small intestine mixes food with digestive secretions and absorbs the products of digestion. Circular muscle contractions mix its contents, while peristalsis, coordinated waves of contraction, moves them onwards. The muscular wall therefore supports both mixing and transport.
Absorbed sugars and amino acids, the building blocks of proteins, enter blood vessels in the villi. Much absorbed dietary fat enters lacteals, specialised lymphatic vessels within the villi. The ileum also absorbs vitamin B12 and recovers bile salts for return to the liver.
How can you explore it in the 3D model?
In the 3D model next to the article, rotate and zoom to compare the upper abdominal position of the duodenum with the more central intestinal loops. Select and isolate a structure, or fade neighbouring structures to examine spatial relationships. Switching layers on and off can help you relate the intestine to the abdominal wall and nearby organs.