Definition of Small Intestine
The small intestine is the site where digestion is completed and nutrients are absorbed. It is the portion of the gastrointestinal tract (G.I.T) below the pyloric sphincter of stomach and the ileocecal valve that opens into the large intestine.
Morphology of Small Intestine
The small intestine is approximately about 3.5m long and 2.4cm wide in a living person. The small intestine is the body’s major digestive organ and the primary site for nutrient absorption.
Function of Small Intestine
The small intestine, or small bowel, is a hollow tube about 20 feet long that runs from the stomach to the beginning of the large intestine. The small intestine breaks down food from the stomach and absorbs much of the nutrients from the food.
Parts of Small Intestine
Based on histological structure as well as function, the small intestine is divided into 3 parts.
- Duodenum
- Jejunum
- Ileum
The duodenum is about 25cm and extends from the pyloric sphincter of stomach to where it becomes the jejunum at duodenojejunal junction. The common bile duct opens into the duodenum from where bile is poured into the duodenum.
Pancreatic secretions are also poured in through the pancreatic duct. These 2 ducts unite to form a common duct called hepatopancreatic ampulla (or ampulla of vater).
Histologically the duodenum differs from the rest of the small intestine by possessing duodenal (Brunner’s) gland in the sub mucosa. This gland secretes abundant mucus mainly to neutralize the acidic chyme entering the duodenum from the stomach.
The jejunum is located in between duodenum and ileum. It is approximately 1m long. The human jejunum is slightly larger and possess more internal folds than the ileum.
The ileum makes up the remaining 2m of the small intestine. The terminal portion of the ileum empties into the medial side of the cecum through the ileocecal valve. Lymph nodules called payer’s patches are abundant on the wall of the ileum and distinguishes it histologically from the rest of the small intestine.
Blood supply to small intestine
The duodenum is supplied by:
- Gastroduodenal artery and its branch (superior pancreaticoduodenal artery). The gastroduodenal artery, a branch from the celiac trunk supplies the duodenum proximal to the entry of the bile duct into the duodenum.
- The inferior pancreaticoduodenal artery (a branch of superior mesenteric artery) supplies the duodenum distal to the entry of the bile duct.
The jejunum and ileum are supplied by:
- the superior mesenteric artery. Its branches include; The inferior pancreaticoduodenal artery.
- Jejunal and ileal branches—supplying the bulk of the small intestine.
- The ileocolic artery, supplying terminal ileum, caecum and commencement of ascending colon and giving off an appendicular branch to the appendix. It is the most commonly ligated intra-abdominal artery.
Venous drainage
The duodenum is drained by the splenic vessels and small mesenteric vessels that eventually empty into the portal vein. The jejunum and ileum are drained by the superior mesenteric vessels.
Lymphatic drainage
- Lymphatic vessels from the anterior part of the duodenum drain into the pancreaticoduodenal lymph nodes and pyloric lymph nodes.
- From the posterior part, the lymphatics drain into the superior mesenteric lymph nodes.
Efferents from these nodes drain into the celiac lymph nodes.
- Lymphatics from the jejunum and ileum drain into
- juxta-intestinal lymph nodes
- Mesenteric lymph nodes
- superior central nodes
- ileocolic lymph nodes
Efferents from here drain into the superior mesenteric lymph nodes.
Nerve supply
- The duodenum is innervated by branches from the vagus nerve and greater and lesser splanchnic nerve via the celiac and superior mesenteric plexuses.
- The jejunum and ileum receive parasympathetic innervation from posterior vagal trunk.
Parasympathetic stimulation generally increases intestinal motility and secretions while sympathetic stimulation does the opposite.
Applied Anatomy of Small Intestine
- Duodenal ulcer: The acidic chyme from the stomach empties into the duodenum. Under certain conditions, this can result in ulceration of the duodenum, especially the first part. Ulceration of the initial part of the first part of the duodenum may affect the transverse colon that lies in relation to it. Duodenal (peptic) ulcers are inflammatory erosions of the duodenal mucosa. 65% of the ulcers occur in the first part of the duodenum (around 3cm from the pyloric sphincter). Sometimes the ulcer perforates the duodenal wall, letting its content into the peritoneal cavity and causing peritonitis. Peptic ulcer may also affect organs closely related to the inflamed duodenum, including the liver, gallbladder, pancreas and produce lesions. Although bleeding from gastric or duodenal ulcers is common, erosion of the gastroduodenal artery results to severe haemorrhage into the peritoneal cavity, causing peritonitis.
- Paraduodenal hernias: A loop of intestine may get strangulated in the paraduodenal fold present around the duodenojejunal junction, resulting in paraduodenal hernia. In repairing this hernia, care must be taken not to injure branches of the inferior mesenteric artery and vessels or the left colic artery and its branches that lie in relation to the paraduodenal fold.
- Ileal diverticulum– An ileal (Meckel) diverticulum is a congenital anomaly that occurs in 1-2% of the population. A remnant of the proximal part of the embryonic yolk stalk usually appears as a finger-like pouch. The diverticulum is usually located 30-60cm from the ileocecal junction in infants and 50cm in adults. It lies freely in 74% of the population and is attached to the umbilicus in the other 26%. An ileal diverticulum may become inflamed and produce pain similar to that produced by appendicitis. If gastric tissue is included, a peptic ulcer may occur here.
Structure of Small Intestine
