1. Gross Anatomy of the Small Intestine

1.1 Duodenum

The term duodenum (a Latin derivation from the Greek dodekadaktulon, "12 fingers") is applied to the most proximal segment of the small intestine because of its length - 12 fingers' breadth. The duodenum is subdivided into four portions: the first portion, which corresponds to the radiologic designation of duodenal bulb or cap; the second (descending) portion; the third (transverse) portion; and the fourth (ascending) portion.

Situated immediately above the first portion of the duodenum are the quadrate lobe of the liver and the gallbladder. The gallbladder normally can impinge on the lesser curve of the duodenal cap to produce the smooth concavity seen in radiographs. Behind the first portion of the duodenum is the head of the pancreas. Because of this relationship, the pancreas is the commonest site of penetration by a duodenal ulcer.

The second portion of the duodenum is concave; it hugs the head of the pancreas. A carcinoma or inflammatory mass in the head of the pancreas can occasionally affect the mucosal pattern along the medial aspect of the second portion of the duodenum. Congenital duodenal diverticula are commonly seen extending from the medial aspect of the second portion.

The third portion of the duodenum lies horizontally at the level of the third lumbar vertebra. The superior mesenteric artery, vein and nerve run anterior to its middle segment. In a thin individual or a person with recent massive weight loss, the superior mesenteric vessel sheath may impinge on the third portion of the duodenum, which is associated with chronic, intermittent obstruction of the duodenum.

The fourth portion of the duodenum as it ascends to the level of the second lumbar vertebra is in intimate contact with the aorta. This intimacy of duodenum and aorta can lead to fatal complications of aortic grafting when the graft erodes the duodenal wall, resulting in hemorrhage. Bleeding may be either catastrophic (due to a tiny connection between the aortic lumen and the duodenal lumen) or chronic with iron deficiency and fever (due to erosion of the duodenal mucosa by the exterior of the graft).

The mucosal pattern of the first portion (the duodenal cap or bulb) can be distinguished radiologically and endoscopically from the remaining duodenum. In the cap, shallow folds run longitudinally and are obliterated as the cap is distended. Beginning at the junction between the first and second portions of the duodenum, permanent transverse conniventes, characteristic of the small intestine, begin.

1.2 Jejunum and Ileum ( electron microscopic photos of small intestine 1, 2A, 2B)

The length of the small intestine is approximately 6 m; the length can vary from 4 to 7 m according to the technique used to make the measurement. The proximal 40% of the small intestine is referred to as the jejunum (from the Latin, meaning "empty"), and the distal 60% is designated as the ileum (from the Greek eilein, "to roll or twist"). The wall of the jejunum is thicker and its lumen wider than are those of the ileum. There is also a gradual diminution in the caliber of the lumen from duodenum to ileum. Because of its narrower lumen, the ileum is more prone to obstruction. There is a characteristic difference in the mesentery between jejunum and ileum. The fat is thicker in the ileal mesentery and extends fully to the intestinal attachment. In Crohn's disease, the thickened mesenteric fat encroaches further beneath the serosa of the small intestine. On x-ray, the mucosa in the proximal and ileal segments of the small intestine also differs. In the jejunum, the valvulae conniventes (also known as plicae circulares) are thick, tall and numerous, giving a feathery mucosal pattern on x-ray. This contrasts with the sausage-shaped ileal loops, where the valvulae conniventes are progressively fewer and less prominent but are more clearly seen as transverse folds on x-ray.