Small Intestine

Structure/Morphology

The small intestine consists of the duodenum, jejunum, and ileum. The overall histology is fairly consistent, with an outer serosal layer (adventitia) covering the muscular layer. In the small intestine, the muscular layer (muscularis externa) consists of longitudinal fibers and a thicker layer of circular fibers. Between these lies the myenteric plexus of enteric nerves. Deep to the circular layer of fibers lays the submucosal plexus of nerves in the submucosa. The innermost portion of the small intestine is the mucosa, consisting of a muscularis mucosa, lamina propria, and the inner epithelial lining of the lumen.

The duodenum and proximal jejunum have large circular folds and extensive intestinal villi, both of which become smaller and less frequent as you travel through the jejunum and ileum. At the distal end of the ileum, both circular folds and villi are small and infrequently found.

Brunner’s glands, which secrete an alkaline substance to protect from stomach acid are a feature found only in the duodenum, while lymphoid Peyer’s patches are found only in the ileum.

Anatomical Relations

The length of the small intestine roughly measures 6–7 m and is typically longer in males than females. It starts at the pylorus, located roughly on or to the right of the midline. The duodenum winds down the right upper quadrant, becoming retroperitoneal as it crosses back over the midline. The duodenum moves back intraperitoneally shortly before ending at the duodenojejunal junction, located in the upper left quadrant. The jejunum winds its way through the abdomen, becoming ileum which lies primarily in the lower quadrants below the transumbilical plane. The small intestine ends where the ileum empties into the cecum at the ileocecal orifice in the lower right quadrant.

Most of the small intestine is intraperitoneal (except the middle portion of the duodenum), connected to the posterior abdominal wall by a mesentery. Anteriorly, it's largely covered by the greater omentum and in the upper quadrant the transverse colon. It’s bounded laterally by the ascending and descending colons, inferiorly by the sigmoid colon, rectum, and pelvis, and superiorly by the transverse mesocolon.

Function

The small intestine is the site for both digestion and absorption of nutrients.

Bile is released into the duodenum to emulsify fats. Pancreatic enzymes are also released into the duodenum to digest proteins, carbohydrates, fats, and in the case of bicarbonate, to neutralize stomach acid.

Absorption of nutrients takes place primarily in the small intestine, with the majority of nutrients being absorbed in the jejunum. Some vitamins and minerals are also absorbed in the duodenum and ileum.

Arterial Supply

The small intestine receives blood from branches of both the celiac trunk and the superior mesenteric artery.

The first and second segments of the duodenum are typically supplied with blood by the celiac trunk and are therefore considered parts of the foregut. The gastroduodenal and superior pancreaticoduodenal arteries (anterior and posterior) supply this tissue.

The third and fourth segments of the duodenum are typically supplied with blood by the superior mesenteric artery, specifically the anterior and posterior branches of the inferior pancreaticoduodenal artery. Branches from the first jejunal artery can also supply the fourth segment of the duodenum.

The jejunum and ileum are also parts of the midgut and are therefore supplied by the superior mesenteric artery. Those branches running to the jejunum are jejunal arteries, and to the ileum are the ileal arteries. The last branch to serve the ileum, the ileocolic artery, also sends blood to the cecum and vermiform appendix.

Venous Drainage

The venous drainage of the small intestine is into the portal circulation.

Blood draining from the duodenum via the posterior superior pancreaticoduodenal vein typically drains into the portal vein. The anterior superior pancreaticoduodenal vein and both the anterior and posterior inferior pancreaticoduodenal veins in contrast drain into the superior mesenteric vein.

Blood draining the jejunum and ileum all drains into the superior mesenteric vein, and from there into the portal vein.

Innervation

The small intestine receives both parasympathetic and sympathetic innervation, sends visceral sensory information back to the CNS, and is a site of extensive enteric innervation.

All portions of the small intestine receive parasympathetic axons via the vagus nerve.

Sympathetic innervation originates at the T5-T9 spinal cord levels, travels via the greater splanchnic nerve to the celiac and superior mesenteric ganglia. Postganglionic fibers then follow the celiac trunk or superior mesenteric artery to the tissues of the small intestine.

Visceral sensory axons from the small intestine travel back the same way, following the vagus nerve or greater splanchnic nerve to the T5-T9 spinal cord levels.

The enteric system consists of two plexuses of densely packed small neurons. Meissner’s plexus lies in the submucosal layer and Auerbach’s myenteric plexus lie between the outer longitudinal and inner circular smooth muscle layers. These systems of nerves control mucosal and peristaltic function.

Lymphatic Drainage

Lymphatic drainage from the small intestine parallels the arterial supply. The duodenum drains into celiac and superior mesenteric lymph nodes. Lymph from the jejunum and ileum drains first to the juxtaintestinal nodes then into the central superior mesenteric lymph nodes. (Földi et al., 2012).

Ultimately all lymph drains via the celiac trunk to the cisterna chyli.

List of Clinical Correlates

- Crohn’s disease

- Celiac disease

- Duodenitis

- Jejunal diverticulosis

- Jejunal atresia

- Pernicious anemia

- Cobalamin deficiency

References

Földi, M., Földi, E., Strößenreuther, R. and Kubik, S. (2012) Földi's Textbook of Lymphology: for Physicians and Lymphedema Therapists. Elsevier Health Sciences.