This section describes the small intestine and its structures.

Small Intestine

Overview

The small intestine is a convoluted tube averaging 6 meters in length. It extends from the pylorus to the ileocecal valve , where it ends in the large intestine. It is contained in the central and lower part of the abdominal cavity, surrounded above and at the sides by the large intestine; a portion of it extends below the superior aperture of the pelvis and lies in front of the rectum. It is in relation, in front, with the greater omentum and abdominal parietes, and is connected to the vertebral column and posterior wall of the abdominal wall by the mesentery . The small intestine is divisible into three portions: duodenum, jejunum, and ileum. The small intestine contains the same four muscular layers seen in the rest of the GI tract: serous, muscular, areolar, and mucosa. However, the mucosa and submucosa layers have modifications that reflect its relative location in the digestive pathway, providing increased surface area for absorption.

Gross Anatomy of the Small Intestine

Duodenum

The duodenum is approximately 25 cm in length. It is the shortest , widest , and most fixed part of the small intestine. It has no mesentery, being only partially covered by peritoneum. It is retroperitoneal for all but the first 2.5 cm. It pursues a C-shaped course around the head of the pancreas beginning at the pylorus on the right side and ending at the duodenojejunal junction on the left side. This junction usually takes the form of an acute angle, the duodenojejunal flexure , which is supported by a suspensory muscle of the duodenum known as the Ligament of Trietz . Contraction of this muscle widens the angle of the flexure, facilitating movement of the intestinal contents. The ducts delivering bile and pancreatic juice from the liver and pancreas, join close to the duodenum at a conversion point called the hepatopancreatic ampulla which empties into the duodenum via the duodenal papilla. The entry of bile and pancreatic juice is controlled by a muscular valve called the Spincter of Oddi . The duodenum is divisible into four portions the superior, descending, horizontal, and ascending. Their relative locations are listed below.

Clinically Oriented Anatomy 4 th Edition, 1999; page 239

Clinically Oriented Anatomy 4 th Edition, 1999; page 239

Clinically Oriented Anatomy 4 th edition, 1999; page 422

Jejunum

The Jejunum is approximately 2.5 meters in length and extends from the duodenojejunal junction to the ileum . It lies coiled in the upper part of the peritoneal cavity occupying the umbilical and left iliac regions. Most of the jejunum lies in the left upper quadrant . Its diameter is approximately 4 cm, and is thicker, more vascular, and of a deeper color than the ileum. It is attached to the posterior abdominal wall on the left side of the aorta by the mesentery. The mesentery allows free motion, so that each coil can accommodate itself to changes in form and position.

Ileum

The Ileum is approximately 3.6 meters in length and joins the large intestine at the ileocecal valve . It lies coiled in the lower part of the peritoneal cavity and in the pelvis, occupying chiefly the umbilical, hypogastic, right iliac, and pelvic regions. Most of the ileum lies in the right lower quadrant . The terminal part of the ileum usually lies in the pelvis. Its diameter is approximately 3.75 cm and its coats thinner and less vascular than those of the jejunum. Like the jejunum, it is also attached to the posterior abdominal wall by the mesentery but on the right side of the aorta.

Microscopic Anatomy

Nearly all nutrient absorption occurs in the small intestine, which is highly adapted for its function. The length alone provides a huge surface area, and its wall has three structural modifications that amplify this absorptive surface enormously: Plicae circulares, villi, and microvilli. Most absorption occurs in the proximal part of the organ, and all of these modifications decrease in number toward the end of the small intestine. Details of the three modifications are listed below:

Plicae circulares : Deep, permanent, circular folds of the mucosa and submucosa. In addition to increasing absorptive surface the plicae force the chyme to move spirally through the lumen, which continually mixes the chyme with intestinal juice and slows its movement through the lumen.

Villi : Fingerlike projections of the mucosa, approximately 1 mm high that give it a velvety appearance and feel. The epithelial cells of the villi are chiefly absorptive columnar cells. With the lamina propria core of each villus is a dense capillary bed and a modified lymphatic capillary called a lacteal . Digested foodstuffs are absorbed through the mucosal cells into both the capillary blood and the lacteal. The villi are large and leaflike in the duodenum (the intestinal site of most active absorption) and gradually become narrower and shorter along the length of the small intestine. Between the villi, the mucosa is studded with pits that lead into tubular glands called intestinal crypts or crypts of Lieberkühn.

Microvilli : Tiny projections of the plasma membrane of the absorptive mucosal cells, give the mucosal surface a fuzzy appearance that is referred to as the brush border. In addition to enhancing absorption the plasma membrane of the microvilli bears intestinal digestive enzymes referred to collectively as brush border enzymes.

Arterial Supply and Venous Drainage

Duodenum

The main arterial supply is from the superior and inferior pancreaticoduodenal arteries . These supply the proximal and distal halves respectively. The pancreaticoduodenal arteries lie in the curve between the duodenum and head of the pancreas and supply both structures. They anastomose to form anterior and posterior arterial arcades. The duodenal veins follow the arteries and drain into the portal vein, some directly and others indirectly, through the superior mesenteric and splenic veins.

Clinically Oriented Anatomy 4 th Edition, 1999; page 240

Jejunum and Ileum

The arteries to the jejunum and ileum arise from the superior mesenteric artery (SMA). The SMA usually arises from the abdominal aorta at the level of the L1 vertebra, approximately 1 cm inferior to the celiac trunk, and runs between the layers of the mesentery sending 15 to 18 branches to the jejunum and ileum. The arteries unite to form loops or arches, arterial arcades, that give rise to straight arteries, the vasa recta. The superior mesenteric vein drains the jejunum and ileum. It lies anterior and to the right of the SMA in the root of the mesentery and unites with the splenic vein to form the portal vein .

Clinically Oriented Anatomy 4 th Edition, 1999; page 245

Lymphatic Drainage

Solitary lymphatic nodules are found scattered throughout the mucous membrane of the small intestine, but are most numerous in the lower part of the ileum. Each consists of a dense interlacing retiform tissue closely packed with lymph-corpuscles, and permeated with an abundant capillary network. The interspaces of the retiform tissue are continuous with larger lymph spaces which surround the gland, through which they communicate with the lacteal system. They are situated partly in the submucous tissue, partly in the mucous membrane, where they form slight projections of its epithelial layer

The aggregated lymphatic nodules , also called Peyer's Patches , form circular or oval patches, from twenty to thirty in number, and varying in length from 2 to 10 cm. They are largest and most numerous in the ileum. In the lower part of the jejunum they are small, circular, and few in number. They are occasionally seen in the duodenum. The lymphatic plexuses are especially abundant around these patches.

The lymphatics of the small intestine follow the blood vessels, and lymph is filtered through several levels of lymph nodes; the first set is located adjacent to the bowel wall, the second set is adjacent to the mesenteric arcades, and the third set lies along the trunk of the superior mesenteric artery. The duodenum may also drain into lymph nodes along the celiac artery. These mesenteric lymph nodes ultimately drain into the cisterna chyli and thoracic duct. The terminal ileum drainage is to the ileocolic lymph nodes.

Innervation

The enteric nervous system can and does function autonomously, but normal digestive function requires communication links between this intrinsic system and the central nervous system. These links take the form of parasympathetic and sympathetic fibers that connect either the central and enteric nervous systems or connect the central nervous system directly with the digestive tract. The nerves of the duodenum derive from the vagus and sympathetic nerves through the celiac and superior mesenteric plexuses on the pancreaticoduodenal arteries. The SMA and its branches are surrounded by a perivascular nerve plexus through which the nerves are conducted to the parts of the intestine supplied by this artery. The sympathetic fibers in the nerves to the jejunum and ileum originate in the T5 through T9 segments of the spinal cord and reach the celiac plexus through the sympathetic trunks and thoracic splanchnic nerves. The parasympathetic fibers in the nerves to the jejunum and ileum derive from the posterior vagal trunks. In general, sympathetic stimulation causes inhibition of gastrointestinal secretion and motor activity, and contraction of gastrointestinal sphincters and blood vessels. Conversely, parasympathetic stimuli typically stimulate these digestive activities.

Study Questions

  1.  List major gross anatomy sections in order of digestion.

  2.  What are the three microstructures of the intestine?  Do they increase or decrease in number as digestion occurs?

Additional Study Questions