Anatomy of the Small Intestine

Topography and Structure of the Small Intestine and Terminal Ileum

The small intestine consists of a retroperitoneal portion, the duodenum, and a mesenteric portion comprising the coils of the jejunum and the ileum ( Fig. 50.1 ). Given that the mesenteric portion of the small intestine is subject to considerable individual and functional variations, its total length varies considerably. The average length for adults is approximately 5 m (15 to 20 feet), 40% of which is accounted for by the upper part, the jejunum, and 60% by the lower part, the ileum.

The jejunum begins at the duodenojejunal flexure on the left side of the second lumbar vertebra or, occasionally, somewhat more cranially (see Section II ). The ileum joins the large intestine in the region of the right iliac fossa. The duodenojejunal flexure is situated high up in the inframesocolic zone of the peritoneal cavity and may be partially concealed by the attachment of the transverse mesocolon. Between the duodenojejunal flexure and the ileocolic junction, the parietal line of attachment of the small intestine mesentery runs obliquely from above on the left to below on the right, passing across the lumbar spine, large prevertebral blood vessels (aorta, inferior vena cava), right psoas major, and right ureter.

Because the mesentery is only approximately 15 to 20 cm (6 to 8 inches) long at its parietal line of attachment, rather than the several meters (corresponding to length of intestine) along its intestinal attachment, it splays fanlike toward the intestine. Mesentery, consisting of two layers of peritoneum, affords the intestinal coils a wide range of movement. The space between the two layers of peritoneum is filled with connective tissue and fat tissue, the latter varying greatly from one person to another. Embedded in this tissue are blood and lymph vessels running between the intestine and the dorsal wall of the abdomen, along with nerves and mesenteric lymph nodes.

The various portions of the large intestine appear as a horseshoe-shaped arch and form a frame enclosing the convolutions of the small intestine (see Section V ). However, this frame may be overlapped ventrally by the coils of the small intestine, particularly on the side of the descending colon. Similarly, depending on their filling and on their relationship to the pelvic organs, the coils of the small intestine may bulge downward into the true pelvis or, if the pelvic organs are greatly distended (e.g., in pregnancy), may be displaced in a cranial direction.

With a greatly variable shape and highly mobile position, the greater omentum hangs like an apron from the greater curvature of the stomach and spreads between the anterior abdominal wall and the coils of the small intestine.

The greater part of the coils of the jejunum lies upward to the left, whereas those forming the ileum are situated lower and to the right side. Because it is attached only to its mesentery, the small intestine is capable of considerable movement. Its coils vary greatly in position even in the same person, depending on the state of intestinal filling and peristalsis and on the position of the body as observed under x-ray examination after oral introduction of a rubber tube. In accordance with its progressively shortened mesentery, the only position that has a more or less “constant” position is the terminal ileum, which passes from the left across the right psoas major to the site of the ileocolic junction.

Gross Structure of the Small Intestine

The freely mobile portion of the small intestine extends from the duodenojejunal flexure to the ileocolic orifice. This portion of the small intestine consists of the jejunum and the ileum, which run imperceptibly into each other; the transition is marked by a gradual change in the diameter of the lumen and by various structural alterations. As with the entire gastrointestinal (GI) tract, the virtually identical walls of the jejunum and ileum consist of five coats: mucosa, submucosa, circular muscularis, longitudinal muscularis, and serosa ( Fig. 50.2 ).

The innermost layer, the mucous membrane, is thickly plicated by macroscopically visible circular or convoluted folds, or plicae, known as circular or Kerckring folds (valves), or valvulae conniventes. These folds vary in height, projecting 3 to 10 mm into the lumen, and run in a transverse direction to the lumen's longitudinal axis. Some plicae extend all the way around the internal circumference, others go only halfway or two-thirds the way around the circumference, and still others spiral around two or even more times. These do not act as a true valve; projecting into the lumen, Kerckring valves will slow down, to a certain extent, the progression of the luminal contents, but their essential function is to increase the absorptive surface area. This principle is all the more obvious because the fold's surface is further equipped with tiny, fingerlike projections, or villi.

Below the epithelial surface of the mucosa, but participating in the formation of Kerckring folds and the villi, is the tunica propria, or lamina propria, a loose coat of predominantly reticular connective tissue, assuming in some parts a lymphatic character. The lamina propria also contains thin fibers of smooth muscle radiating from the muscularis mucosae and extending upward to the tips of the villi, which have an even surface when these fibers are relaxed but become jagged or indented when the fibers contract. The muscular fibrils act as motors maintaining the pumping function of the villi. Situated in the lamina propria, and especially in the stroma of the villi, are the terminal ramifications of the blood vessels, the central lacteal or lymph vessels of the villi, and nerve fibers. Many solitary lymph nodes are embedded in the lamina propria, which may reach far into the submucosal layer.

The muscularis mucosae separates the mucous membrane from the submucosal coat and is composed of two thin, nonstriated muscle layers that keep the movable muscle layer in place. The outer longitudinal layer is thinner than the inner circular layer from which the muscle fibers in the core of the villi emanate. Tunica submucosa consists of collagen connective tissue, the fibers of which form a network of meshes. By altering the angles of its meshes, the submucosal network is able to adapt to changes in the diameter and length of the intestinal lumen. The submucosa contains a rich network of capillaries and larger vessels, numerous lymphatics, and the submucous nerve plexus of Meissner. The muscle layer is made of smooth muscle cells. The thick inner circular layer and the thinner outer longitudinal layer are connected by convoluted transitional fascicles where the layers border on each other. Between the two layers is spread a network of nonmyelinated nerve fibers and ganglion cells, the myenteric plexus of Auerbach.

Serosa is composed of a layer of flat, polygonal epithelia and a subserosa of loose connective tissue. It covers the entire circumference of the intestinal tube, except for a narrow strip at the posterior wall, where the visceral peritoneum connects with the two serous layers of the fan-shaped mesentery.

The jejunum and ileum differ in size and appearance. The ileal lumen is narrower and the diameter of the total ileal wall is thinner than in the jejunum. The average diameter of the jejunum is 3 to 3.5 cm, whereas the ileum is 2.5 cm or less. Because of this difference, the intestinal contents show up more clearly through the ileum than through the jejunum. When the abdomen is opened, the jejunum has a whitish red hue, whereas the ileum, during life and after death, takes on a darker appearance. The folds and the villi become smaller and decrease in number as the small intestine continues. In the lower reaches of the ileum, the folds appear only sporadically.

In the jejunum, lymphatic tissue is encountered only in the form of solitary nodules, which appear as pinhead-sized elevations on the surface of the mucosa. They become more numerous and more pronounced as they near the large intestine. In addition, aggregate nodules (Peyer patches) appear, confined to the ileum. Averaging 1 to 1.5 cm wide, Peyer patches are 2 to 10 cm long and vary greatly in number, usually 20 to 30. The ileal mesentery contains more fatty tissue and appears to be thicker than the jejunum.

Microscopic Structure of the Small Intestine

The entire mucosal surface of the small intestine is covered with projections 0.5 to 1.5 mm long, the intestinal villi ( Fig. 50.3 ). The mass of these villi (estimated at four million for the jejunum and the ileum) accounts for the velvetlike appearance of the intestinal mucosa. In the jejunum, villi are longer and broader than in the ileum. Valleys, or indentations, between the villi result in nonramified pits, each of which harbors one or two tubular structures, the intestinal glands, or crypts of Lieberkühn.

The entire inner surface of the small intestine is covered by a single line of epithelial cells, most of which are cylindrical, highly prismatic columnar cells with a well-developed cuticular border on the surface. Between these columnar cells are interspersed three other types of cells: goblet cells, Paneth (oxyphilic granular) cells, and enterochromaffin cells. Goblet cells secrete an alkaline mucous fluid that coats the entire mucosa. As the small intestine moves closer to the large intestine, healthy anaerobic bacterial organisms tend to live in this mucus and function as probiotics. Most goblet cells are found in the crypts of Lieberkühn or along the lower parts of the villi, but some are located in the upper parts of the villi. The characteristic elements of the floor of the crypts are Paneth cells, also called oxyphilic granular cells because of the staining qualities of their granules. They secrete antimicrobial and growth protein substances. The third cell type is enterochromaffin cells (argentaffin or argyrophilic), which contain basal staining granules with a high affinity for silver and chromium. Their habitat is the crypts of Lieberkühn, where it is now believed stem cells exist and give rise to all intestinal cells. These cells have a definite neuroendocrine function.

Within the tunica (lamina) propria is a great variety of cells, most of which originate from reticular cells. In addition to the usual connective tissue cells, lymphocytes and plasma cells are present. Lymphocytes show a marked tendency to migrate through the epithelium toward the lumen. These cells make up the largest mass of immunoprotective tissue in the body. Mast cells are also present in the lamina propria and react to antigens. The interstitial cells of Cajal are present in the wall.

The principal task of the GI tract is to serve as an organ of nutrition to satisfy caloric and nutritional requirements. Key steps in digestion occur within the lumen of the small bowel, and then absorption occurs through these epithelial cells. Villi, covered by the epithelial cells, function as the organelles of absorption.

The luminal surface of the epithelial cell is covered with fine, projecting rods called microvilli. Each epithelial cell contains approximately 1000 microvilli, which increases the cellular surface approximately 24 times. The average length of a microvillus is 1 µm, and the width is 0.07 µm. Covered by a continuation of the cell membrane, microvilli contain, in the core, fine fibrils connected by a network of fibrils called the terminal web. Microvilli form a sheet that can be seen under the microscope and that is often lost when the epithelium is damaged, as in celiac disease.

Shortly after the ingestion of a fatty meal, fine lipid droplets are observed in the intermicrovillus spaces, which then are seen in the terminal web; pinocytotic activity subsequently occurs. The droplets seem to proceed and can be found in the main body of the epithelial cell, where they coalesce into large units in vesicles connected to each other by intracellular tubules. The system is referred to as the endoplasmic reticulum. Through this reticulum, fat droplets pass toward the lateral cell surfaces, and from the intercellular spaces the droplets traverse the basement membrane to enter the central lacteals of the villi. In the region below the microvilli, the profile of the lateral surface is irregular because of end plates. Toward the base of each cell, the membrane is plicated, or underplayed, which means the adjacent cells become interdigitated.