The small intestine is the portion of the alimentary tract extending from the pylorus to the ileocecal valve, and it consists of three segments: the duodenum, the jejunum (upper two-fifths), and the ileum (lower three-fifths). The anatomy, physiology, and pathology of the duodenum are discussed in Chapter 25.
The duodenum is a retroperitoneal structure that is approximately 25 cm long and is divided into four segments: the bulb and the descending, transverse, and ascending segments. Both the celiac trunk, through the gastroduodenal artery, and the superior mesenteric artery (SMA) supply the duodenum.
The jejunum begins at the ligament of Treitz. The jejunum and ileum are suspended on a mobile mesentery covered by a visceral peritoneal lining that extends onto the external surface of the bowel to form the serosa. There is no sharp demarcation between the jejunum and the ileum; as the intestine proceeds distally, the lumen narrows, the mesenteric vascular arcades become more complex, and the circular mucosal folds become shorter and fewer.
The mesentery contains fat, blood vessels, lymphatic channels and nodes, and nerves. The jejunum and ileum are supplied by the SMA. Branches within the mesentery anastomose to form arcades, and small straight arteries from these arcades enter the mesenteric border of the gut. Venous blood is drained through the superior mesenteric vein, which then joins the splenic vein behind the pancreas to form the portal vein.
Lymphatic drainage is abundant, with several levels following the blood vessels. Elliptical, lymphoid aggregates (Peyer’s patches) are present in the submucosa on the antimesenteric border along the distal ileum, and smaller follicles are evident throughout the remainder of the small intestine. The lymph is drained into the lymph nodes that follow the mesenteric arcade and then into the lymphatic vessels along the SMA. Eventually, the lymph is drained into the cisterna chyli that is located below the diaphragm.
The innervation for the small intestine is both sympathetic (fibers from the greater and lesser splanchnic nerves) and parasympathetic (from the right vagus nerve). Although both types of autonomic nerves contain efferent and afferent fibers, only the sympathetic afferents appear to mediate intestinal pain.
The wall of the small intestine consists of four layers: mucosa (innermost), submucosa, muscularis, and serosa (outermost).
The mucosa is characterized by circular folds about 10 mm high, named valvulae conniventes, that are taller and more numerous in the proximal jejunum and project into the lumen (Figure 31–1). These folds, combined with the presence of villi on the surface of the valvulae conniventes, increase the absorptive surface area about eight times. There are approximately 20-40 villi per square millimeter. They are about 0.5-1 mm long, and their walls are made up of epithelial cells with tiny projections named microvilli. ...