Chapter 29: Small Bowel

ANATOMY

Gross Anatomy

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 23.

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 superior mesenteric artery (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 (SMV), which then joins the splenic vein behind the pancreas to form the portal vein.

Lymphatic drainage is abundant. 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. Regional lymph nodes follow vascular arcades and drain toward the cisterna chyli.

The nerve supply for the small bowel 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.

Microscopic Anatomy

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 29–1). 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/mm². They are about 0.5-1 mm long and their walls are made up of epithelial cells with tiny projections named microvilli. The epithelial cells enclose a central axis that contains an arteriole surrounded by blood and lymphatic capillaries, known as a lacteal, and fibers from the muscularis mucosae. The microvilli (1 μm in height) amplify the potential absorptive surface area up to 200-500 m² (Figure 29–2).