The small intestine is the portion of the alimentary tract extending
from the pylorus to the cecum. The structure, function, and diseases
of the duodenum are discussed in Chapter 23;
the jejunum and ileum are described in this chapter.
The small intestine in an adult is 5–6 m long from the
ligament of Treitz to the ileocecal valve. The upper two fifths
of the small intestine distal to the duodenum is termed the jejunum, and
the lower three fifths is the ileum. There is no sharp demarcation
between the jejunum and the ileum; however, as the intestine proceeds
distally, the lumen narrows, the mesenteric vascular arcades become more
complex, and the circular mucosal folds become shorter and fewer
(Figure 29–1). In general, the jejunum
resides in the left side of the peritoneal cavity, and the ileum
occupies the pelvis and right lower quadrant.
Blood supply and luminal surface of the small bowel.
The arterial arcades of the small intestine increase in number from one
or two in the proximal jejunum to four or five in the distal ileum,
a finding that helps to distinguish proximal from distal bowel at
operation. Plicae circulares are more prominent in the jejunum.
The small bowel is attached to the posterior abdominal wall by
the mesentery, a reflection from the posterior parietal peritoneum.
This peritoneal fold arises along a line originating just to the
left of the midline and passing obliquely to the right lower quadrant.
Although the mesentery joins the intestine along one side, the peritoneal
layer of the mesentery envelops the bowel and is called the visceral
peritoneum, or serosa.
The mesentery contains fat, blood vessels, lymphatics, lymph
nodes, and nerves. The arterial blood supply to the jejunum and
ileum derives from the superior mesenteric artery. Branches within
the mesentery anastomose to form arcades (Figure
29–1), and small straight arteries travel from these
arcades to enter the mesenteric border of the gut. The antimesenteric border
of the intestinal wall is less richly supplied with arterial blood
than the mesenteric side, so when blood flow is impaired, the antimesenteric
border becomes ischemic first. Venous blood from the small intestine
drains into the superior mesenteric vein and then enters the liver
through the portal vein.
Submucosal lymphoid aggregates (Peyer patches) are much more
numerous in the ileum than in the jejunum. Lymphatic channels within
the mesentery drain through regional lymph nodes and terminate in
the cisterna chyli.
Parasympathetic nerves from the right vagus and sympathetic fibers
from the greater and lesser splanchnic nerves reach the small intestine
through the mesentery. Both types of autonomic nerves contain efferent
and afferent fibers, but intestinal pain appears to be mediated
by the sympathetic afferents only.