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SECTION F: SMALL INTESTINE

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Chapter 48

Anatomy and Physiology of the Small Intestine

E. Ramsay Camp, Kevin F. Staveley-O’Carroll, Niraj J. Gusani, Jussuf T. Kaifi, and Eric T. Kimchi

Key Points

1 The normal adult anatomy of the small intestine is the result of a complex cascade of embryologic

events which result in 270 degrees of total rotation of the bowel around its axis. A failure of these

precise steps produces a spectrum of anatomical variants which are grouped together as malrotation

of the intestinal tract.

2 There is no clear anatomic boundary between the jejunum and ileum. The proximal two-fifths of the

small intestine distal to the ligament of Treitz have been arbitrarily defined as jejunum and the distal

three-fifths as ileum.

3 The enteric nervous system contains two major plexuses:

a. the myenteric (Auerbach) plexus, located between the longitudinal and circular muscle layers

b. the submucosal (Meissner) plexus

4 The small intestine is the largest endocrine organ in the human body. The secretion of numerous

hormones and neurotransmitters are specific to distinct anatomic zones within the small intestine.

5 The coordinated movement of the gastrointestinal tract is necessary for the proper digestion of food.

Well-timed contraction and relaxation patterns are initiated in gastrointestinal nervous system

causing coordinated electrical activity and muscular movements.

6 The lumen of the gastrointestinal tract is connected to the outside environment and comes in direct

contact with many potentially pathogenic microorganisms. Consequently, the small intestine needs a

complex defense mechanism to battle against these exposures in different ways.

7 The small intestine reabsorbs nearly 80% of the fluid that passes through it. This dynamic process is

accomplished by a rapid bidirectional movement of fluid in the intestinal lumen. This ebb and flow

of fluid in the intestinal lumen is critical in maintaining normal homeostasis. Minor changes in

intestinal permeability or rate of flow of the intestinal contents can result in net secretion and

diarrheal states.

8 While the exact mechanisms of many of the interactions between the gut microflora and the small

intestinal microenvironment are still speculative, it has become evident that a symbiotic

environment is present which, at least in part, is responsible for proper homeostasis of the small

intestine.

The small intestine’s intrinsic design serves to provide a maximum amount of surface area for

absorption of nutrients, water, and electrolytes. Specialized areas provide neurohormonal stimulation to

the digestive tract. Its structure and vast surface area also provide an important physical barrier to

potential pathogens and certain areas are critical in immune surveillance.

GROSS ANATOMY AND EMBRYOLOGY

1 The small intestine spans from the pylorus to the ileocecal valve, and includes three distinct regions:

the duodenum, the jejunum, and the ileum. These areas combined measure approximately 200 to 300

cm in a newborn and 5 to 7 m in an adult; comprising nearly 62% of the entire length of the alimentary

tract. The gastrointestinal tract is formed from the endodermal layer of the developing embryo. The

small intestine is derived from the distal foregut (proximal duodenum), midgut, and the adjacent

splanchnic mesenchyme. Epithelium and glands develop from the embryonic endoderm, while

connective tissue, muscle, and serosa develop from the mesoderm. During the 5th and 6th weeks of

development, the duodenal lumen is temporarily obliterated due to proliferation of its mucosal lining.

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