Medical Physiology, 3rd Edition

Functional Anatomy

The small intestine and large intestine have many similarities in structure and function. In some cases, different regions of the intestinal tract carry out certain functions in much the same manner. In other cases, however, substantial heterogeneity exists between different intestinal segments (e.g., ileum versus jejunum) or between different mucosal areas (e.g., villus versus crypt) in one intestinal segment.

As discussed in Chapter 41, the basic structure of the intestine is a hollow cylinder with columnar epithelial cells lining the lumen, with circular and longitudinal layers of smooth muscle in the wall, and with endocrine and neural elements (see Fig. 41-2). Enteric neurons, as well as endocrine and paracrine agonists, regulate both epithelial transport and motor activity during both the interdigestive and the postprandial periods. As a result, the intestines propel their contents in a caudad direction while either removing fluid and electrolytes from the intestinal lumen (i.e., absorption) or adding these substances to the lumen (i.e., secretion).

Both the small and large intestine absorb and secrete fluid and electrolytes, whereas only the small intestine absorbs nutrients

Among mammals, absorption of dietary nutrients is an exclusive function of the small intestine. Only during the neonatal period does significant nutrient absorption take place in the large intestine. The small intestine absorbs nonelectrolytes after extensive digestion of dietary nutrients by both luminal and brush-border enzymes, as discussed in Chapter 45. In contrast, both the small intestine and the large intestine absorb fluid and electrolytes by several different cellular transport processes, which may differ between the small intestine and large intestine and are the subject of this chapter.

Another vitally important function of the intestinal epithelium is the secretion of intestinal fluid and electrolytes. Teleologically, fluid secretion may be considered an adaptive mechanism of the intestinal tract that protects from noxious agents, such as bacteria and bacterial toxins. In general, the cellular mechanisms of intestinal electrolyte secretion in the small intestine and colon are similar, if not identical. Frequently, the adaptive signal that induces the secretory response also induces a motor response from the intestinal muscle, resulting in a propagated propulsive response that promotes dilution and elimination of the offending toxin.

The small intestine has a villus-crypt organization, whereas the colon has surface epithelial cells with interspersed crypts

Both the small intestine and the large intestine have a specialized epithelial structure that correlates well with epithelial transport function.

The small intestine (Fig. 44-1A) consists of finger-like projections—villi— surrounded by the openings of glandular structures called crypts of Lieberkühn, or simply crypts. Both villi and crypts are covered by columnar epithelial cells. The cells lining the villi are considered to be the primary cells responsible for both nutrient and electrolyte absorption, whereas the crypt cells primarily participate in secretion.

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FIGURE 44-1 Microscopic view of the anatomy of the small and large intestine. A, The surface area of the small intestine is amplified at three levels: (1) macroscopic folds of Kerckring, (2) microscopic villi and crypts of Lieberkühn, and (3) submicroscopic microvilli. B, The surface area of the colon is amplified at the same three levels as the small intestine: (1) macroscopic semilunar folds, (2) crypts (but not villi), and (3) microvilli.

The colon (see Fig. 44-1B) does not have villi. Instead, the cells lining the large intestine are surface epithelial cells, and interspersed over the colonic surface are numerous apertures of colonic crypts (or glands) that are similar in function and structure to the small-intestinal crypts. Not surprisingly, the surface epithelial cells of the colon are the primary cells responsible for colonic electrolyte absorption, whereas colonic gland cells are generally believed to mediate ion secretion.

The intestinal mucosa is a dynamic organ with continuous cell proliferation and migration. The zone of cell proliferation is at the base of the crypt in both the small and large intestine, and the program of events is similar in both organs. The progenitor cell is a stem cell that differentiates into several specialized cells (e.g., vacuolated, goblet, and Paneth cells) that line the villi and crypts in the small intestine and the surface and glands in the colon. The vacuolated cell migrates along the crypt-villus axis and becomes a villous absorptive cell after undergoing substantial changes in its morphological and functional characteristics. In the small intestine, these villous cells migrate until they reach the tips of the villi, undergo apoptosis (see p. 1241), and then slough into the lumen of the intestine. The overall period from the initiation of cell proliferation to sloughing is ~48 to 96 hours. The overall rate of cell migration may increase or decrease: decreased cell turnover occurs during starvation, whereas increased cell turnover occurs during feeding and lactation, as well as after intestinal resection. The compensatory response that follows intestinal resection involves both luminal and hormonal factors.

The surface area of the small intestine is amplified by folds, villi, and microvilli; amplification is less marked in the colon

An additional hallmark of both the small and large intestine is the presence of structures that amplify function by increasing the luminal surface area. These structures exist at three levels. In the small intestine, the first level consists of the macroscopic folds of Kerckring. The second level consists of the microscopic villi and crypts that we have already discussed. The third level is the submicroscopic microvilli on the apical surfaces of the epithelial cells. Thus, if the small intestine is thought of as a hollow cylinder, the net increase in total surface area of the small intestine (versus that of a smooth cylinder) is 600-fold. The total surface area of the human small intestine is ~200 m2, or the surface area of a doubles tennis court (Table 44-1). The colonic surface area is also amplified, but to a more limited extent. Because the colon lacks villi, amplification is a result of only the presence of colonic folds, crypts, and microvilli. Amplification is an effective means of increasing the surface area that is available for intestinal absorption, the primary function of the small and large intestine.

TABLE 44-1

Structural and Functional Differences Between the Small and Large Intestine

 

SMALL INTESTINE

LARGE INTESTINE

Length (m)

6

2.4

Area of apical plasma membrane (m2)

~200

~25

Folds

Yes

Yes

Villi

Yes

No

Crypts or glands

Yes

Yes

Microvilli

Yes

Yes

Nutrient absorption

Yes

No

Active Na+ absorption

Yes

Yes

Active K+ secretion

No

Yes