Essentials of Anatomy and Physiology

A summary of the digestive secretions and their
functions is found in Table 16–1. Regulation of these
secretions is shown in Table 16–2.

ABSORPTION

Most absorption of the end products of digestion takes
place in the small intestine (although the stomach does
absorb water and alcohol). The process of absorption
requires a large surface area, which is provided by sev-

eral structural modifications of the small intestine;

these are shown in Fig. 16–8. Plica circulares, or cir-

cular folds, are macroscopic folds of the mucosa and

submucosa, somewhat like accordion pleats. The

mucosa is further folded into projections called villi,

which give the inner surface of the intestine a velvet-

like appearance. Each columnar cell (except the

mucus-secreting goblet cells) of the villi also has

microvillion its free surface. Microvilli are micro-

scopic folds of the cell membrane, and are collectively

The Digestive System 383

Small intestine

Plica

circulares

Intestinal

gland

Microvilli

Absorptive cell

Goblet cell

Lacteal

Capillary

network

Enteroendocrine cell

A

B

Figure 16–8. The small intestine. (A) Section through the small intestine showing plica

circulares. (B) Microscopic view of a villus showing the internal structure. The enteroen-

docrine cells secrete the intestinal hormones.

QUESTION:What is the purpose of the villi? What other structures have the same purpose?