chyme. The pyloric sphincter controls the passage of chyme from the stom-
ach to the small intestine allowing only relatively small amounts of chyme
through at any one time. This control is necessary because the digestive activ-
ity within the small intestine is time-consuming and because its capacity is
limited. The activities of the stomach and small intestine are coordinated by
the nervous and endocrine systems.
Small Intestine
The final stages of digestion and the absorption of its products occur in the
small intestine (Figure 11.1(A)). Contractions in the small intestine help to
break food up, mix it with digestive juices and propel it towards the colon.
The small intestine is about 23 feet (7 m) long in an adult. It is divided into
three sections: the duodenum constitutes the first 250 mm, followed by the
jejunum then the ileum. Digestion occurs primarily in the duodenum and
jejunum and absorption in the ileum.
The walls of the duodenum contain goblet cells that secrete mucus that
protects it against damage by acidic chyme. Crypts of Lieberkuhn are pits
within the wall of the small intestine with secretory cells that release intes-
tinal juice and Paneth cells that secrete lysozyme. Intestinal juice is largely
water with mucus and buffered to a pH of about 7.6. Along with pancreatic
juice (see below), it neutralizes chyme and provides a liquid medium that
aids absorption of nutrients. Lysozyme is an antibacterial enzyme. The epi-
thelial lining consists of cells called enterocytes organized into small pro-
jections into the lumen called villi (Figure 11.11). In turn, each enterocyte
DIGESTION
CZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY '-*
Goblet cell
Lacteal
Capillary
Epithelium
(mucous membrane)
Connective tissue
including smooth muscle fibers
Crypt of Lieberkuhn
Paneth cells
Brunner's gland
Muscularis mucosa
GIT lumen
Figure 11.11A villus with its associated crypts of
Lieberkuhn.
Margin Note 11.3 Disaccharides
Disaccharides, as their name implies,
are composed of two simple sugars
(monosaccharide residues) cova-
lently bound together by a glycosidic
bond. A variety of disaccharides are
known, which can be found naturally
or produced by the action of diges-
tive enzymes on polysaccharides.
Disaccharides differ from one another
in the nature of their constituent
monosaccharides and the type of
glycosidic bond joining them together
(Table 11.1).
i