mucus-secreting cells and two special types of enteroendo-
), as shown in Figure 15-8A. These
enteroendocrine cells regulate gastric (HCl) secretion.
(Figs. 15-9 to 15-15C) is a hollow organ of
small diameter that is typically 6 to 7 m long. It is the major
site for the absorption of nutrients. Important features of the
small intestine are
, which increase surface
area for absorption. Intestinal glands called
are located in the lamina propria of the small
intestine. Villi project into the lumen of the intestine; the glands
of Lieberkühn open into the mucosa at the base of the villi
(Fig. 15-11A,B). The small intestine can be divided into three
is the shortest segment of the small intestine,
about 20 to 25 cm long. It has small openings called
[Figs. 16-9A and 16-15A]), which
allow pancreatic juice and bile to enter the digestive tract.
It has a similar general structure to other parts of the small
intestine (Fig. 15-9). However, the
secreting gland) in the submucosa are a unique feature of the
duodenum (Fig. 15-10A,B).
is much longer than the duodenum, about
2.5 m long (two ± fths of the rest of the small intestine). It has
long villi and a somewhat increased number of goblet cells. It
has neither Brunner glands nor Peyer patches (Fig. 15-15A).
is the longest segment, about 4 m long (three ± fths
of the rest of the small intestine). It has short villi with sig-
cantly increased numbers of goblet cells on the surface
of the mucosa. There are clusters of lymphatic nodules in
the lamina propria of the ileum; sometimes these lymphatic
nodules extend into the submucosal layer. These clusters of
lymphatic nodules are called
and are unique to
the ileum (Fig. 15-15B,C).
(Figs. 15-16 to 15-19B) is a hollow organ
with a relatively large diameter compared to the small intestine
and is about 1.5 m long. It is the last region of the digestive tract
and is the major site for absorption of water and salts. It also
forms, stores, and eliminates feces. Most of the regions of the
large intestine have tunics that are similar to those of the small
intestine, but there are no villi in the mucosa. There are large
in the large intestine. These cells produce
mucus, which helps in the formation of the feces and protects and
lubricates the surface of the intestinal wall. The large intestine
is the most proximal region of the large intestine.
It is a small, blind pouch of the large intestine where the
ileum connects to the ascending colon. A sphincter muscle, a
thickening of the muscularis mucosae, is called the
and is located at the junction of the ileum and cecum.
It prevents the contents of the large intestine from backing
up into the small intestine (Fig. 15-16).
is a small, blind tube that attaches to the
posterior-medial wall of the cecum. It has the general tunic
structure of the intestine and a small irregular lumen. There
lymphatic nodules in the lamina propria (Fig.
is the longest segment of the large intestine. It
. The proximal half of the
colon is responsible for the majority of the absorption
of water and salt; the distal half of the colon has only a
small absorptive function and is predominantly for pro-
cessing and storing feces. The colon does not have villi; it
has a smoother surface than the small intestine. Columnar
absorptive cells and goblet cells line the mucosa. The large
intestinal glands, the glands (crypts) of Lieberkühn, con-
. Lymphatic nodules may also be found
in the lamina propria. The muscularis externa consists
of inner circular layers of muscle; the outer
smooth muscle layer becomes three
rectum and anal canal
are the last segments of the large
intestine. The junction between the rectum and the anal
canal is called the “anorectal junction.” The mucosa of the
rectum is similar to that of the colon but has fewer glands
of Lieberkühn. The main function of the
is the tem-
porary storage of feces. The sensory receptors in the rectum
send signals to the brain when feces need to be evacuated.
is the distal end of the large intestine. Most
of the anal canal is lined by strati±
ed squamous epithelium,
although simple cuboidal epithelium may be present at the
anorectal junction. Sebaceous glands and hair follicles may
be found at or near the anal opening. There are many veins
in the lamina propria and submucosa of the anal canal. The
refers to the condition in which these
veins become chronically swollen and inﬂ
amed in the rectal
and anal regions (Fig. 15-19B).