282
UNIT 3
Organ Systems
mucus-secreting cells and two special types of enteroendo-
crine cells:
gastrin-secreting cells
(
G cells
) and
somatostatin-
secreting cells
(
D cells
), as shown in Figure 15-8A. These
enteroendocrine cells regulate gastric (HCl) secretion.
Small Intestine
The
small intestine
(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
villi
and
microvilli
, which increase surface
area for absorption. Intestinal glands called
glands
(
crypts
)
of Lieberkühn
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
parts: the
duodenum
,
jejunum
, and
ileum
.
1.
The
duodenum
is the shortest segment of the small intestine,
about 20 to 25 cm long. It has small openings called
duodenal
papillae
(
minor
and
major
[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
Brunner glands
(mucus-
secreting gland) in the submucosa are a unique feature of the
duodenum (Fig. 15-10A,B).
2.
The jejunum
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).
3.
The ileum
is the longest segment, about 4 m long (three ± fths
of the rest of the small intestine). It has short villi with sig-
ni±
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
Peyer patches
and are unique to
the ileum (Fig. 15-15B,C).
Large Intestine
The
large intestine
(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
numbers of
goblet cells
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
includes the
cecum
,
appendix
,
colon
,
rectum
, and
anal canal
.
1.
The cecum
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
ileocecal
valve
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).
2.
The appendix
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
are many
lymphatic nodules in the lamina propria (Fig.
15-19A).
3.
The colon
is the longest segment of the large intestine. It
includes the
ascending colon
,
transverse colon
,
descend-
ing colon
, and
sigmoid colon
. 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-
tain primarily
goblet cells
,
columnar cells
,
enteroendocrine
cells
, and
stem cells
. Lymphatic nodules may also be found
in the lamina propria. The muscularis externa consists
of inner circular layers of muscle; the outer
longitudinal
smooth muscle layer becomes three
teniae coli
(Figs. 15-16
and 15-17A–C).
4.
The
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
rectum
is the tem-
porary storage of feces. The sensory receptors in the rectum
send signals to the brain when feces need to be evacuated.
The
anal canal
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
term
hemorrhoids
refers to the condition in which these
veins become chronically swollen and infl
amed in the rectal
and anal regions (Fig. 15-19B).
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