Small intestine Flashcards
The small bowel increases in length from about
250 cm in the term newborn to about 600 to 800 cm in the adult.
The caliber of the small intestine gradually diminishes from proximal to distal, and there is a fourfold reduction in surface area from the distal duodenum to the terminal ileum.
The duodenum is the most proximal portion of the small intestine.
It begins with the duodenal bulb, travels in the retroperitoneal
space around the head of the pancreas, and ends on its return to the
peritoneal cavity at the ligament of Treitz
The biliary and pancreatic
ducts usually join together 1 to 2 cm from the outer margin
of the duodenal wall and drain into the medial wall of the second
portion of the duodenum through the ampulla of Vater.
In 5% to
10% of individuals, an accessory pancreatic duct, also known as the
duct of Santorini, enters separately through the minor papilla 1 to 2
cm proximal to the ampulla of Vater
The proximal 40% of the mobile
small intestine is the jejunum, which occupies the left upper portion
of the abdomen. The remaining 60% of small intestine is the
ileum, and it is normally situated in the right side of the abdomen
and upper part of the pelvis
There is no distinct anatomic demarcation
between the jejunum and ileum, but the jejunum tends to be
thicker, is more vascular, and has a greater diameter than the ileum.
The luminal surface of the small intestine has visible mucosal
folds called the plicae circularis or folds of Kerckring.
They are more numerous in the proximal jejunum, decrease in number
distally, and are absent in the terminal ileum.
The jejunum and ileum are freely mobile in the abdominal
cavity and are attached to the posterior abdominal wall by the
intestinal mesentery
The entire length of jejunum and ileum is
suspended in this mesentery, except for the distal terminal ileum at
the cecum, which is retroperitoneal
The mesentery is formed by
a fan-shaped anterior reflection of the posterior peritoneum that
extends from the left side of the body toward the right sacroiliac
joint.
The small bowel transitions to the colon at the ileocecal (IC) valve,
which consists of 2 semilunar lips that protrude into the cecum.
The colon is a tubular structure about 30 to 40 cm in length at
birth and measuring some 150 cm in the adult, quarter the length of the small intestine. The colon begins at the
IC valve and ends distally at the anal verge
It consists
of 4 segments: cecum and vermiform appendix, colon (ascending,
transverse, and descending portions), rectum, and anal canal
The
diameter of the colon is greatest in the cecum (7.5 cm) and narrowest
in the sigmoid (2.5 cm)
It is larger in caliber, mostly fixed in position, and
has outer longitudinal muscle fibers that coalesce into 3 discrete
bands called taeniae: the taenia liberis (free tenia), taenia omentalis
(omental tenia), and taenia mesocolica (mesenteric tenia).
Taeniae
are located at 120-degree intervals around the colonic circumference
and extend from the cecum to the proximal rectum
Outpouchings, or haustra, occur between the taeniae, and their
mucosal surface is sectioned by semilunar folds to give the serosa
a sacculated and puckered appearance
The mesentery fully suspends
the transverse colon and sigmoid colon, while the remainder of
the colon has mesentery only on its free anterior surface
The cecum is the most proximal portion of the colon. It is
about 6 to 8 cm in length and breadth and lies in the right iliac
fossa, projecting downward as a blind pouch below the entrance
of the ileum.
The cecum is normally nonmobile because it is fixed in position
by a small mesocecum; anomalous fixation, however, occurs in
10% to 20% of the population, predominantly women, predisposing
them to cecal volvulus
The IC valve passes perpendicularly through the posteromedial
wall of the cecum and consists of a superior and inferior fold
arranged in an elliptical manner at the IC orifice.
orifice is roughly 2.5 cm inferior to the IC valve, and the vermiform
appendix
The ascending colon is narrower than the cecum and extends
about 12 to 20 cm from the level of the IC valve to the inferior
surface of the posterior lobe of the liver
IC valve to the inferior
surface of the posterior lobe of the liver, where it angulates left
and forward, forming the hepatic flexure. The ascending colon is
covered with peritoneum in about 75% of individuals and thus is
usually considered to reside in the retroperitoneum
The transverse is the longest (40 to 50 cm)
and most mobile segment of the colon.
The descending colon is about 25 to 45 cm in length and travels
posteriorly and then inferiorly in the retroperitoneal compartment
to the pelvic brim.
It emerges from the retroperitoneum
into the peritoneal cavity as the sigmoid colon redundant segment of variable length, tortuosity, and mobility.
S-shaped
The mobility of the sigmoid colon renders it susceptible to volvulus, and because it is the narrowest part of the colon, tumors
and strictures of this region typically cause obstructive symptoms early in the course of disease.
The rectum is 10 to 12 cm in length and begins at the peritoneal
reflection, follows the curve of the sacrum passing down
and posteriorly, and ends at the anal canal. The rectum narrows
at its junction with the sigmoid, expanding proximal to the anus.
The anorectal junction is 2 to 3 cm anterior to the tip of the coccyx.
2 to 3 cm anterior to the tip of the coccyx. The rectum does
not have sacculation, appendices epiploicae, or mesentery. The
outer rectal wall is progressively thickened with prominent and
anterior bands of muscle as it descends toward the anus. The
luminal surface of the rectum has 3 transverse folds called the
valves of Houston.
The anal canal is 2 cm long in the infant and 4.5 to 5 cm long
in the adult. It occupies the ischiorectal fossa, passing inferiorly
and outward toward the anal opening
The external sphincter is made up of striated muscle; it
surrounds the anal canal, and its fibers blend with those of the
levator ani muscle to attach posteriorly to the coccyx and anteriorly
to the perineal body.
Distally, the anal verge represents
the transition of anoderm to true skin
. The mucosa of the distal
3 cm of the rectum and anal canal contains 6 to 12 redundant
longitudinal folds called the columns of Morgagni, which terminate
in the anal papillae.
These columns are joined together
by mucosal folds called the anal valves, which are situated at
the dentate line.
The zona alba is a white zone that demarcates
the transition to typical squamous epithelium
The proximal duodenum receives arterial blood from the right gastric artery, supraduodenal artery, right gastroepiploic artery, and superior and inferior pancreaticoduodenal arteries.
Venous drainage is via the SMV and the splenic and portal veins.
The
SMA delivers oxygenated blood to the distal duodenum, jejunum
and ileum, ascending colon, and proximal two thirds of the transverse colon.
Branches of the inferior mesenteric artery
supply the remainder of the colon. The arterial supply of the anal area is from the superior, middle, and inferior hemorrhoidal arteries, which are branches of the inferior mesenteric, hypogastric,
and internal pudendal arteries, respectively
Venous drainage of the anus is by both the systemic and portal systems.
The internal hemorrhoidal plexus drains into the superior rectal
veins and then into the inferior mesenteric vein, which, with the
SMV, joins the splenic vein to form the portal vein.
The vascularity
of the distal anus drains by the external hemorrhoidal
plexus through the middle rectal and pudendal veins into the
internal iliac vein.
Lymphatic drainage courses through the mesentery from villus
lacteals and lymphatic follicles and converges at preaortic lymph
nodes around the SMA and celiac artery.
The lymphatic drainage
of both the small intestine and colon follows their respective
blood supplies to lymph nodes in the celiac, superior preaortic,
and inferior preaortic regions. Lymphatic drainage proceeds to
the cisterna chyli and then via the thoracic duct into the left subclavian
vein`
Proximal to the dentate line, lymphatic drainage is
to the inferior mesenteric and periaortic nodes, whereas distal to
the dentate line it flows to the inguinal lymph nodes.
Therefore,
inflammatory and malignant disease of the lower anal canal can
manifest with inguinal lymphadenopathy.
The autonomic nervous system—sympathetic, parasympathetic,
and enteric—innervates the GI tract. The sympathetic and parasympathetic
nerves constitute the extrinsic nerve supply and connect
with the intrinsic nerve supply, which is composed of ganglion
cells and nerve fibers within the intestinal wall
The wall of the small intestine and colon is composed of
4 layers: mucosa (or mucous membrane), submucosa, muscularis
(or muscularis propria), and serosa
The mucosa is thick
and highly vascularized, although less so in distal portions. It has
concentric folds (plicae circulares) that are also referred to as the
valves of Kerckring. The surfaces of the mucosal folds are studded
with villus projections, and these features combine to produce a
400- to 500-fold increase in mucosal surface area. An intestinal
villus will typically project 0.5 to 1.5 mm into the lumen, and
the height of the villus decreases from proximal to distal small
intestine.
Villi are wider and more leaf-shaped in the duodenal
bulb and proximal duodenum, becoming more finger-like in the
distal duodenum, proximal jejunum, and remainder of intestine.
Mucosal
epithelial cells turn over every 5 to 7 days. Intestinal epithelial
cells are mature by the time they reach the upper third of the
villus. Paneth cells are the only cells that do not migrate. Undifferentiated
cells have fewer intracellular organelles and microvilli
than absorptive cells.
Goblet cells are mucin-producing cells that are scattered
among intestinal villi but are more common in the distal ileum
and large intestine
Goblet cells are oval or round with flattened
basal nuclei (Fig. 98.4A); their cytoplasm is basophilic, metachromatic
(see Fig. 98.4B), and PAS-positive (see Fig. 98.4C) and
consists mostly of mucin-secreting granules