Download HIINDGUT LARGE INTESTINE Where water is absorbed from indigestible

HIINDGUT
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LARGE INTESTINE
o Where water is absorbed from indigestible residues of liquid chyme,
converting it to feces, which are stored temporarily and allowed to
accumulate until defecation occurs
o Consists of
 Cecum; appendix, ascending, transverse, descending, and sigmoid
colon; rectum; and anal canal
o Can be distinguished from small intestine by
 Omental appendices
 Small fatty omentum-like projections
 teniae coli
 3 longitudinal bands
o Mesocolic tenia to which transverse and sigmoid
mesocolons attach
o Omental tenia, to which omental appendices attach
o Free tenia, to which neither mesocolons nor omental
appendices are attached
 These are made of smooth muscle
 They begin at the base of the appendix as the thick longitudinal
layer of the appendix separates into 3 bands and run the length
of the large intestine
 They broaden and merge with one another again at the
rectosigmoid junction into a continuous longitudinal layer
around the rectum
 Their tonic contraction shortens the wall with which they are
associated, and the wall becomes baggy looking, forming…
 Haustra
 Sacculations of the wall of the colon between the teniae
 It’s bigger around
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CECUM AND APPENDIX
o Cecum is the first part of the large intestine—continuous with ascending colon
 Is a blind intestinal pouch
 Lies in the iliac fossa of the right lower quadrant of the abdomen, inferior to the
junction of the terminal ileum and cecum
 May be palpable through the anterolateral abdominal wall if distended with gas
or feces
 Lies close to inguinal ligament
 Almost entirely enveloped by peritoneum, but has no mesentery
 Commonly bound to lateral abdominal wall by one or more cecal folds of
peritoneum
 Terminal ileum enters the cecum obliquely and partly invaginates into it
 Ileal orifice enters cecum between ileocolic lips (superior and inferior)
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Folds that meet laterally forming ridges called the frenula of the ileal
orifice
 Is usually closed, appearing as ileal papilla on the cecal side
Appendix is a blind intestinal diverticulum that contains masses of lymphoid tissue
 Is on the posteromedial aspect of cecum, inferior to the ileocecal junction
 Has short triangular mesentery, the mesoappendix, which derives from the
posterior side of the mesentery of the terminal ileum
 Attaches to the cecum and proximal part of appendix
 Position of the appendix is variable, but it is usually retrocecal
ARTERIAL SUPPLY
 Ileocolic artery, terminal branch of the SMA
 Appendicular artery, branch of ileocolic artery, supplies the appendix
VENOUS DRAINAGE
 Tributary of SMV, the ileocolic vein
NERVES
 Superior mesenteric plexus
 Sympathetic fibers are from lower thoracic part of spinal cord
 Afferent nerve fibers accompany these to T10 spinal segment
 Parasympathetics are from vagus nerves
COLON
o Four parts
 Ascending
 Second part of the large intestine
 Passes superiorly on the right side of abdominal cavity from the
cecum to the right lobe of the liver, where it turns to the left at
the RIGHT COLIC FLEXURE (hepatic flexure)
o Flexure lies deep to ribs 9-10 and is overlapped by
inferior part of liver
 Is narrower than the cecum and is secondarily retroperitoneal
along the right side of the posterior abdominal wall
 (awesome picture of arteries to colon on pg. 250)
 Is usually covered by peritoneum anteriorly and on its sides,
BUT it has a short mesentery in 25% of people
 Is separated from anterolateral abdominal wall by the greater
omentum
 Deep vertical groove lined with parietal peritoneum, the RIGHT
PARACOLIC GUTTER, lies between the lateral aspect and the
adjacent abdominal wall
 ARTERIAL SUPPLY OF ASCENDING COLON AND RIGHT COLIC
FLEXURE
o Branches of SMAILEOCOLIC AND RIGHT COLIC
ARTERIES
o These anastomose with each other and with the right
branch of the MIDDLE COLIC ARTERY
This is the first of many anastomotic arcades, that is
continued by LEFT COLIC and SIGMOID ARTERIES to
form the MARGINAL ARTERY/JUXTACOLIC ARTERY
 This artery parallels and extends the length of
colon
 VENOUS DRAINAGE
o Ileocolic and right colic veins tributaries of SMV
 NERVES from superior mesenteric nerve plexus
TRANSVERSE
 Third, longest, and most mobile part of the LI
 Crosses the abdomen from right colic flexure to left colic
flexure, where it turns inferiorly to become descending colon
 LEFT COLIC FLEXURE/SPLENIC FLEXURE is usually more
superior and acute and less mobile that the right colic flexure
o Lies anterior to inferior part of left kidney and attaches
to the diaphragm through phrenicocolic ligament
 Transverse colon and its mesentery, the transverse mesocolon,
loop down, often inferior to level of iliac crests
o Mesentery is fused with posterior wall of omental bursa
 Root of transverse mesocolon is continuous with parietal
peritoneum posteriorly
 ARTERIAL SUPPLY
o MIDDLE COLIC ARTERY, branch of SMA
 VENOUS DRAINAGE
o SMV
 NERVES from superior mesenteric nerve plexus via right and
middle colic arteries—transmit sympathetic, parasympathetic
(vagal), and visceral afferents
DESCENDING
 Is secondarily retroperitoneal
 Continuous with sigmoid colon
 Has short mesentery in 33% of people
 Passes anterior to lateral border of left kidney
 Has LEFT PARACOLIC GUTTER on its lateral aspect
SIGMOID
 Links descending with rectum
 Rectosigmoid junction is indicated by termination of the teniae
coli
 Usually has a long mesentery—sigmoid mesocolon
o Is free to move at its middle part, so can result in volvuli
 Root of sigmoid colon has inverted V-shaped attachment
extending medially and superiorly along the external iliac
vessels and then medially and inferiorly from the bifurcation of
the common iliac vessels to the anterior aspect of the sacrum
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Left ureter and division of left common iliac artery lie
retroperitoneally, posterior to apex of the root of the sigmoid
mesocolon
 Omental appendices of sigmoid colon are long and disappear
when sigmoid mesentery terminates
 ARTERIAL SUPPLY
o LEFT COLIC AND SIGMOID ARTERIES—branches of IMA
o At left colic flexure, second transition occurs in blood
supply of abdominal part of the alimentary canal: SMA
(midgut) to IMA (hindgut)
 VENOUS DRAINAGE
o IMV, flowing to splenic vein and hepatic portal vein after
that
 NERVES
o Sympathetics from lumbar part of sympathetic trunk via
lumbar splanchnic nerves, superior mesenteric plexus,
and periarterial plexuses following IMA and branches
o Parasympathetics from pelvic splanchnic nerves via
inferior hypogastric plexus independent of arterial supply
to this part of the digestive tract
o Orad to the middle of the sigmoid colon—Pain sensations
travel with sympathetics while reflex information follows
parasympathetics to the vagal sensory ganglia
o Towards end of sigmoid colon—all visceral afferents
follow parasympathetic fibers to sensory ganglia of S2S4
RECTUM AND ANAL CANAL
o Rectum is fixed (primarily retroperitoneal and subperitoneal) part of LI
o Is continuous with sigmoid colon at S3 level
o Is continuous inferiorly with anal canal
Clinical correlates 254-261