Gut Book 4: Infracolic Compartment: Intestines, Superior and Inferior Mesenteric Vessels, Hepatic Portal Vein Flashcards
Small bowel
small intestine distal to duodenum
Jejunoileum
that portion of the small intestine (jejunum and ileum) that connects the duodenum with the cecum of the colon
jejunoileum relations
anteriorly- to anterior body wall peritoneum
medially- to the cecum, ascending and descending colon (sometimes partly overlapping these structures anteriorly, too)
superiorly- to the transverse colon
inferiorly- to the sigmoid colon
posteriorly- through posterior body wall peritoneum to retroperitoneal strutures, i.e. aorta, IVC, kidneys, parts II & III of the duodenum and lower half and uncinate process of the pancreas
Why is the common term jejunoileum used?
because the transition from jejunum to ileum is so gradual that their definitive structural differences are most easily observed by comparing their combined proximal and distal regions.
Compared to ileum, the jejunum has
greater diameter thicker wall increased number of circular folds few lymphoid nodules increased vascularity deeper color less mesenteric fat fewer arterial arcades longer vasa recta
Thickness of intestinal wall decreases from above downward due to
- decrease in thickness of both the inner circular and outer longitudinal muscular layers
- decrease in the number of plica circularis
Jejunoileum general characteristics & location
approx 20 feet in length
Upper 2/5 (8 ft) is jejunum; lower 3/5 (12 feet) is ileum
begins at the point of re-peritonealization of part IV of the duodenum on the left of LV2
Upper 1/3 of the jejunoileum located in the upper left quadrant
middle 1/3 located in umbilical region
lower 1/3 in pelvis and right iliac fossa
Note: most of the jejunum lies in the left upper quadrant, while most of the ileum resides in the lower right quadrant. The terminal ileum ascends from the pelvis to become continuous with the right colon at the cecum (ileocecal junction)
Jejunoileum: mesentery
The jejunoileum is anchored to the posterior body wall by the MESENTERY PROPER (dorsal mesentery). The actual point of attachment or “root” begins at the point of re-peritonealization of the distal duodenum (part 4 located to the left of LV2) and extends across the posterior body wall in an inferior oblique direction to the level of the right sacroiliac joint, a course of six to seven inches. In its course it crosses the third part of the duodenum, aorta, IVC and the right ureter, right gonadal vessels and the right psoas major muscle.
To accommodate such an abbreviated mesenteric attachment the jejunoileum, as well as it accompanying mesentery…
are thrown into folds similar to those of a collapsible paper fan.
mesentery proper: contents
blood vessels, nerves and lymphatic vessels traverse the mesentery to reach the jejunoileum.
The mesentery is also a fat storage area, the amount of fat increasing as the ileum is approached.
Meckel’s diverticulum
the remnant of the yolk stalk, if present (1-2%), is located within the terminal meter of ileum on the anti-mesenteric border.
Rule of 2s for Meckel’s diverticulum:
located within approx 2 ft of distal ileum
Occurs in 2% of population
2 cm in length
usually discovered by the age of 2
May contain 2 types of ectopic tissue: gastric and pancreatic mucosa
Note from lecture: this extra pancreatic tissue can lead to it digesting itself
Peyer’s Patches
Collections of lymphocytic nodules that exist on the anti-mesenteric border of the terminal ileum
Large Bowel description
approx 5 ft in length
extends from cecum to anus forming an arch which borders the small intestine to the right, superiorly, to the left and partially inferiorly
Large bowel divisions
cecum with appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anal canal
Large bowel differs from the small bowel how?
- is more distensible than the small bowel although the diameter may appear the same
- muscular layers are thinner, to the extent that the outer longitudinal layer is incomplete. It consists of three narrow bands (approx 1 cm. in width), the teniae coli.
They are shorter by 1/6 than the length of the colon. This causes the colon to be sacculated forming the HAUSTRA COLI (pouches) - fat storage appendages are present
- circular folds are replaced by semilunar folds
Epiploic (omental) appendages
fat storage bodies enclosed and suspended within the peritoneum of the large bowel.
Plica semilunares
Internally, the plica circularis of the small bowel are replaced by plica semilunares (semilunar folds) in the large bowel.
Microscopically, the mucosa of the large bowel is…
evenly developed and devoid of villi
Large bowel function
remove water from food residues and store and compact stool.
Mesenteries of the large bowel
mesoappendix, transverse mesocolon, sigmoid mesocolon
fusion fascia
The ascending and descending colon are secondarily retroperitoneal. The area dorsal to them represent areas of fusion between two planes of fascia: the formerly peritonealized colon and the peritoneum of the posterior body wall. This fascial area is referred to as FUSION FASCIA. It is unique because the ascending and descending colon can be safely approached surgically through these areas as no vessels or nerves traverse these planes of fascia.
Peritonealized portions of the large intestine include:
appendix, cecum, transverse colon, sigmoid colon, and superior rectum
Mesoappendix
extension of dorsal mesentery to the appendix
transverse mesocolon
suspends the transverse colon from the posterior body wall along a transverse line at approx LV1-LV2
Sigmoid mesocolon
suspends the sigmoid colon across the left pelvic brim to the third sacral segment
Large bowel subdivisions
cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal
Cecum
blind pouch at the proximal portion of the ascending colon
Located in RLQ, peritonealized
receives the terminal ileum and appendix on its posteromedial surface
ileocecal valve
Terminal ileum invaginates into the cecum to form the superior and inferior lips of the ileocecal valve.
Note: although the circular muscle layer of the ileum is continued into the lips of the ileocecal valve, the valve is not competent. That is, contraction of the terminal ileum is more important in preventing reflux of cecal contents into the ileum.
Appendix
blind-ending diverticulum; average length is 8 cm.
affixed to the posterior body wall by the mesoappendix
occupies a retrocecal position (64%)
possesses a complete coat of longitudinal muscle formed by the teniae coli which begin at the appendix
mucosa and submucosa of appendix
filled with lymphoid nodules
Appendicitis
inflammation due to blockage of lumen cause by overgrowth of epithelial lining or impaction via coprolith (fecal stone).
Referred pain via GVA fibers to T10 dematome at the umbilicus
Where inflamed appendix contacts parietal peritoneum of posterior body wall, pain felt at McBurney’s point: point 2/3 the distance from the umbilicus to the ASIS
Ascending colon
- extends from the cecum to the right colic flexure
- retroperitoneal
- the right paracolic gutter is located between the ascending colon and the right lateral body wall
- narrower than the cecum
ascending colon relations
anterior- small intestine and the greater omentum
lateral- transversus abdominis
medial- posterior body wall, small intestine
posterior- posterior body wall, right kidney
Transverse colon extent and attachments
extends from the right colic flexure (hepatic) to the left colic flexure (splenic)
suspended from posterior body wall by the transverse mesocolon
attached to the stomach by the gastrocolic ligament
Omental apron of greater omentum is attached to its anterior surface
left colic flexure is attached to the left diaphragm by the phrenicocolic ligament
transverse colon relations
superior- liver, gall bladder, stomach and spleen
anterior- anterior body wall
inferior- small intestine
posterior- transverse mesocolon
Descending colon
extends from the left colic flexure to the pelvic brim
retroperitoneal
descends on the posterior body wall musculature along the lateral border of the left kidney
The left paracolic gutter is located between the descending colon and the left lateral body wall.
descending colon relations
anterior- small intestine
lateral- transversus abdominis
medial- small intestine and left kidney
posterior- posterior body wall
Sigmoid colon
extends from the descending colon to the rectum
S-shaped, beginning on the left side extending to the right and ending on the midline where it becomes continuous with the rectum
diverticulosis
evaginations of the colonic mucosa occur most often in the descending and sigmoid colon. Caused by an increase in the pressure generated by the colon to move feces of low fiber content, the wall outpockets at points where it is weakest (where blood vessels penetrate the colon wall). Material becomes lodged in these pockets and the pockets become inflamed.
Rectum divisions
divided into thirds on the basis of its partial peritonealization:
1- proximal third- continuous with the sigmoid, covered with peritoneum laterally and anteriorly
2- middle third- covered with peritoneum ANTERIORLY only
3- distal third- rectal ampulla: dilated portion resting on pelvic diaphragm which exists below peritoneum. Peritoneum above the ampulla is reflected to the pelvic viscera or lateral and posterior pelvic wall.
Rectum musculature
teniae coli fan out to form outer longitudinal muscle bands anterior and posterior to the rectum
inner circular layer continuous with that of colon
rectum internal surface modifications (2)
transverse rectal folds and ampulla
transverse rectal folds
typically three shelves composed of mucosa, submucosa and some circular muscle protrude from the walls of the rectum. They cause the external surface of the rectum to be thrown into curvatures or flexures; 2 on the left and one on the right. Functionally, they assist in supporting the fecal mass.
rectal ampulla
dilated portion superior to pelvic diaphragm
Anal canal location
Located in anal triangle between opposing ischioanal fossae; extends from pelvic diaphragm to anus.
Located inferior to the ampulla of the rectum; angles 90 degrees posteriorly due to the passage of the puborectal sling at point of junction of rectum and anal canal
what allows for expansion of anal canal during the elimination of feces
it is surrounded by ischioanal fat
internal anal sphincter
continuous with the inner circular muscle layer; involuntary in nature and autonomically innervated
external anal sphincter
circularly arranged skeletal muscle innervated by the inferior rectal branches of the pudendal n. (S2,3,4)
anal columns
vertical folds caused by veins deep to the mucous membrane
anal sinuses
depression between anal columns at anal valves
anal valves
formed by venous anastomoses that connect the bases of contiguous anal columns and bridge across individual anal sinuses
pectinate line
created by the course of the anal valves
Note from lecture: pectinate means “teeth of a comb.”
Hilton’s White Line
= intersphincteric line
demarcates the point where the external sphincter meets the internal sphincter. ALSO, demarcates the anal verge
anal verge
area where transitional epithelium changes to perianal skin
transitional area (anal pectin)
area between pectinate line above and Hilton’s white line below; demarcates area of epithelial change of the mucosa of the anal canal
Changes occurring at the pectinate line: epithelium
above- columnar
below- stratified squamous
Changes occurring at the pectinate line: blood supply
above- superior rectal artery
below- inferior rectal artery
Changes occurring at the pectinate line: venous drainage
above- portal
below- systemic
Changes occurring at the pectinate line: dilated veins
above- internal hemorrhoid
below- external hemorrhoid
Changes occurring at the pectinate line: sensory nerves
above- visceral
below- somatic
Changes occurring at the pectinate line: lymphatic drainage
above: internal ilac, inf. mesenteric nodes
below: superficial inguinal nodes
Internal hemorrhoids
- prolapse (downward displacement) of rectal mucosa affecting the internal rectal venous plexus
- veins of this plexus can become trapped by the contracted anal sphincters resulting in engorged and ulcerated veins
- due to elaborate anastomoses in this region, bleeding is mostly bright red.
external hemorrhoids
thrombosed (clotted) veins of the external rectal venous plexus
located directly under the skin
painful
general cause of hemorrhoids
in general, the result of increased intra-abdominal pressure, i.e. chronic constipation, straining at stool, pregnancy and portal hypertension.
Superior mesenteric artery (SMA) origin
second unpaired ventral branch from the abdominal aorta
arises in front of LV1 directly inferior to the origin of the celiac artery
SMA distribution
entire small intestine (except for the proximal half of the duodenum), cecum, ascending colon and the proximal 2/3 of the transverse colon
SMA course
arises behind the neck of the pancreas
crosses the uncinate process and the third part of the duodenum to enter the mesentery of the small intestine
follows a course parallel to the root of the mesentery inferiorward toward the cecum.
branches of the superior mesenteric artery will distribute to the intestines by traversing the mesentery which support them.
superior mesenteric vein
accompanies the superior mesenteric artery in its course, lies to the right and ventral to the artery.
Superior mesenteric plexus
The SMA is invested with a thick network of nerve fibers, the superior mesenteric plexus, which conducts autonomic sympathetic and parasympathetic nerve fibers to all areas of the small and large bowel supplied by the SMA.
Branches of the SMA
inferior pancreaticoduodenal a. intestinal aa (jejunal & ileal aa) ileocolic a. right colic a. middle colic a.
Inferior pancreaticoduodenal a.
first branch of the SMA (note- often arises from the 1st jejunal a.)
arises at the pancreatic incisure (separates uncinate from neck)
runs a short course (1-2 cm) to the right where it divides into: anterior and posterior inferior pancreaticoduadenal aa.
anterior and posterior inferior pancreaticoduodenal aa.
divisions of the inferior pancreaticoduodenal a.
anastomose with anterior and posterior SUPERIOR pancreaticoduodenal aa., respectively forming anterior and posterior pancreatic ARCADES.
These anastomoses provide blood to the head of the pancreas and to parts I, II, & III of the duodenum
Intestinal arteries (jejunal & ileal aa.)
major blood supply to entire small bowel
approximately 12-15 in number
arise on the left lateral aspect of the SMA
course in the mesentery proper
anastomose forming several generations of arterial arcades
gives rise to VASA RECTA which penetrate the wall of the intestines
Ileocolic a.
Terminal branch of the SMA
Parallels the terminal intestinal a.
Courses toward the right iliac fossa to terminate at the ileocecal junction
branches of ileocolic a.
ascending (colic) branch anterior cecal artery posterior cecal artery appendicular artery ileal branch
ascending (colic) branch of ileocolic a.
ascends the medial border of the proximal ascending colon to anastomose with the descending branch of the right colic a.
anterior cecal artery (branch of ileocolic a.)
passes anterior to the cecum within the vascular cecal fold
posterior cecal artery (branch of ileocolic a.)
passes posterior to the cecum
appendicular artery (branch of ileocolic a.)
passes posterior to the cecum to enter the mesoappendix
ileal branch of the ileocolic artery
passes from the ileocecal junction along the distal portion of the ilum to anastomose with the last intestinal (ileal) branch
Right colic artery
inconstant branch from the right lateral side of the SMA; may arise as a branch from the ileocolic, middle colic or be entirely absent (13%)
Follows a retroperitoneal course to the right colon where it provides: descending branch (will anastomose with the ascending branch of the colic a.) ascending branch (will anastomose with the descending branch of the middle colic a.)
Middle colic artery
2nd branch of the SMA
arises from the front of the SMA
ascends within the transverse mesocolon to gain the transverse colon
usually bifurcates into RIGHT and LEFT branches joined by an arcade which provide direct branches to the transverse colon
Right branch will anastomose with the ascending branch of the right colic a.
Left branch will anastomose with the ascending branch of the left colic a.
Clinical note re: surgical approach to the pancreas
Care must be taken to not cut the middle colic vessels when surgically approaching the pancreas through the transverse mesocolon.
Inferior mesenteric artery (IMA): origin
Third unpaired ventral branch from the abdominal aorta
arises in front of LV3
IMA: course and distribution
courses in a retroperitoneal position across the left posterior abdominal wall to descend into the pelvis
distributes to the distal 1/3 of the transverse colon, descending colon, sigmoid colon and rectum
inferior mesenteric plexus
the surface of the IMA is invested by the inferior mesenteric plexus, which conducts sympathetic nerve fibers to the distal transverse colon, descending and sigmoid colon
Branches of IMA
Left colic artery
sigmoidal arteries (2-3)
superior rectal artery
Left colic artery origin
arises from the IMA in retroperitoneal position to course to the middle of the medial border of the descending colon
Left colic artery branches
ascending branch- ascends the medial border of the descending colon to anastomose with the left transverse branch of the middle colic a.
descending branch- descends the medial border of the descending colon to anastomose with the ascending branch of the first sigmoid a.
Sigmoidal arteries (2-3)
Arise from the IMA in retroperitoneal position
Cross the posterior abdominal wall anterior to the left psoas major, ureter and gonadal vessels to enter the sigmoid mesocolon
provide ascending & descending branches forming arcades from which vasa recta supplly the sigmoid colon
The ascending branch of the first sigmoid a. will anastomose with the descending branch of the left colic a.
The descending branch of the last sigmoid a. rarely will anastomose with the superior rectal a.
Superior rectal artery (terminal branch)
terminal branch of the IMA
Crosses the left common iliac vessels
Courses within the sigmoid mesocolon to reach the rectosigmoid junction, where it bifurcates to form right and left branches which course on either side of the rectum.
Marginal artery (of Drummond)
ALL colic branches of the SMA and IMA (ileocolic, right, middle and left, colic, and sigmoid aa.) anastomose along the mesenteric border of the entire colon forming the marginal artery. This anastomosis helps to prevent necrosis of the bowel should one of the colic arteries become occluded.
Arterial supply to the rectum & anal canal
Superior rectal a. (branch of the IMA, supplies most of the muscular layer and mucosal lining of the rectum and upper portion of the anal canal)
Middle rectal a. (branch of the internal iliac a., supplies the lower portion of the rectum, predominantly the muscular portion)
Inferior rectal a. (branch of the internal pudendal a., provides major supply to the mucosa and muscular layer of the anal canal)
Three rectal aa. relationship to one another
All three anastomose forming an intricate vascular supply to the rectum and anal canal.
Portal System
a vascular system that begins and ends in capillaries
Hepatic portal system beginning and end
begins in the capillary beds of the digestive system and its accessory glands, including the gall bladder, pancreas and spleen, ends in the capillary-like venous sinusoids of the liver.
Tributaries of the hepatic portal vein include:
Inferior mesenteric vein with its rectal, sigmoidal and colic tributaries
Superior mesenteric vein with its intestinal, colic and pancreatic tributaries
Splenic vein with its gastric and pancreatic tributaries
Note: several veins train directly to the portal vein, among them gastric, cystic and pancreaticoduodenal veins
Hepatic portal vein
the definitive portal vein is formed dorsal to the neck of the pancreas at LV2 through the union of the superior mesenteric and splenic veins. The inferior mesenteric is most often a tributary of the splenic vein.
The portal vein courses posterior to the first part of the duodenum and enters the hapatoduodenal ligament posterior to the hepatic artery and common bile duct
It then enters the liver at the porta and disperses its branches along the intrahepatic biliary duct network.
where do hepatic arteries and portal veins drain to?
the hepatic venous sinusoids
Portal-caval (sytemic) anastomoses
Collateral circulation between tributaries of the portal vein and the venae cavae (SVC & IVC) utilized when resistance to flow within the portal vein or liver becomes too great
Examples of resistance to flow within the portal vein or liver
Cirrhosis, heart disease, hepatic tumor, pancreatic carcinoma, thrombosis
Alternate routes of hepatic flow (barring intrahepatic anastomoses)
Portal-umbilical
portal-hemorrhoidal
portal-azygos
portal-retroperitoneal
Note: the functionality of these anastomoses exist due to the lack of valves within these veins and the concomitant dependence of the anastomoses on the reversal of venous flow.
portal-umbilical flow
paraumbilical veins anastomose with veins of the anterior abdominal wall (superior & inferior epigastric) which eventually drain to the IVC and SVC
Note- can lead to “Caput Medusae”
portal-hemorrhoidal flow
venous plexuses of the rectum and anal canal communicate between superior rectal, middle and inferior rectal veins which are tributaries to veins which join the IVC
portal-azygos flow
esophageal tributaries to the left gastric vein anastomose with esophageal veins which are drained by the azygos system to the SVC.
Note: due to the added stress upon the esophageal veins varices can develop.
portal-retroperitoneal flow
retroperitoneal veins draining the colon (ileocolic, right, middle and left colic), anastomose with other retroperitoneal veins (gonadal veins, veins of the pararenalfat) to drain to renal veins and ultimately the IVC.
Venous metastasis from the colon to the liver
metastasis from primary colorectal cancer to the liver follows the specific path of venous drainage of the different regions of the colon:
a. metastasis from the appendix, cecum, ascending colon, hepatic flexure and proximal transverse colon primarily seed the right lobe of the liver via the vein of the midgut: superior mesenteric v.
b. metastasis from the rectum, sigmoid, descending colon, splenic flexure, and distal transverse colon primarily seed the left lobe of the liver via the vein of the hindgut: inferior mesenteric v.
c. the caudate and quadrate lobe receive a mixture of blood from veins of the foregut, midgut and hind gut.
