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