PROCEDURE - Superior Mesenteric
This lab examines the vascular supply in the region of the intestinal tract corresponding to the embryonic midgut (supplied by the superior mesenteric artery) and hindgut (supplied by the inferior mesenteric artery).
1. Reflect the greater omentum and transverse colon
superiorly to expose the mesentery of the transverse colon, the mesocolon.
(Figure330)
2. Move the coils of jejunum and ileum to the left side of
the abdomen such that you can observe the root of the mesentery attached to the
posterior abdominal wall.
(Figure330)
3. On any part of the mesentery connecting to the small intestines, blunt dissect an opening into the peritoneum and begin removing a region of the connective tissue.
With good preservation you may be able to grip the peritoneum and peel the anterior layer away.
All of the arteries distributing out to the intestines travel within the folds of mesentery, which needs to be opened/removed to view the arteries.
4. Continue to scrape and clean peritoneum to find an artery coursing within the
mesentery.
(Netter294)
(Photo3007)
All of the arteries supplying the small intestines originate from the superior mesenteric artery and we can use any of those to follow back to the superior mesenteric artery.
The most common branching pattern is described below, however be aware that there can be considerable differences between individuals.
The mesentery can contain several hundred lymph nodes that you may encounter during dissection. There are also numerous nerves that are typically too small to observe traveling out through the mesentery to the intestines.
5. Using blunt dissection follow any vessel you found superiorly
to identify the superior mesenteric artery and its origin from the aorta.
(Figure339) (Netter294)
(Photo3007)
The superior mesenteric artery is the second anteriorly directed major blood supply from the aorta to the abdominal organs. The artery branches directly anteriorly from the aorta ~1cm inferior to the celiac trunk and posterior to the neck of the pancreas (vertebral level L1).
The superior mesenteric artery crosses anterior to the third part of the duodenum connecting to the anterior surface of the aorta behind the neck of the pancreas. You may need to mobilize and push the pancreas superiorly to see this connection.
The artery will be surrounded by a plexus of autonomic nerve fibers traveling along the vessels out to the intestines. These can be removed as necessary.
Close to the origin of the artery at the aorta will be the superior mesenteric ganglion innervated by the lesser splanchnic nerves, although this ganglion is small and can be difficult to find (Netter304).
6. Follow the superior mesenteric artery inferiorly
isolating and following branches arising from the artery out to their intestinal
targets as listed below.
(Figure339) (Netter294)
(Photo3007)
Middle Colic Artery - This is the first major branch which typically arises from the anterior side of the superior mesenteric artery and supplies the transverse colon.
Right Colic Artery - A second large branch, the right colic artery, arises from the right side of the superior mesenteric artery and supplies the ascending colon.
Ileocolic Artery - This is the lowest branch of the superior mesenteric artery coursing to the right to supply the region of the cecum (the ileocolic junction as the artery name implies).
Intestinal Arteries - An additional 15-18 intestinal arteries branch off from the superior mesenteric artery to provide blood supply to the jejunum and ileum (Photo3008).
Arterial Arcades - As the intestinal arteries approach the intestines they are cross-connected with a series of vessel anastomoses forming intermesenteric arterial 'arcades' (Photo3008).
Vasa rectae (intestinal) - Past the arcades vessels form straight terminal branches called intestinal vasa rectae connecting to the intestines. These are also commonly known as arteriae recta or straight arteries (Photo3008).
Vasa rectae to the jejunum are longer and more regular than vasa rectae to the ilium, but this is not sufficiently consistent to be an identification feature.
7. Numerous mesenteric veins drain blood back from the
intestines. Examine the following veins.
(Figure340) (Netter296)
(Netter297)
(Netter299)
Note that the veins carry the same names as their respective arteries and course together with the arteries.
Superior mesenteric vein - Veins from the intestines converge into the superior mesenteric vein which runs along the right side of the superior mesenteric artery.
Hepatic portal vein - The superior mesenteric vein passes posterior to the pancreas where it will join with the splenic vein forming the hepatic portal vein. Do not overly damage the pancreas at this point, but if you gently mobilize the pancreas you may be able to see the junction.
PROCEDURE - Inferior Mesenteric
8. To examine the inferior mesenteric vascular supply of the large intestines,
start by moving the small intestines to the right such that the descending colon
is visible.
(Figure333)
9. Open the peritoneum close to the sigmoid mesocolon and
find any of the arterial terminal branches feeding the colon.
(Figure341) (Netter295)
(Photo3009)
10. Dissect and
follow that vessel superiorly past any other branches until you reach the origination
point at the anterior surface of the aorta.
(Figure341) (Netter295)
(Photo3009)
This is the inferior mesenteric artery which is the third anteriorly directed blood supply from the aorta to the abdominal organs, located approximately at vertebral level L2-3 posterior to the third part of the duodenum.
Near the junction of the inferior mesenteric artery with the aorta is the inferior mesenteric ganglia that is innervated by the least splanchnic nerves. This is usually a cluster of very small separated ganglion that are essentially impossible to identify (Netter305).
11. Work your way back down the inferior mesenteric artery
identifying and following branches arising from the artery out to their
intestinal targets as listed below.
(Figure341) (Netter295)
(Photo3009)
Left Colic Artery - The first major branch is typically the left colic artery which supplies the left third of the transverse colon and the descending colon.
Sigmoidal Arteries - Working inferiorly, three to four sigmoidal artery branches arise from the inferior mesenteric artery supplying the sigmoid colon.
Superior Rectal Artery - The terminal most inferior branch is the superior rectal artery which supplies the rectum (do not follow that into the pelvic region at this time).
12. Follow the left colic artery to the region near to the
colon and dissect in the mesentery parallel to the transverse colon and parallel
to the descending colon for the marginal artery.
(Figure341) (Netter295)
Marginal Artery - The marginal artery runs in the mesentery parallel and close to the colon. This is part of the vascular arcade that connects (anastomoses) regions supplied by the superior mesenteric artery with regions supplied by the inferior mesenteric artery.
The marginal artery allows blood to 'shunt' from one part of the colon to another part should one of the main supply vessels be compromised. This arterial anastomosis continues all the way around the colon and connects to the arterial arcades of the small intestines.
13. Numerous mesenteric veins drain blood from the
colon back to the inferior mesenteric vein, follow one of these to find the main
vein.
(Figure340) (Netter298)
(Netter299)
Note that the veins carry the same names as their respective arteries and course together with the arteries.
Inferior mesenteric vein - This vein ascends just to the left of the inferior mesenteric artery, posterior to the pancreas, and can join either the splenic vein (most common) or superior mesenteric vein (less common).
Post-Lab Daily Clinical Review Cases
Each lab has several multiple choice practice cases to reinforce understanding of the material within the lab and associated lectures. These are optional to review at home for consolidating and testing understanding.
CHECKLIST
Soft Structures
Superior mesenteric artery
Middle
colic artery
Right colic artery
Ileocolic artery
Intestinal
arteries
Arcades
Inferior mesenteric artery
Left colic
artery
Marginal artery
Sigmoidal arteries
Superior
rectal artery
Hepatic portal vein
Splenic vein
Superior mesenteric vein
Inferior mesenteric vein