Abdomen Lab 6

Lab 6B - IMA, Hindgut, and removal of the mid and hindgut

Lab Overview

Dissect the branches of the IMA and the organs of the hindgut.
Appreciate the clinical importance of the marginal artery as as anastomosis between the SMA and IMA
Find the hepatic portal vein and trace its tributaries from the digestive tract.
Remove the intestines and arrange them in anatomical position on a tray.

Viscera of the Hindgut

Overview

The hindgut begins at the distal 1/3 of the transverse colon, and then includes the splenic flexure, descending colon, sigmoid colon, and rectum.
The descending colon is secondarily retroperitoneal, so it will be directly attached to the posterior body wall. Leave the descending colon in place for now, we want to find the vasculature of the hindgut (following section) before we free the descending colon.

Assuming the midgut was explored in an earlier lab, you should be familiar with the transverse mesocolon and the transverse colon suspended within it.

The midgut stops and the hindgut begins at a point approximately 2/3 of the way along the transverse colon. It ascends to the splenic flexure, and then turns inferiorly and is secondarily retroperitoneal. The descending colon is the secondarily retroperitoneal part of the hindgut, and transitions to sigmoid (“S-shaped”) colon, which is suspended by a mesentery (the sigmoid mesocolon). The sigmoid colon transitions to rectum. The distal rectum is retroperitoneal.

Note the presence of haustra, taeniae coli, and epiploic appendices (appendeces epiploic if you prefer) along the hindgut. These should be familiar from the midgut.

Search for evidence of diverticulosis in the sigmoid and distal descending colon.

Arteries of the hindgut

Before You Begin...

The artery of the hindgut is the inferior mesenteric artery (IMA).

Because the descending colon is secondarily retroperitoneal, it has no mesentery, and so the IMA is in the retroperitoneal space, as is the abdominal aorta.

Find the IMA and its branches by carefully exploring the retroperitoneal space. The extraperitoneal fat will hide the IMA and its branches to some extent.

Carefully mobilize the descending colon, safeguarding the IMA and its branches.

Hindgut: Inferior Mesenteric Artery

The inferior mesenteric a. is the third unpaired vessel arising from the abdominal aorta at the level of the L3 vertebra.

Push the small intestine into the upper right quadrant of the abdomen to identify the following branches to the distal portion of the colon. Use blunt dissection through the peritoneum or mesocolon and scrape excess fat.

Inferior mesenteric a. - use forceps to dissect over the inferior end of the abdominal aorta, about 3 cm superior to the aortic birucation; usually arises to the left of the midline of the aorta
- Left colic a. - supplies the descending colon and the left portion of the transverse colon; anastomoses with the middle colic a. (from the superior mesenteric a.)
- Sigmoid aa. - within the sigmoid mesocolon; usually four to five branches that form arches; supply blood to the sigmoid colon
- Superior rectal a. - supplies the proximal portion of the rectum; usually a large vessel (the terminal branch of the inferior mesenteric a.)

- Marginal a. (of Drummond) - Note that the marginal artery (artery of Drummond) typically forms a robust anastomosis between the SMA and IMA and between the colic branches of the SMA.
Inferior mesenteric lymph nodes - look for lymph nodes in the mesentery; named according to the organ than they drain.

These lymph nodes may be hard to find, but it is important to note that as a general rule lymphatics follow the vasculature and drain into the closest lymph nodes.

FIGURE 7.2 Branches of the mesenteric a.

FIGURE 7.3 Inferior mesenteric lymph nodes

Clinical Correlation: Referred pain from the appendix.

Acute inflammation of the appendix is a common cause of acute abdomen (severe abdominal pain arising suddenly). The pain of appendicitis usually commences as a vague pain in the periumbilical region because afferent pain fibers enter the spinal cord at the T10 level. Later, severe pain in the lower right quadrant results from irritation of the parietal peritoneum lining the posterior abdominal wall.

The hepatic portal system and removal of the intestines

Portal System of Veins

The portal system of veins drains poorly oxygenated but nutrient-rich venous blood from the gastrointestinal tract to the liver. As in other parts of the body, expect variation in the pattern of veins. The three principal tributaries to the hepatic portal v. are the superior mesenteric v., inferior mesenteric v. and splenic v.

Lift up the greater omentum and transverse colon and push the small intestine inferiorly to identify the following veins, (usually accompanying previously dissected arteries):

Portal v. - locate the portal v. in the porta hepatis; usually about 5 cm long and formed posterior to the neck of the pancreas via the union of the superior mesenteric v. and the splenic v.
Superior mesenteric v. - drains blood from the midgut; located to the right of the superior mesenteric a.
Splenic v. - courses inferior to the splenic a. and posterior to the pancreas

Clean and follow the splenic v. and superior mesenteric v. to their union at the portal v.

Inferior mesenteric v. - usually drains into the splenic v.; to the left of the superior mesenteric v. (expect variation!)
Right and left gastric vv. - drain blood from the lesser curvature of the stomach to the hepatic portal v.; course with the corresponding arteries
Superior rectal v. - drains the rectum to the inferior mesenteric v.; courses with the corresponding artery; do not attempt to follow the superior rectal a. and v. distally! (These will be dissected in a future laboratory session).

FIGURE 7.5 Portal system of veins

FIGURE 7.6 Portal venous system

FIGURE 7.7 Portocaval anastomoses

Clinical Correlation: Portocaval anastomoses

In portal hypertension (as in hepatic cirrhosis), the portal blood cannot pass freely through the liver, and the portocaval anastomoses become engorged, dilated, or even varicose. As a consequence, these veins may rupture. The sites of the portocaval anastomoses shown are between (1) left gastric vein (portal) anastomoses with the esophageal veins (systemic) draining into the azygos veins and then to the IVC (esophageal varices result if these vessels are dilated; see Fig. 7.8.3); (2) superior rectal vein (portal) anastomoses with the inferior and middle rectal veins (systemic) draining into the IVC (hemorrhoids result if these vessels are dilated); (3) paraumbilical vein (portal) anastomoses with small epigastric veins of the anterior abdominal wall (systemic), then to the iliac veins and then to IVC (when varicose form “caput medusae” so named because of the resemblance of the radiating veins to the serpents on the head of Medusa, a character in Greek mythology; Fig 7.8.1 and Fig 7.8.2); and (4) twigs of colic veins (portal) anastomose with systemic retroperitoneal veins.

Figure 7.8.1 Caput medusae

Figure 7.8.2 Caput medusae

FIGURE 7.8.3 Esophageal varices

Removal of the Gastrointestinal Tract

HELPFUL HINTS

Be sure to read all of the directions before beginning to remove the GI tract!

To more clearly view and dissect the posterior abdominal wall, you may choose to remove most of the GI tract. However, you should leave the celiac trunk and its associated organs (foregut) intact.

Follow these instructions to remove the GI tract:

Locate the duodenojejunal junction and the rectum; use your fingers to free up a segment of each part of the gut.
Tie two pieces of string tightly around the duodenojejunal junction, about 2-3 cm apart
Free the rectum from the peritoneum.
Tie two pieces of string tightly around the rectum about 2-3 cm apart; this helps limit the amount of feces that will escape when you cut the rectum.
Use scissors to cut through the GI tract between the adjacent pairs of ties.
Use scissors to cut the superior mesenteric a. about 2 cm distal to its origin from the abdominal aorta, inferior to the pancreas (the short stump on the aorta will serve as a handy reference in the future).
Use scissors to cut the inferior mesenteric a. about 2 cm distal to its origin from the abdominal aorta, inferior to the pancreas (again, the short stump on the aorta will serve as a handy reference)
Use scissors to cut the superior mesenteric v. distal to its union with the splenic v.
Use your fingers and forceps to bluntly dissect the peritoneal attachments of the GI tract between the pairs of string ties while removing the GI tract.
Leave as much mesentery (and its vasculature) as possible attached to the excised small intestine. The following vessels should be identifiable on the removed gut: SMA (and its associated vessels), middle colic artery (and its associated branches), SMV and IMV.

Note that the celiac trunk and splenic vein should remain in the abdominal cavity. Store the removed GI tract in a plastic bag on the shelf below your table.

FIGURE 7.9 Removal of the GI tract. Transection of the duodenojejunal junction (A) and transection of the rectum (B)

HELPFUL HINTS

Review the variations that are possible with the IMV, SMV, and splenic vein and how they feed into the portal vein. For the dissection, you basically need to detach only the IMV and SMV from the portal vein, however they may be attached to the portal vein. The splenic vein need not be cut.

Place the removed intestines on a tray in proper anatomical position. As time allows, you should return to this anatomical preparation and continue to clean the dissection and define the branches of the superior and inferior mesenteric arteries. You can slip the entire tray into a clear plastic bag and store it on the shelf beneath your table.

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