Abdomen Lab 4A

Distal to the pyloric sphincter the gut tube becomes the first part of the small intestine, the duodenum. The duodenum is the shortest region of the small intestine, but it receives bile from the liver and digestive enzymes and bicarbonate ions from the pancreas. Both the liver and the pancreas are derived from the foregut (endoderm and splanchnic lateral plate mesoderm), and are still connected to it by their respective ducts, which empty into the duodenum.

The duodenum becomes secondarily retroperitoneal in its middle part. Note where it disappears into the retroperitoneum, and then move the transverse colon and greater omentum out of the way to find the spot where the duodenum emerges from the retroperitoneum and transitions to the jejunum. 

Mobilize the jejunum and the ileum to appreciate the anatomy of the mesentery, and follow the small intestine to its termination at the ileocecal junction. There is no grossly visible transition between the jejunum and the ileum

The ileum attaches to the cecum at the iliocecal junction. The cecum is a pouch that is suspended from the ascending colon. Mobilize the cecum and search for the appendix on its postero-medial surface.  

The ascending colon is secondarily retroperitoneal, and therefore it is directly attached to the posterior body wall (it has no mesentery). Follow the ascending colon superiorly to the right (hepatic) flexure, where it transitions to the transverse colon.

The transverse colon is suspended by the greater omentum and the transverse mesocolon, so the greater omentum must be reflected superiorly to see it. 

The transverse colon typically ascends as it moves toward the left side of the abdominal cavity. It terminates at the splenic (left colic) flexure, where it transitions to the descending colon. 

The descending colon is secondarily retroperitoneal, so it will be applied to the posterior body wall. It has no mesentery, same as the ascending colon. Just superior to the pelvis the descending colon transitions to the sigmoid colon. 

The sigmoid colon does have a mesentery. It travels toward the midline and transitions to the rectum, which is the terminal part of the large intestine. 

The first part of the rectum is intraperitoneal but as it passes into the pelvis it become retroperitoneal. 

The arterial supply to the GI tract arises from three unpaired branches of the abdominal aorta. They are: 

• Celiac trunk

• Superior mesenteric a.

• Inferior mesenteric a. 

These branches supply the foregut, midgut, and hindgut respectively. 

Use forceps and your fingers to shred the lesser omentum and then dissect down toward the aorta to find the celiac trunk. You’ll be dissecting through the tough celiac plexus of autonomic nerves.

Another approach to finding the celiac trunk is to find the splenic artery coursing through the pancreas and follow it medially to the celiac trunk. Again, feel free to resect portions of the liver to clear the dissection field. See Figure 4.2 and Figure 4.3 for help.