Abdomen Lab 5B

Lab 5B - Foregut, Midgut, and SMA

Lab Overview

Mobilize and identify the viscera of the foregut:
- Spleen (NOT a digestive system structure)
- Liver
- Duodenum and pancreas
- Stomach
Review the celiac trunk and its branches
Dissect the superior mesenteric artery (SMA) and its branches

Short List of Structures - 5B

Lobes and Surfaces of the Liver
- Right and Left Lobes
- Diaphragmatic and Visceral surfaces
Porta Hepatis
Superior Mesenteric Artery
- Duodenal and Jejunal branches
- Ileocolic artery

Viscera of the foregut

Here we’ll look at the Foregut, i.e. the abdominal organs served by the branches of the celiac trunk, in more detail.

Duodenojejunal junction

Suspensory ligament of the duodenum

Spleen and Associated Structures

Identify the following (move the liver, stomach, and other viscera to get a better view):

Spleen - located intraperitoneally in the left upper quadrant; rests superior to the left colic flexure; the tail of the pancreas touches the spleen at its hilum
- Hilum - analogous to the root of the lung, where the splenic vessels exit and enter; peritoneum covers the entire surface of the spleen, except at this point

HELPFUL HINTS

Although the spleen is intraperitoneal, it is not part of the digestive tract. It is an outgrowth of the cardiovascular system from the posterior body wall, and no part of it is derived from endoderm.

Liver

Diaphragmatic surface - region of the liver in contact with the diaphragm
Visceral surface - region of the liver in contact with viscera (stomach, duodenum, colon, right kidney)

Slip your hands between the right and left lobes of the liver and the diaphragm. Your hands now fill the subphrenic recess. You cannot touch one hand to the other because of ligaments (reflections of the peritoneum from the liver to the diaphragm).

Falciform ligament - reflections of the parietal peritoneum around the obliterated umbilical v.; attaches the liver to both the diaphragm and anterior abdominal wall; the free (inferior) border of the falciform ligament contains the round ligament of the liver (obliterated umbilical v.).

Anterior view of the liver (diaphragm reflected)

Liver-Inferior (Visceral) Surface

Lift the liver superior to identify the following structures on its inferior surface (it may help to orient the liver exactly as shown in the figures):

Right lobe - located to the right of the grooves filled by the gallbladder and inferior vena cava
Left lobe - located to the left of the falciform ligament, round ligaments, and groove for the ligamentum venosum
Quadrate lobe - located anteriorly; bordered by the gallbladder, porta hepatis, and round ligament of the liver
Caudate lobe - located posteriorly; bordered by the groove for the ligamentum venosum, groove for the inferior vena cava, and porta hepatis

Posteroinferior view of the liver

Porta Hepatis and Biliary Tree

Lift the liver superiorly. Push the stomach inferiorly to identify the porta hepatis, located between the left and right lobes of the liver, within the hepatoduodenal ligament (the free edge of the lesser omentum). The porta hepatis contains the common hepatic duct, hepatic a. proper, and the portal v.

Use forceps and your fingers to bluntly dissect or use the open scissor technique to remove the remaining peritoneum from the region of the hepatoduodenal ligament to identify the following contents:

Common bile duct - green and usually on the superficial right side of the porta hepatis; dissect this duct toward the liver and gallbladder to reveal the

Cystic duct - duct from the gallbladder which joins the common hepatic duct to form the common bile duct.

Follow the common hepatic duct to the liver to locate the right and left hepatic ducts.

Hepatic a. proper - branch from the common hepatic a. (after the latter gives rise to the gastroduodenal a. and right gastric aa.) on the superficial left side of the porta hepatis

Follow the common hepatic duct to the liver to locate the right and left hepatic ducts.

Portal v.- deep and posterior to the common bile duct and hepatic a. proper; the largest of the three structures

Porta hepatis. Left anterolateral view. The liver has been lifted.

Clinical Correlation: Liver biopsy

Hepatic tissue may be obtained for diagnostic purposes by liver biopsy. The needle puncture is commonly made through the right tenth intercostal space in the midaxillary line. Before the physician takes the biopsy, the person is asked to hold his or her breath in full expiration to reduce the costodiaphragmatic recess and to lessen the possibility of damaging the lung and contaminating the pleural cavity.

Clinical Correlation: Gallbladder and gallstones

Gallstones are concretions, pebble(s), in the gallbladder or extra-hepatic biliary ducts. When they become lodged in the ducts the surrounding smooth muscle spasms and causes pain (biliary colic).

Clinical Correlation: effect of cirrhosis on blood flow through the liver

There is progressive destruction of hepatocytes in cirrhosis of the liver and replacement of them by fibrous tissue. This tissue surrounds the intrahepatic blood vessels and biliary ducts, making the liver firm and impeding the circulation of blood through it.

Clinical Correlation: Hepatic segmentation

Since the right and left hepatic arteries and ducts and branches of the right and left portal veins do not communicate, it is possible to perform hepatic lobectomies (removal of the right or left part of the liver) and segmentectomies.

Vasculature: Celiac Trunk Review

Lift the liver superiorly. Push the stomach out of the way (this may be superiorly or inferiorly, depending on its size and shape). With the foregut dissected, it is easier to review the celiac trunk and its branches. Identify the following:

Find the union of the three arteries. The point of union is the celiac trunk. Note that dissection of the celiac trunk is challenging because it is surrounded by very tough tissue called the celiac ganglion (an autonomic ganglion). Also, be aware that as in the rest of the body, there are many variations in the pattern of the vasculature. Do not get bogged down if your cadaver’s vasculature does not look like these images.

Duodenum and Pancreas

Use forceps to dissect and identify the following, starting with the duodenum. Find the pancreas just inferior to and along the splenic artery. The pancreas has a rough “scrambled eggs” surface appearance. Bluntly dissect the common bile duct from the surrounding pancreatic tissue. The following structures are diffult to find, so we do not require you to find them, but they are important in understanding the anatomy and function of the digestive system:

Circular folds - invaginations (flaps) of the mucosal lining, increase the surface area of the duodenal epithelium.
Major duodenal papilla - elevations where the pancreatic and common bile ducts enter the duodenal lumen
Pancreaticoduodenal ampulla - outside the second part of the duodenum, the bulbous junction of the common bile and pancreatic ducts; it opens into the lumen of the duodenum via the major duodenal papilla
Duodenal sphincter - circularly arranged smooth muscle fibers in the wall of the pancreaticoduodenal ampulla
Main pancreatic duct - use the ampulla as your guide to dissect the pancreatic tissue to reveal the main pancreatic duct

FIGURE 5.14 Interior view of the duodenum showing circular folds, major duodenal papilla, the pancreaticoduodenal ampulla after blunt dissection of pancreas, and the main pancreatic duct extending through the pancreas

Clinical Correlation: Radiographs of esophagus, stomach, duodenum (barium swallow)

A hiatal--or hiatus--hernia is a protrusion of part of the stomach into the mediastinum through the esophageal hiatus of the diaphragm. The hernias occur most often in people after middle age, possibly because of weakening of the muscular part of the diaphragm and widening of the esophageal hiatus.

SMA and Vessels of the Midgut

Before You Begin...

Many of the structures below reside within the mesentery, it will be very important to only use blunt dissection to peel back the mesentery and visualize these easily damaged structures; the second large, unpaired artery of the gut, the superior mesenteric artery (SMA), and its branches. Here is a summary of two major steps you will complete in the sections below:

Locate the duodenojejunal junction - this is a good landmark for locating the SMA, but if all else fails, you should be able to follow the aorta from the thorax down. Once you have found the SMA, carefully dissect along its length to find the (small) branches of the SMA.
In order to better visualize the colic arteries (SMA branches) you will need to bluntly dissect underneath the ascending and descending colon to mobilize them. This blunt dissection should be done entirely with your hands and will be directly deep to the colon, but superficial to the kidneys and the associated adipose of the posterior body wall. Call a member of the teaching staff over if you have any difficulty with this.

Midgut: Superior Mesenteric Artery

Lift up the greater omentum and transverse colon, push aside the small intestine, and bluntly dissect the mesentery to identify the following structures that are draped by mesentery:

Superior mesenteric a. - the second unpaired artery arising from the abdominal aorta, just inferior to the celiac trunk, at the level of the L1 vertebra

Superior mesenteric v. - courses along the right side of the superior mesenteric a.

Palpate to the right of the duodenojejunal junction for the superior mesenteric a. and v. Use forceps to bluntly dissect through one layer of the peritoneum that makes up the mesentery to expose the superior mesenteric a. and v.; both vessels emerge from beneath the pancreas and cross over distally to a portion of the small intestine and the third part of the duodenum.

Superior mesenteric plexus - plexus of autonomic nerves that surround the superior mesenteric a. You may experience the plexus as a mat of connective tissue surrounding the vessels.

Note that you will encounter nerve plexuses that wrap around the celiac trunk, superior mesenteric a. and inferior mesenteric a. While functionally important and distinct, you will have to sacrifice them for the sake of demonstrating the vasculature adequately.

To expose the branches of the superior mesenteric vessels, use forceps and fingers to peel back the anterior layer of the mesentery (keep the vessels intact!). Note that the names of the branches of the superior mesenteric a. correspond to the structures the vessel supplies.

Identify the following branches to the small intestine:

Jejunal and ileal aa. - 15 to 18 branches of the superior mesenteric a.; supply the jejunum and ileum; pass between the two layers of the mesentery
Arterial arcades - arteries form arches within the mesentery that eventually form straight terminal branches (vasa recta) to the jejunum and ileum)

Peel back the mesentery that is attached to a segment of the proximal jejunum and distal ileum to identify the arterial arcades and the vasa recta. Note that, compared the the jejunum, the ileum has more arterial arcades that are stacked upon one another; consequently, the vasa rectae of the ileum are shorter than those of the jejunum.

FIGURE 5.18 Jejunal and ileal arteries

Use blunt dissection through the peritoneum or mesocolon and remove excess fat to continue revealing the branches of the superior mesenteric a. to identify the following branches to the proximal portion of the colon:

*note that you will likely need to mobilize the descending and ascending colon to visualize these branches (see "Before you Begin" section, step #2 for details.)

Ileocolic a. - terminal branch of the superior mesenteric a.; supplies the cecum, appendix, and distal portion of the ileum; anastomoses with ileal branches and the right colic a.
- Appendicular a. - terminal branch of the ileocolic a.; supplies the appendix
Right colic a. - origin is variable, in that in may arise from the superior mesenteric a., the ileocolic a., or the middle colic a.; supplies the ascending colon; anastomoses with the ileocolic and middle colic aa.
Middle colic a. - supplies the right half of the transverse colon; anastomoses with the right colic a. and the left colic a. (branch of the inferior mesenteric a.)
Marginal a. - Artery of Drummond. Anastomosis between the SMA and IMA. Connects the middle colic and left colic aa..
Superior mesenteric lymph nodes - look for lymph nodes in the mesentery; names according to the organ that they drain

Note that the tributaries of the superior mesenteric v. drain the same regions supplied by the superior mesenteric a.

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