Abdomen Lab 5A

Lab 5A - IMA, Hindgut, and Posterior Body Wall

Lab Overview

Dissect the branches of the IMA and the organs of the hindgut and posterior body wall.
Appreciate the clinical importance of the marginal artery as an anastomosis between the SMA and IMA
Find the hepatic portal vein and trace its tributaries from the digestive tract.
Have a detailed look at the diaphragm muscle and the muscles and nerves of the posterior body wall

Short List of Structures - 5A

Viscera of the Hindgut

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 the following structures along the hindgut:

Haustra - The small pouches that form along the large intestine.
Taenia Coli mm. - 3 bands of longitudinal muscle, also characteristic of the large intestine.
Epiploic (Omental) appendices - the blobs of adipose that ornament the large intestine.

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 anorectal a. - supplies the proximal portion of the rectum; usually a large vessel (the terminal branch of the inferior mesenteric a.)

- Marginal a. - 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.

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.

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 7.7 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

Organs of the posterior body wall

Overview

Using blunt dissection and scraping, remove the remaining peritoneum and extraperitoneal fat to identify the following:

Kidneys - lateral to vertebrae T12-L3; use scissors and your fingers to remove the parietal peritoneum and perirenal fat from over and around the kidneys respectively
Ureters - use forceps to free the ureters from the parietal peritoneum and fat; extend from the renal pelvis to the urinary bladder, you will not be able to follow the ureters to the bladder at this time.
Adrenal (suprarenal) glands - positioned on the superior pole of each kidney; pale in color and somewhat triangular shaped
Abdominal aorta - use forceps to dissect through the parietal peritoneum to reveal the aorta (and neighboring vena cava and branches); the abdominal aorta courses longitudinally to the left of the midsagittal line, on the anterior surface of the vertebral bodies
Inferior vena cava - courses longitudinally to the right of the midsagittal line, also on the anterior surface of the vertebral bodies
Iliac crest - palpate its internal surface
Diaphragm - move the liver and stomach to different positions to see the diaphragm

Appreciate the distribution of the lymphatics of the posterior abdominal wall. Lymph nodes may be difficult to find. Recall that lymphatics usually follow the course of venous drainage and thus, you should probably be able to intuit where the lymph in these lymph nodes came from.

FIGURE 8.1 Overview of the posterior abdominal wall

Kidneys and Adrenals in Situ

Kidneys - left and right; observe that the right kidney is slightly lower than the left kidney due to the presence of the liver in the upper right quadrant of the abdomen (we’ll cut open the kidneys in the next block)
Perirenal fat - each kidney is embedded in a substantial layer of fat; with your fingers, shell out the kidneys from the perirenal fat
Adrenal glands - each adrenal gland is separated from the superior pole of each kidney by a layer of connective tissue; use forceps or your fingers to separate the adrenal glands from the kidneys, but leave the adrenal blood vessels intact!
Ureters - cross the psoas major m. and course deep to the gonadal vessels en route to the true pelvis (urinary bladder); use forceps to delineate both ureters, but do not cut the ureters!

HELPFUL HINTS

It may be difficult to differentiate the adrenal glands from the perirenal fat pad so look carefully.

Identify the following:

Adrenal (suprarenal) vessels: you should know that there are 3 suprarenal aa., but you will likely not be able to find all 3.
- Superior suprarenal a. - arises from the inferior phrenic a.
- Middle suprarenal a. - arises from the aorta
- Inferior suprarenal a. - arises from the renal a.
- Suprarenal v. - usually drains to the renal v.; only one vein compared to the three arteries; the veins are asymmetric

Abdominal Aorta

Use forceps to reveal and identify the following:

Abdominal aorta - courses longitudinally to the left of the midsagittal line, on the anterior surface of the vertebral bodies; the bifurcation of the abdominal aorta is at the L4 vertebra
- Celiac trunk - T12 vertebral level; unpaired artery; supplies the foregut
- Superior mesenteric a. - L1 vertebral level; unpaired artery; supplies the midgut
- Inferior mesenteric a. - L3 vertebral level; unpaired artery; supplies the hindgut
- Middle suprarenal a. - paired arteries from the abdominal aorta, near the level of the celiac trunk; supplies the suprarenal glands
- Renal aa. - L1 vertebral level; paired arteries that originate between the superior and inferior mesenteric aa.; the left renal a. is shorter than the right renal a.; multiple arteries are frequently encountered (expect variation); give rise to the inferior suprarenal a.
- Gonadal (testicular/ovarian) aa. -(these are very small!) L2 vertebral level; paired arteries that arise inferior to the renal aa. and superior to the inferior mesenteric a.; cross anterior to the ureters and external iliac vessels
- Lumbar aa. - four paired arteries that course laterally across the sides of vertebrae L1-L4; segmental blood supply of the lumbar region
- Median sacral a. - unpaired artery that arises from the abdominal aorta at its bifurcation into the common iliac aa.
- Common iliac aa. - paired arteries from the distal end of the aorta at the L4 vertebral level; each bifurcates into the external and internal iliac aa.

Inferior Vena Cava

Use forceps to reveal and identify the following branches of the inferior vena cava:

Right renal v. - shorter than the left renal v.
- Right suprarenal v. - drains the right adrenal gland; only one suprarenal v. drains each adrenal gland compared to the three adrenal aa. that supply each gland
Left renal v. - crosses anterior to the aorta and just inferior and posterior to the superior mesenteric a.
- Left suprarenal v. - one vein also drains the left adrenal gland
- Left gonadal (testicular/ovarian) v. - drains the left gonad into the inferior border of the left renal v.
Right gonadal v. - drains the right gonad into the inferior vena cava
Lumbar vv.
Middle sacral v.
Common iliac vv.

Clinical Correlation: Abdominal Aortic Aneurysm (triple-A)

Rupture of an aneurysm (localized enlargement) of the abdominal aorta causes severe pain in the abdomen or back. If unrecognized, a ruptured aneurysm has a mortality of nearly 90% because of heavy blood loss. Surgeons can repair an aneurysm by opening it, inserting a prosthetic graft (such as one made of Dacron) and sewing the wall of the aneurysmal aorta over the graft to protect it. Aneurysms may also be treated by endovascular catheterization procedures.

Figure 8.13 Abdominal Aortic Aneurysm

FIGURE 8.14 Abdominal Aortic Aneurysm

Muscles and Associated Structures of the Posterior Abdominal Wall

Before You Begin...

In this section, we will be looking at the muscles of the posterior abdominal wall, including the diaphragm, as well as their associated structures.

If you haven’t done so already, you should cut out part of the liver to free up some space and to better visualize the adjacent viscera. Preserve the part of the liver attached to the porta hepatis and the gallbladder.

Posterior Abdominal Wall Muscles

The diaphragm is a dome shaped muscle that separates the thoracic cavity from the abdominal cavity. If not already cut, cut the peritoneal reflections from the diaphragm muscle to the liver. To cut those reflections, use your fingers to locate them and scissors to cut them. This procedure will mobilize (free) the liver so that it may be moved away from the diaphragm.

Use forceps and your fingers to remove the parietal peritoneum and fat from both sides of the posterior abdominal wall to identify the following:

Psoas major m. - long muscle attached to the bodies and transverse processes of L1-L5 vertebrae
- Overlaps the medial portion of the quadratus lumborum m. and crosses anterior to the sacroiliac joint; passes inferior to the inguinal ligament to insert on the lesser trochanter of the femur
- The lumbar plexus of nerves arises from the lumbar transverse processes, exits the intervertebral foramina, and emerges between the psoas major muscle fibers.
Psoas minor m. - this thin muscle courses anteriorly on and medial to the psoas major m.; inserts distally on the pecten pubis (the psoas minor m. is sometimes absent)
Quadratus lumborum m. - quadrilateral-shaped muscle that is lateral and deep to the psoas major m.
- Iliolumbar ligament - courses from the ilium to the lumbar vertebrae along the attachment of the quadratus lumborum m.
Iliacus m. - located in the iliac fossa of the pelvis; attaches to the iliac fossa and courses distally to the lesser trochanter of the femur (may be covered by a considerable amount of fat)

Nerves of the Posterior Abdominal Wall

Use forceps to remove residual fat from the posterior abdominal wall to dissect the following nerves. Dissect all of the following nerves long their entire course, then return to the nerves in the listed sequence to identify them by name. Again, expect variation.

Subcostal n. (T12 spinal cord segment) - scrape/dissect through the peritoneum and fascia inferior to rib 12; emerges inferior to the lateral arcuate ligament and descends on the anterior surface of the quadratus lumborum m. before passing through the transversus abdominus m. laterally
Lumbar plexus - emerges laterally from the muscle belly of the psoas major m.
Femoral n. (L2-L4 spinal cord segments) - located deep and lateral to, and often overlapped by, the psoas major m. near its union with the iliacus m.; courses deep to the inguinal ligament en route to the thigh
Obturator n. (L2-L4 spinal cord segments) - probe the fat deep and medial to the inferior portion of the psoas major m. near the sacral promontory to reveal the obturator n.

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