Anatomic Consideration

The large intestine consists of the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum. The ascending colon, descending colon, and rectum are considered extraperitoneal organs because the ascending and descending colon lie in the anterior pararenal space and are covered by a single layer of the posterior peritoneum, while the rectum is surrounded by extraperitoneal perirectal fat in the pelvis. The transverse colon and sigmoid colon are suspended in the peritoneal cavity by the mesocolon, which is formed by two layers of the peritoneal linings, and the cecum is attached to the ileocolic mesentery in the right iliac fossa.12

The arterial supply to the cecum, ascending colon, and transverse colon derives from the superior mesenteric artery (SMA), whereas the blood supply to the descending colon, sigmoid colon, and upper rectum is from the inferior mesenteric artery (IMA) and that to the lower rectum from the internal iliac arteries. Similarly, the venous drainage of the cecum, ascending co

Fig. 6—11. Splenic lymphoma with involvement of nodes (arrows) along splenic artery in splenorenal ligament. These nodes drain into the celiac node and left inferior phrenic node (N).

Fig. 6—11. Splenic lymphoma with involvement of nodes (arrows) along splenic artery in splenorenal ligament. These nodes drain into the celiac node and left inferior phrenic node (N).

Left Inferior Phrenic Vein

lon, and transverse colon drains into the superior mesenteric vein (SMV), and that of the descending colon, sigmoid colon, and rectum drains into the inferior mes-enteric vein (IMV). In general, the artery and vein supplying and draining each segment of the colon accompany each other in the mesocolon, with the marginal artery and vein forming the arcade along the mesocolic side of the colonic wall before giving off the branches of vasa recta to penetrate into the colonic wall. These vessels in the mesocolon and the marginal vessels along the mesocolic side of the colonic wall form the anatomic landmarks to localize the mesocolon.

Cecum, Ascending Colon, Ileocolic Mesentery, and Descending Colon and Mesocolon

Along the mesocolic margin of the ascending colon and the descending colon lie the marginal vessels. As these vessels course in the cephalocaudal direction, the vessels can be identified as "dots" of vessels medial to the ascending and descending colon on axial images, with branches of the vasa recta supplying the colon (Fig. 612). These vessels can be better defined on CT images scanned at 3- to 5-mm scanning collimation. Medial to

Fig. 6—12. CT anatomy of the ascending mesocolon.

(a) The ileocolic vein (arrow), a branch of the superior mesenteric vein (SMV) (arrowhead), and the marginal vessels (open arrow) of the ascending colon (AC) form the plane of the ascending mesocolon.

(b) The right colic vein (open arrows) is a branch of the ileocolic vein (arrow), communicating with the marginal vessels.

(c) The plane of the ileocolic vein (arrow) is anterior to the right gonadal vein (open arrow) and the right ureter (arrowhead). Note thrombosis of the gonadal vein.

(d) The distal branches of the ileocolic vessels (arrow) terminate in the cecum, appendix, and the terminal ileum.

Fig. 6—12. CT anatomy of the ascending mesocolon.

(a) The ileocolic vein (arrow), a branch of the superior mesenteric vein (SMV) (arrowhead), and the marginal vessels (open arrow) of the ascending colon (AC) form the plane of the ascending mesocolon.

(b) The right colic vein (open arrows) is a branch of the ileocolic vein (arrow), communicating with the marginal vessels.

(c) The plane of the ileocolic vein (arrow) is anterior to the right gonadal vein (open arrow) and the right ureter (arrowhead). Note thrombosis of the gonadal vein.

(d) The distal branches of the ileocolic vessels (arrow) terminate in the cecum, appendix, and the terminal ileum.

the marginal vessels, we should identify the ileocolic vessels for the ascending mesocolon and mesocecum, and the inferior mesenteric vessels for the descending mesocolon.1315

The ileocolic vessels are the branches of the superior mesenteric vessels located near the base of the mesentery. They run in the oblique direction from near the midline, anterior to the third portion of the duodenum, toward the cecum in the right iliac fossa. As they are located near the base of the mesentery, their course is constant and not flopping in the peritoneal cavity like other branches of the SMA or SMV supplying the ileum or the jejunum. After crossing the third portion of the duodenum, they can be identified anterior to the right ureter and the gonadal vessels and can be traced toward the cecum. The plane between the marginal vessels of the ascending colon and the ileocolic vessels defines the plane of the ascending mesocolon.1314

The inferior mesenteric vessels run in the cephalo-caudal direction in the anterior pararenal space slightly anterior and left to the aorta. The vessels can be identified anterior to the left ureter and the gonadal vessels and the left renal vein.15 The IMV drains into the splenic vein, the confluence of the SMV and splenic vein, or directly into the SMV. Before it enters that vein, it forms the left duodenomesocolic fold at the left paraduodenal space, which is where the duodenum exits the retro-peritoneum and enters into the peritoneal cavity. Therefore, the anatomic landmarks for the left paraduodenal space include the inferior mesenteric vein forming the left and superior boundaries, the superior mesenteric artery and vein forming the right boundary, with the duodenum at the center. The plane between the inferior mesenteric vein and the marginal vessels of the descending colon forms the plane of the descending mesocolon.

The Transverse Colon and Mesocolon

The transverse colon is suspended in the peritoneal cavity by the transverse mesocolon. The transverse mesocolon is formed by two posterior peritoneal layers, with the root of the transverse mesocolon traversing across the second portion of the duodenum and the head of the pancreas and along the inferior border of the body and the tail of the pancreas. Within the transverse me-socolon are branches of the middle colic artery, which most commonly arises from the anterior surface of the proximal segment of the SMA, and branches of the middle colic veins, which drain into the SMV or IMV.

The marginal vessels along the mesocolic side of the colon can be identified as longer segments of the vessels as compared to dots of marginal vessels of the ascending or descending colon. This is because the marginal vessels of the transverse mesocolon run in the transverse direction. The position of the vessels varies depending upon the position of the transverse colon. If the transverse colon hangs down toward the umbilicus as most of the midportion of the transverse colon does, the marginal vessels can be seen cephalad to the wall of the colon. However, if the transverse colon suspends or floats up toward the diaphragm as most splenic flexure does, the marginal vessels will be caudal to the colonic wall.

The transverse mesocolon can be traced toward the root of the mesocolon by following the marginal vessels to the middle colic vein draining into the SMV. The typical anatomy will show the marginal vessels from the right and left transverse colon forming the middle colic vein and joining the right gastroepiploic vein to become the gastrocolic trunk draining into the SMV anterior to the head of the pancreas.5,14 However, there are many anatomic variations that demonstrate several isolated branches of the middle colic vessels in the transverse mesocolon draining into the SMV or IMV, but these vessels are all coursing toward the head and the body of the pancreas into which the root of the transverse me-socolon inserts.

The Sigmoid Colon and Mesocolon

The sigmoid colon is another segment of the colon that is suspended in the peritoneal cavity. The sigmoid me-socolon is formed by the two peritoneal layers attached to the third sacral segment and extending in the cephalad direction toward the left external iliac vessel where it joins another leaf of the sigmoid mesocolon, which extends from the descending mesocolon, suspending the proximal sigmoid colon. These two leaves of sigmoid mesocolon ascend toward the anterior surface of the abdominal aorta where the IMA arises. The length and arrangement of the mesocolon vary depending upon the length and redundancy of the sigmoid colon.

The anatomic landmarks of the sigmoid mesocolon are the superior hemorrhoidal vessels, the marginal vessels, the sigmoidal vessels, and the IMA and IMV.13,14 The superior hemorrhoidal vessels are plexuses of vessels on both sides of the rectum ascending to form the inferior mesenteric vessels. The IMV is located anterior to the left external iliac vessels, the left ureter, and the left gonadal vein. The marginal vessels form the arcade along the mesocolic side of the colon with multiple branches of the sigmoidal vessels communicating the marginal vessels to the IMV. However, because of anatomic variations in the length and redundancy of the sigmoid colon, the positions of the marginal vessels and the sigmoidal vessels are not constant. On the other hand, the anatomy of the IMA and IMV is more con stant. The origin of the IMA almost always arises from the anterior wall of the abdominal aorta at approximately the level of L3, then courses to the left and gives off a branch, the left ascending colic artery, that ascends along the IMV toward the left paraduodenal space. The other branches form the sigmoidal arteries and the superior hemorrhoidal artery.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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