Small Bowel and Large Bowel Resection and Anastomosis

Regional Vascular Anatomy of the Small Bowel and Large Bowel

The celiac artery and superior mesenteric artery (SMA) are the two principal visceral vessels supplying the liver, the biliary tree, the spleen and pancreas, the omentum, and the remainder of the gastrointestinal tract, except for the esophagus, the rectum, and the colon distally to the midtransverse colon ( Fig. 17.1 ). The inferior mesenteric artery (IMA) supplies the splenic flexure of the colon, the descending colon, the sigmoid colon, and the rectum ( Fig. 17.2 ). The rectum also receives blood supply from the internal iliac arteries. The celiac axis is the largest branch of the aorta at the level of the 12th thoracic vertebra and arises at a right angle from the anterior aspect of the abdominal aorta. The SMA arises only 1 to 2 cm below the origin of the celiac artery. The celiac artery gives rise to the common hepatic artery, the splenic artery, and the left gastric artery ( Fig. 17.3 ). The common hepatic artery gives rise to the gastroduodenal artery and the proper hepatic artery, which branches into right, middle, and left hepatic arteries. Sometimes the right hepatic artery is a branch of the SMA. The splenic artery runs along the superior aspect of the pancreas and gives rise to the dorsal pancreatic artery, which supplies the body of the pancreas. The dorsal pancreatic artery anastomoses with the superior mesenteric circulation via the pancreaticoduodenal artery, a major communication between the celiac and the superior mesenteric circulation. Other branches of the splenic artery penetrate though the body of the pancreas, and, distally, short gastric arteries (the number varies from 2 to 15) arise from the splenic artery. The gastroepiploic artery usually arises from the splenic artery late just before the latter enters the splenic hilum. The left gastroepiploic artery anastomoses with the right gastroepiploic artery, a branch of the gastroduodenal artery.

The SMA provides the entire blood supply to the jejunum and ileum. It arises from the aorta at the level of the first lumbar vertebra and measures approximately 1 cm in diameter. It lies behind the neck of the pancreas; it then passes over the uncinate process, anterior to the third part of the duodenum, before entering the root of the small bowel mesentery. Constant branches of the SMA include the inferior pancreaticoduodenal, the middle colic, the right colic, the ileocolic, and the intestinal arteries. The common inferior pancreaticoduodenal artery is the first branch given off and supplies the retroperitoneal duodenum. The middle colic artery is the second branch of the SMA ( Fig. 17.4 ). This vessel is an important landmark in the management of superior mesenteric arterial occlusive problems. The remaining small intestine is supplied by the jejunoileal arteries. They arise from the left side of the SMA after it enters the mesentery. These consist of 12 to 20 branches. These branches extend into the mesentery, where they form arcades. From these arcades, the vasa recta arise and pass to the mesenteric border of the bowel without anastomosing with one another. The avascular spaces are called the windows of Deaver. The vasa recta continue to form the subserosal plexus. These are sufficient to supply 6 to 8 cm of small intestine if the adjacent vasa recta have been occluded or ligated. The SMA also gives rise to the middle colic, right colic, and ileocolic arteries, arising from the right side of the SMA, which supply the cecum, ascending colon, and transverse colon, respectively. The space of Treves is an avascular space between the SMA and ileocolic artery, which may result in an inconsistent blood supply to the terminal ileum. Venous drainage of the small intestine is composed of direct tributaries that correspond to the branches of the SMA to form the superior mesenteric vein. This joins the splenic vein to form the portal vein.

The IMA arises from the left aspect of the aorta, usually at the level of L3, averaging 3 to 4 cm in length before branching ( Fig. 17.5 ). The IMA is 5 mm in diameter; it gives rise to the left colic branch, three or four sigmoidal branches, and the superior rectal (also known as superior hemorrhoidal) artery.

Important collateral pathways of the mesenteric circulation include the communicating network between the SMA and IMA. One important communication between the SMA and the IMA is the arc of Riolan, also known as the meandering mesenteric artery (see Fig. 17.4 ). This vessel connects the ascending branch of the left colic artery with a branch from the SMA just proximal to the origin of the middle colic artery. The meandering mesenteric artery is not to be confused with the marginal artery of Drummond, which is located more peripherally in the mesentery. The marginal artery of Drummond connects the left branch of the middle colic artery (from the SMA) with the ascending branch of the left colic artery (from the IMA). This is a series of arcades along the mesenteric border of the entire colon beginning at the ileocolic artery and running to the sigmoid arteries; thus, it connects the vasculature of the SMA and IMA. The marginal artery gives rise to the vasa recta, which enter the colon wall and form intramural anastomoses. These intramural anastomoses are not as extensive as in the small bowel, and they can supply only 2 to 3 cm of colon compared with the 6 to 8 cm in the small intestine. In approximately 5% of patients, the marginal artery is deficient in one or more of the following areas: the last 6 to 8 cm of the terminal ileum, between the ileocolic and right colic arteries, between the middle and left colic arteries (termed the avascular area of Riolan), and between the superior rectal and last sigmoid arteries. The surgeon should be aware of these deficiencies because they may lead to compromise of intestinal integrity. Identification of the major arteries before resection of a portion of the colon can help avoid the unfavorable situation of unrecognized vascular insufficiency.

The middle and inferior rectal (hemorrhoidal) branches of the internal iliac arteries anastomose with the superior rectal (hemorrhoidal) branch of the IMA ( Fig. 17.6 ). This circuit provides an important collateral circulation to the distal colon and rectum. The collateral mesenteric communications are important in the setting of occlusive disease. An occlusion of the IMA is compensated for by the SMA and the iliac arteries. An SMA occlusion is compensated for by the collateral circulation from the celiac artery via the pancreaticoduodenal arcade and the meandering mesenteric artery and the marginal artery of Drummond from the IMA.

The venous drainage of the colon follows the course of the arterial system and is composed of the superior mesenteric vein and the inferior mesenteric vein (IMV) (see Figs. 17.4 and Fig. 17.5 ). The IMV empties into the splenic vein, which then joins the superior mesenteric vein to form the portal vein. The portal vein ascends to the liver behind the bile duct and hepatic artery at the free edge of the lesser omentum. At the porta hepatis, the portal vein divides into the left and right branches, which eventually empty into the hepatic sinusoids. The portal system does have several anastomoses with the systemic venous system, which can compensate for obstructed portal venous return.

The lymphatic system of the colon also follows the arterial supply previously described. Intramural lymphatic channels drain into extramural lymph vessels, which empty into the colonic lymph nodes.

Anatomy of the Small Intestine

The small intestine extends from the pylorus to the cecum and consists of the duodenum, jejunum, and ileum. This chapter focuses on the last two of these components of the small intestine.

The absorption of water, electrolytes, and nutrients is a major function of the small intestine ( Table 17.1 ). In addition, the small intestine contributes to the immune system and endocrine system. Together, the jejunum and ileum measure approximately 6 to 7 m in length, with the jejunum comprising the proximal 40% and the ileum accounting for the distal 60%. Despite the fact that there is no sharp morphologic distinction between the jejunum and ileum, there are certain characteristics that help distinguish between the two:

• 1.

  The jejunum has a thicker wall owing to the fact that the circular folds, or plicae circulares, are larger and well developed in the proximal end of the small bowel; these folds are small in the superior part of the ileum and absent in the terminal ileum.

• 2.

  The jejunum is of greater diameter.

• 3.

  The jejunal mesentery contains less fat than that of the ileum, and the arterial arcades are easier to visualize than in the ileum.

• 4.

  The jejunum has fewer arterial arcades with longer vasa recti compared with the ileum.

Anatomy of the Large Intestine

The large intestine is approximately 1.5 m in length, beginning at the cecum and ending with the anus in the perineum.

The large bowel is characterized by full-thickness infoldings of the bowel wall, called haustra ( Fig. 17.7 ). These infoldings correspond to transverse folds in the bowel lumen called plicae semilunares. The large bowel has three thickened bands of longitudinal muscle that run its length from the appendix to the rectum called taeniae coli (taenia omentalis, taenia libera, and taenia mesocolica). Contraction of the taeniae causes the haustra to become more prominent. The large bowel has small pouches of peritoneum filled with fat called appendices epiploicae that are most prominent on the descending and sigmoid colon.

The cecum is the most proximal portion of the large intestine ( Fig. 17.8 ). It is a blind pouch that is 5 to 7 cm in length and projects caudal to the ileocecal junction in the right iliac fossa of the right lower quadrant. Usually, the cecum is entirely enveloped by peritoneum but does not have a mesentery. It is the widest part of the large intestine but also has the thinnest wall; thus, it is at highest risk for perforation in cases of large bowel obstruction. The appendix arises 2 to 3 cm inferior to the ileocecal junction. The appendix has its own short mesentery called the mesoappendix, which connects it to the inferior part of the mesentery of the ileum. The appendiceal artery is a branch off the ileocolic artery and runs in the mesoappendix. The ileum leads into the cecum and is separated by the ileocecal valve. Its function is to limit the reflux of colonic contents into the ileum. Approximately 2 L of fluid enters the colon daily through the ileocecal valve.

The ascending colon measures approximately 15 to 20 cm long and runs from the ileocecal valve to the hepatic flexure. It ascends as a retroperitoneal structure covered by peritoneum only on its anterior and lateral surfaces. It lies anterior to the quadratus lumborum, psoas, and transversus abdominis muscles; inferior pole of the right kidney; and descending portion of the duodenum. Lateral to the ascending colon is the white line of Toldt, which represents the fusion of the colonic mesentery with the parietal peritoneum. There may be congenital adhesions between the anterior aspect of the ascending colon and the right abdominal wall (Jackson membrane). The hepatic flexure may have several attachments to the liver and gallbladder. The ascending colon and hepatic flexure are supplied by the ileocolic and right colic arteries, and the venous drainage is through the ileocolic and right colic veins, which drain into the superior mesenteric vein. The lymphatic drainage of the ascending colon is via the paracolic and epicolic lymph nodes, which empty into the superior mesenteric lymph nodes.

The transverse colon is the portion of large bowel that lies between the hepatic and splenic flexures. It is the longest portion of the large bowel and usually measures 30 to 60 cm in length. Occasionally, a redundant transverse colon will reach into the pelvis. Unlike the ascending and descending colon, the transverse colon has its own mesentery, which is longest in the center, and is therefore considered an intraperitoneal structure. The root of the mesentery of the transverse colon covers the descending part of the duodenum, the pancreas, and a portion of the left kidney. At the hepatic and splenic flexures, the mesentery is very short and may place the transverse colon in contact with the duodenum and the head of the pancreas, which may be injured during mobilization of the hepatic flexure. The splenic flexure is connected to the diaphragm by the phrenocolic ligament and to the spleen by the lienocolic ligament. The transverse colon is attached to the greater curvature of the stomach by the cephalic portion of the greater omentum, which is also referred to as the gastrocolic ligament.

The descending colon is approximately 20 to 25 cm in length; it begins at the splenic flexure and ends at the pelvic brim with the start of the sigmoid colon, which is demarcated by its intraperitoneal mesentery. The proximal part of the descending colon is attached to the peritoneum overlying the left kidney by the phrenicocolic ligament. Similar to the ascending colon, the descending colon is a retroperitoneal structure that is covered by peritoneum only on its anterior and lateral surfaces. Lateral to the descending colon is the white line of Toldt, which demarcates the correct plane to enter the retroperitoneal space to mobilize the descending colon.

The sigmoid colon begins at the pelvic brim, curves inferiorly along the left pelvic side wall over the bifurcation of the left common iliac artery, and finally runs in the midline. It is usually 45 cm long, but variations in length are very common. The taenia coli are wider than in the rest of the colon. The sigmoid mesocolon begins at the pelvic brim and becomes longer to the midpoint of the sigmoid colon and then decreases in size as it travels inferiorly. Thus the sigmoid colon is significantly longer than its mesentery.

The sigmoid colon becomes the rectum in front of the sacrum at the level at which the sigmoid colon mesentery ends and the appendices epiploicae disappear. The taeniae coli become more diffuse around the rectum and form a complete outer layer of longitudinal muscle. The longitudinal muscles merge with the perineal body and with muscles of the external sphincter. The rectum is 15 to 18 cm in length. Proximally it is of similar diameter as the sigmoid colon, but more distally it widens to the infraperitoneal ampulla, which is capable of significant distention. The upper third of the rectum is covered by peritoneum anteriorly and laterally, the middle third is covered only anteriorly, and the lower third is without peritoneal covering. The peritoneum is tented forward to the upper vagina to form the rectouterine pouch of Douglas. The rectum passes inferiorly through a ligamentous gate formed by the left and right ligamentum sacrouterinum and travels posterior to the vagina. This surgical plane between the posterior vagina and the rectum is an important landmark during surgical procedures for gynecologic cancer because here the retroperitoneum can be entered below the peritoneal reflection and the perirectal fat and the anterior rectum can be developed in preparation for the resection.

Surgical Techniques