Gross Anatomy: Intestines & Portal System Notes 2024 PDF

Gross Anatomy: Intestine & Portal System Page 15 of 29 Dr. Dennis Goebel 1. General characteristic features of the large intestine include: a. Large diameter (ranging from ~4-6 cm) b. Thin walls c. Taeniae coli (also...

Gross Anatomy: Intestine & Portal System Page 15 of 29 Dr. Dennis Goebel 1. General characteristic features of the large intestine include: a. Large diameter (ranging from ~4-6 cm) b. Thin walls c. Taeniae coli (also called teniae coli): Identified as 3 equally spaced (~ 12-18 mm apart), condensed longitudinal smooth muscle bands. These are characteristic to the cecum, ascending-, transverse-, descending- and sigmoid- colon, and are most easily identified on the outer surface of cecum, the ascending colon and the transverse colon (See Figure 10). i. Taeniae coli are absent on the appendix, the rectum and the anal canal, as each display a continuous outer longitudinal smooth muscle layer (See Figure 10). d. Haustra: These are sacculations (out-pocketing’s) of the circular smooth muscle layer of the colon which are located between the taenia coli. These segmented structures are further separated by a condensation of equally spaced circular smooth muscle bands that form concentric constrictions, which regularly space the haustra on the external surface of the colon (See Figure 10). Internally, theses constrictions can be visualized via endoscopy and are called semilunar folds (not shown). e. Epiploicae (epiploic or omental appendices, also called appendages): Consists of small fat-filled pouches that are invested by peritoneum. These are suspended off of the external surfaces of the large intestine (See Figure 10). B. Divisions of the large intestine 1. Cecum: The sac-like cecum (Latin- “blind” ended) is the first part of the large intestine. The cecum is completely encircled by visceral peritoneum (but usually lacks a mesentery), and is thus defined as being a peritoneal structure (See Figure 10). a. It is located in the right lower quadrant and lies in the iliac fossa (See Figure 1). b. It is a broad blind-ended pouch measuring 5-7 cm in length and width, and is continuous with the ascending colon. Its superior boundary is defined by a horizontal plane positioned just above the entrance of the ileum (See dashed line in Figure 10). c. The ileum enters the cecum obliquely on its medial side, and forms the ileocecal valve. i. The ileocecal valve has limited sphincter activity due to poorly developed circular muscles. However, reflux of fecal materials from the cecum into the ileum usually does not occur. This is due impart by the large luminal volume of the cecum/large intestine, as compared to the ileum. Under normal conditions, this reduces back-pressure and reflux into the ileum. Gross Anatomy: Intestine & Portal System Page 16 of 29 Dr. Dennis Goebel 2. Appendix: The (vermiform) appendix is a diverticulum (derived from the colon) that extends off of the blind-ended surface of the cecum (See Figure 10). i. The appendix is usually a peritoneal structure that is suspended by a mesentery (called the meso-appendix). The meso-appendix is anchored mostly to the terminal end of the ileum (See Figures 10 &11). ii. The appendix is usually positioned posterior to the cecum within the retrocecal recess (See Figure 11). iii. The appendix muscular layers consist of an inner circular and an outer longitudinal-smooth muscle layer. Note that on the appendix, the outer longitudinal layer is continuous, and is the result of the merger of the three taenia coli, from the cecum (Figure 11). Look for this in lab. Figure 11 3. Ascending colon: The ascending colon is usually a retroperitoneal structure that is associated with the right posterior wall of the abdominal cavity. a. It typically has a narrower diameter than the cecum. b. It extends from the cecum to just inferior to the right lobe of the liver, where it transitions into the transverse colon at a sharp medial bend to the left. This sharp bend is called the right colic (or hepatic) flexure (See Figures 10 and 12). 4. Transverse colon: The transverse colon is a peritoneal structure that is suspended off of the posterior abdominal wall by the transverse mesocolon. Gross Anatomy: Intestine & Portal System Page 17 of 29 Dr. Dennis Goebel a. It is approximately 45 cm in length, and traverses from the right to the left side of abdomen beginning at the right colic (hepatic) flexure and ending at the left colic (splenic) flexure (See Figure 10) where, it transitions into the descending colon. b. The three taenia coli on the located on the transverse colon are clearly displayed (look for these in lab) and are defied as: i. Mesocolic: The taenia coli associated with the attachment of the transverse mesocolon (See Figure 12). ii. Omental: The taenia coli associated with the attachment of the greater omentum (See Figure 12). iii. Free: The taenia that lacks a mesentery attachment. The free taenia coli is located on the posterior surface of the transverse colon (See Figure 12). Figure 12 5. Descending colon: The descending colon is usually a retroperitoneal structure that begins at the left colic (splenic) flexure and descends on the left side of the posterior abdominal wall into the left iliac fossa, where it then transitions to become the sigmoid colon (See Figures 10 &12). Gross Anatomy: Intestine & Portal System Page 18 of 29 Dr. Dennis Goebel 6. Sigmoid colon: The sigmoid colon is roughly 40 cm in length. It is a peritoneal structure that is suspended off of the posterior abdominal wall (in the left iliac fossa region) by an extended mesentery called the sigmoid mesocolon. This provides the sigmoid colon with a considerable amount of freedom of movement (Figure 12). a. The sigmoid colon forms an S-shaped loop and is located between the descending colon and the rectum (Figure 12). b. Omental appendages of the sigmoid colon are usually very long and fat filled. c. The sigmoid colon extends from the pelvic brim to ~3rd vertebral segment of the sacrum (Figure 12). d. The transition between the sigmoid colon and the rectum is hallmarked by: i. The longitudinal fibers of the three taenia coli of the sigmoid colon fanning out to form a continuous outer smooth muscle layer on the distal rectum. ii. The large intestine going from peritoneal (sigmoid colon) to retroperitoneal (rectum). B. Arteries of the Colon 1. Superior mesenteric artery (SMA): The SMA supplies the proximal half of the colon (from the cecum to the left colic (splenic) flexure of the transverse colon). a. The SMA usually gives rise to three primary branches that come off of the right side of the artery. They are the middle colic A., right colic A. and the ileocolic A. (See Figure 13 on the next page). Note, variability of the origins and branching patterns of these arteries is common, and that they are named based upon the region that they supply, rather where their origin arises. The descriptions that follow represent the most common branching pattern displayed by the SMA. i. Middle colic artery: The middle colic artery is usually the second branch coming off on the right side of the SMA. a. It provides the main arterial supply to the transverse colon. b. As it approaches the colon, it bifurcates into right and left branches, whereby the right branch will anastomose with ascending branches of the right colic artery and the left branch will anastomose with ascending branches of the left colic artery (See Figure 13). ii. Right colic artery: The rt colic artery is usually the third branch arising off of the SMA on the right side. a. It supplies most of the ascending colon., and usually bifurcates into an ascending and descending branch (See Figure 13). iii Ileocolic artery: This is usually the 4th branch coming off of the right side of the SMA (See Figure 13 and 14). Gross Anatomy: Intestine & Portal System Page 19 of 29 Dr. Dennis Goebel Figure 13 a. It supplies the distal part of the ileum, cecum, the appendix and the proximal region of the ascending colon. b. The ileocolic A. gives rise to the following 5 branches (See Figure 14). These include the following. 1. Ascending colic artery, forms an anastomosis with the descending branch of the right colic artery. 2. Anterior cecal artery supplies the anterior portion of the cecum. 3. Posterior cecal artery supplies the posterior cecum. Gross Anatomy: Intestine & Portal System Page 20 of 29 Dr. Dennis Goebel 4. Appendicular artery usually passes posterior to the ileum and travels within the mesoappendix to supply the appendix. 5. Ileal branches supplying the most distal end of the ileum. Figure 14 2. The Inferior mesenteric artery (IMA) is the third unpaired visceral artery of the abdominal aorta (See Figure 13). It arises at the level of LV3 (roughly at the level of the inferior border of the duodenum). This artery supplies the descending colon, sigmoid colon and the superior region of the rectum. The IMA bifurcates close to its origin to give rise to the left colic artery and a descending trunk that will give off 3-4 sigmoidal branches and a superior rectal artery. a. Left colic artery: This artery bifurcates into ascending and descending branches suppling the descending colon. i. The ascending branch anastomoses with the left branch of the middle colic artery. ii. The descending branch anastomoses with the ascending branch from the sigmoid arteries. b. Sigmoid arteries (3-4 branches coming off of the IMA). These arteries form a series of arcades that supply the sigmoid colon and anastomose with the descending branch of the left colic artery. c. Superior rectal artery is the most medial branch of the IMA and provides blood supply to the superior aspect of the rectum. It forms anastomotic connections with the middle and inferior rectal arteries. These branches will be described in a later enrichment presentation. Gross Anatomy: Intestine & Portal System Page 21 of 29 Dr. Dennis Goebel 3. Marginal artery (‘of Drummond’): The marginal artery is a “physiological” artery resulting from the anastomoses of the ascending and descending branches of the ileocolic, right colic, middle colic (rt & left branches) and left ascending and descending colic, and sigmoid branches (See Figure 15). a. It provides a redundant blood supply via the superior- and inferior-mesenteric arteries to the entire large intestine, to assure that in the event where blockage of a distal branch of the SMA or IMA occurs, that oxygenated blood supply will reach the area normally provided by the blocked branch. b. It gives rise to the straight arties (vasa recti) supplying the colon (Figure 15). c. Understanding the anatomy of the marginal artery is of surgical importance, in the event of surgical removal (resection) of a diseased section of the sigmoid colon and/or the upper rectum (which are the most common sites for colorectal cancer). These surgeries require the ligation of part, or all of the IMA, to cut off the primary blood supply to these regions. With the goal to retain as much of the non-diseased descending colon as possible, and to have it fitted for a colostomy1,2, preserving the marginal arterial supply is essential to assure that the distal colon will receive oxygenated blood supply from the SMA through the marginal artery’s anastomotic connections. Figure 15 Gross Anatomy: Intestine & Portal System Page 22 of 29 Dr. Dennis Goebel C. Venous return of the large intestine 1. Like the arteries that supply the large intestine, the venous return is also regionally provided by two major veins, the superior mesenteric vein and the inferior mesenteric vein (See Figure 16). Both veins direct the venous return to the liver by way of the hepatic-portal vein. The hepatic portal system is described in section V of these notes. a. The superior mesenteric vein (SMV) i. In addition to providing venous return for most of the small intestines, the SMV also provides for venous return for the proximal half of the large intestine up to the splenic flexure. This includes the cecum, ascending colon, transverse colon, and the appendix (See Figure 16). Figure 16 Gross Anatomy: Intestine & Portal System Page 23 of 29 Dr. Dennis Goebel b. The inferior mesenteric vein (IMV) i. The IMV provides venous drainage into the hepatic portal system from the splenic flexure of the colon, to the superior portion of the rectum. This includes the descending colon, sigmoid colon and the proximal half of the rectum. (Figure 16). D. Innervation of the large intestine 1. Enteric plexus: As noted in the proximal regions of the GI tract (e.g. the esophagus, stomach, and small intestines), the large intestines also contain an intrinsic neuronal network called the enteric plexus. The enteric plexus is controlled by both sympathetic (SANS) and parasympathetic (PANS) input, to regulate motility and glandular secretion. 2. Regional Sympathetic innervation (SANS) of the large intestines: a. SANS Innervation proximal to the splenic flexure of the large intestine: Preganglionic SANS fibers exit the sympathetic chain as the thoracic splanchnic nerves. These fibers join a dense neural plexus called the abdominal prevertebral plexus which is associated with the aorta. The thoracic splanchnic axons synapse on postganglionic SANS neurons located in celiac and superior mesenteric ganglia that are associated with the main visceral branches of the aorta (e.g., celiac- superior mesenteric- arteries). i. Postganglionic SANS axons exit the superior mesenteric ganglia and travel along their corresponding visceral arterial branches to innervate the small- and large-intestines as far distally as the left colic (splenic) flexure. See Figure 17 on the next page. b. SANS Innervation distal to the splenic flexure of the large intestine: Preganglionic SANS innervation of the large intestine distal to the splenic flexure (descending colon and sigmoid colon) is provided by lumbar splanchnic nerves that originate from L1-2 spinal cord levels. The lumbar splanchnic branches synapse on postganglionic neurons located in the inferior mesenteric ganglia. Postganglionic axons then travel with the branches of the IMA arteries to innervate the descending and sigmoid colon. See Figure 17 on the next page. 3. Regional Parasympathetic innervation of the large intestine: Preganglionic parasympathetic innervation (PANS) of the large intestines originates from two sources, the anterior and posterior vagal trunks (CN X), and the Rt & L. pelvic splanchnic nerves (ventral rami from sacral levels S2-S4). See Figure 17 on the next page. a. Parasympathetic (PANS) innervation of the proximal to the splenic flexure of the colon: Para-sympathetic innervation of the esophagus, stomach, small intestine and large intestine up to the splenic flexure of the transverse colon is provided by the anterior & posterior vagal trunks. Gross Anatomy: Intestine & Portal System Page 24 of 29 Dr. Dennis Goebel i. The preganglionic axons from the rt & left vagal trunks converge once the enter the abdominal cavity and join the sympathetic fibers (pre- and post- ganglionic) to form the abdominal prevertebral plexus. ii. The PANS preganglionic fibers from CN X use the visceral branches of the aorta (celiac and superior mesenteric arteries) to make their way to their designated visceral targets (e.g. stomach, small & large intestine, up to the splenic flexure of the transverse colon), where they will then synapse on postganglionic parasympathetic neurons located within the enteric plexus. Figure 17 Gross Anatomy: Intestine & Portal System Page 25 of 29 Dr. Dennis Goebel b. Parasympathetic innervation of the large intestine distal to the splenic flexure of the colon: Preganglionic PANS fibers arise from pelvic splanchnic nerves (branch from ventral rami of sacral nerves S2-S4) to innervate the descending colon, sigmoid colon, and rectum (discussed later). The pelvic splanchnic axons utilize the right and left inferior hypogastric plexus (See Figure 17), which joins a single superior hypogastric plexus located midline to travel to the inferior mesenteric artery (See Figure 17). They will use the IMA branches to reach the descending colon, sigmoid colon and rectum where they will synapse on postganglionic neurons located within the enteric plexus (See Figure 17). 4. Visceral pain of the large intestines: Remember that visceral afferent sensory fibers from the small and large intestines travel with the sympathetic splanchnic nerves, and provide a route for gut pain signals (bowl distension and ischemia) to reach the spinal cord. Referred pain resulting from ischemia or distension of the large intestine is relayed to spinal dermatome levels L1-2. Patients will complain of extreme tenderness and sharp pain localized to the body wall (back, side or front), anywhere along L1-2 dermatomes. E. Other Clinical issues with the colon: 1. Bowel obstruction: Common causes result from impact feces in the colon, colon cancer, inflammatory bowel disease (Crohn’s disease), volvulus (twisting of the large intestine, mainly the sigmoid colon). 2. Peritonitis: Intestinal ischemia, infection and possible rupture of the blocked intestine can lead to infections gaining access into the peritoneal cavity. This can lead to death, if not attended to. 3. Diverticular disease (“Diverticulitis”): Bulging pouches occurring within the small intestine (e.g. Meckel’s diverticulum is the most common), and throughout the large intestine (mostly involving out-pockets projecting from the haustra of the sigmoid colon). The disease manifests itself by causing inflammation and or localized infection within these out-pocketing’s. 4. Intussusception: Is a rare but serious disorder in children, where one part of the small or large intestine slides inside an adjacent part. This disorder can cause bowel obstruction, ischemia/inflammation/infection and is life-threatening, if not attended to. (See Figure 18). Figure 18 Gross Anatomy: Intestine & Portal System Page 26 of 29 Dr. Dennis Goebel 5 Blockade of chronic visceral pain: Recall that the visceral pain fibers (whose cell bodies reside in the dorsal root ganglia) all pass through their respective sympathetic prevertebral ganglia (e.g., the Celiac-, Superior-, and Inferior-mesenteric ganglia) in route to the spinal cord. These ganglia (mainly celiac and superior mesenteric ganglia) are often targeted for nerve blockade, to reduce chronic visceral pain associated with cancer of the viscera. III. THE HEPATIC PORTAL SYSTEM A. The hepatic portal system serves to deliver nutrient-rich venous return from the GI- tract to the liver, where it will be further processed. Venous return from the entire digestive track is funneled into the hepatic portal vein that enters the liver on its visceral surface. 1. The Hepatic portal vein is formed by the union of the splenic and the superior mesenteric veins. This merger occurs on the right side of the posterior abdominal wall at the level of LV1, anterior to the IVC, and posterior to the neck/body region of the pancreas. a. The inferior mesenteric vein (IMV) drains into the splenic vein, approximately 2 cm to the right of where the splenic vein and the superior mesenteric vein join to form the HPV (See Figure 19 on the next page). i. Recall that the IMV is contained within the paraduodenal fold, and that it passes posterior to the pancreas in route to the splenic vein (See Figure 19 on the next page). b. Post-processed venous blood exits the liver on its diaphragmatic surface, via the hepatic veins (3-4 in number). These veins empty directly into the inferior vena cava. B. PORTAL-SYSTEMIC ANASTOMOSES (PSA) 1. The portal venous system communicates with the systemic system venous (inferior vena cava) in several locations. Under normal conditions, these connections are not physiological. However, when the portal circulation is obstructed, (defined as portal- hypertension) owing most commonly to liver disease, blood returning from the GI tract can still reach the heart, by way of the inferior vena cava, through a number of PSA collateral routes. Clinically relevant anastomotic connections occur at: a. Para-umbilical region: Paraumbilical veins (PUV, portal) anastomose with systemic veins draining the anterior abdominal wall in the area surrounding the umbilicus. Portal hypertension can lead swelling and pooling of the venous blood connections between the PUV and the systemic abdominal veins in this region and result in the pooling of venous blood just under the surface of the skin surrounding the umbilicus (See Figure 20, on page 28 of these notes). b. Anal-rectal region: PSA occurs between the superior rectal veins (portal), and the middle-rectal veins (systemic). Portal hypertension causes backflow of portal blood from the superior rectal veins into the middle rectal veins (systemic) (See Figure 20 on the next page) This condition usually accompanies swelling Gross Anatomy: Intestine & Portal System Page 27 of 29 Dr. Dennis Goebel of the venous plexus between the superior and middle rectal veins into the anal canal and form internal hemorrhoids which will bulge into the rectal canal. i. Due to the lack of sensations of touch in the large intestine (including the rectum), swelling of internal hemorrhoids resulting from portal hypertension go undetected until these swelling rupture, most commonly caused by passage of a hardened stool, resulting in significant bleeding to accompany it. c. Gastro-esophageal region: A portal systemic anastomosis occurs on the external surface of the in the esophagus at the junction between the abdominal and thoracic cavities. The esophageal tributaries of the left gastric vein (portal) anastomose with the esophageal vein, which empty into the azygos (systemic) vein (See Figure 20 on the next page). Figure 19 Gross Anatomy: Intestine & Portal System Page 28 of 29 Dr. Dennis Goebel Figure 20 d. Clinical Relevance: Patients with cirrhosis of the liver sustain progressive destruction of hepatic parenchymal cells which are replaced with fibrous tissue. Over time, this would obstruct venous blood flow exiting the liver into the IVC resulting in portal hypertension. i. When portal venous blood is not allowed to drain into the inferior vena cava it is diverted to the systemic veins. The blood diverted to system veins cause enlargement of the veins and is called varicose veins. The varicose veins in the anal region are called hemorrhoids, those in the gastro-esophageal regions are called esophageal varices, and those in the umbilical area are called caput medusae. The enlarged varicose veins could rupture and bleed. Gross Anatomy: Intestine & Portal System Page 29 of 29 Dr. Dennis Goebel ii. The portal vein and all of its tributaries lack internal valves. Therefore, the direction of blood flow is fully dependent upon a high-to-low venous pressure gradient. Surgeons are mindful of this when resecting and removing sections of diseased colon or small intestine by first, ligating all portal connections between the healthy and diseased tissues, before removing the diseased section(s) of intestine. Trauma to the GI tract or accidental cutting of a major portal tributary will result in bidirectional portal hemorrhaging from the liver, as well as from the cut portal tributaries coming from the viscera. References: 1. Billings, PJ and Nicholls, JC., The surgical anatomy of the marginal artery. Anals. of the Royal College of Surgeons of England. 1984, 66:334-335. 2. Gangam, R.R., Sharon, S.S., Surgical significance of the inferior mesenteric artery and its branching pattern. International J. Anatomy and Research, 2016, 4:2062-68. 3. Atlas of Human Anatomy, 6th Ed., 2014, Sanders/Elsevier. 4. Gray’s Anatomy for Students, 3rd Ed., 2015, Churchill Livingstone/Elsevier.