20 The Gastrointestinal System – Hindgut
GASTROINTESTINAL SYSTEM
The Hindgut
Learning Objectives
By the end of the course students will be able to:
- Describe the basic anatomy of the large intestines, including blood supply and internal structure
Reference: Moore, Clinically Oriented Anatomy, chapter 2.
Particularly relevant Blue Boxes in Moore:
●Colonoscopy, p.260
●Diverticulosis, p. 261
♦ Colitis, Colectomy, Ileostomy and Colostomy p. 260
● Volvulus of Sigmoid Colon p. 26
To access the Netter Presenter Database click here
Grant’s Dissector, 15th Edition, pp 89 – 99
To access Gray’s Photographic Dissector section on the Gastrointestinal Tract click here
To access the Primal Pictures software click here
Check out the Primal Pictures model of the Abdomen and Intestines
THE HINDGUT
The Large Intestine (Netter 276, 288; Moore 246-253)
The large intestine has a greater diameter than the small intestine and courses in a loop to connect the ileum to the rectum. The ascending portion of the colon is approximately 13 cm. long (5 inches), the transverse colon is approximately 38 cm. long, the descending colon is approximately 25 cm. long, and the pelvic sigmoid colon is about 25 – 38 cm. long.
Portions of the large intestine have peritoneal attachments and portions do not. Recall that the mesentery of the ascending and descending colon were absorbed so that these structures now lie in a retroperitoneal position. The transverse mesocolon (Netter 265) attaches the transverse colon to the posterior abdominal wall, and the mesosigmoid attaches the pelvic sigmoid to the posterior wall.
The large intestine takes origin at the cecum (Netter 274), which is the sac that lies below the ilio-cecal junction. When the ascending colon reaches the right colic flexure, it becomes the transverse colon and courses across the abdomen to the left colic flexure, which is more superior than the right colic flexure. The descending colon proceeds from the left colic flexure to the pelvic brim where it becomes the sigmoid (pelvic) colon. The sigmoid colon continues to the rectum.
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Structurally, the colon differs from the small intestine in that the external longitudinal muscle coat has been divided into three muscle bands, the teniae coli – Netter 276. The teniae coli take origin at the ROOT of the appendix (i.e., the longitudinal muscle coat of the appendix is intact). The teniae coli are shorter than the colon itself, so there is a”purse-string” effect and the colon develops little out-pouchings along its course. The pouches are known as haustra coli. The teniae coli have different names which are derived from their course along the transverse colon: the teniae mesocolon, the teniae libra , and the teniae omentalis. |
Small fat containing pouches, the appendices epiploicae , are located along the colon with the exception of the cecum, appendix, and the rectum.
The junction of the ileum to the cecum is the site of a functionless valve, the ileocecal valve. The superior and inferior lips of the ileo-cecal valve come together laterally to form the frenulum, a ridge around the cecum, which separates the cecum from the colon.
The vermiform appendix (Netter 274, 275) varies in length from 8-13 cm. and can lie in just about any position. The base of the appendix is attached to the cecum, and the appendix, itself, is attached to the mesentery of the ileum by its own mesentery, the mesoappendix.
The blood supply to the large intestine (Netter 288) is from both the superior mesenteric artery and the inferior mesenteric artery.
●The superior mesenteric artery supplies that part of the colon that developed from the midgut (the proximal 2/3 of the transverse colon), and the inferior mesenteric artery supplies that part of the colon which developed from the hindgut (the distal 1/3).
● The blood supply to the cecum is from the anterior and posterior cecal branches which arise from the ileo-colic artery, a branch of the superior mesenteric artery. The blood supply to the appendix, which is often double, is from the appendicular branch of the posterior cecal artery or an appendicular branch of the ileal artery.
●The ascending colon is supplied by the right colic artery, a branch of the superior mesenteric artery.
●The proximal 2/3 of the transverse colon is supplied by the middle colic artery, which is also a branch from the superior mesenteric artery.
●The inferior mesenteric artery takes origin from the aorta about 3-4 cm. above the aortic bifurcation. The inferior mesenteric artery gives off a superior and inferior left colic artery and a sigmoid artery which may branch to form several sigmoid arteries. The superior left colic artery brings blood to the distal 1/3 of the transverse colon and the superior portion of the descending colon. The inferior left colic artery supplies the lower portion of the descending colon.
Innervation of the Large Intestine
●Parasympathetic innervation of the proximal 2/3 of the large intestine is from the vagus nerve (Netter 302, 303). The pelvic splanchnic nerves, from S2 – S4, supply the distal 1/3 of the transverse colon, descending colon and rectum. Sympathetic innervation to the proximal 2/3 of the transverse colon is from the lesser splanchnic nerve (T10, T11) while the distal 1/3, descending colon and rectum is from lumbar splanchnic nerves (L1, L2)
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Clinical Considerations The terminal end of the inferior mesenteric artery becomes the superior hemorrhoidal artery which supplies the superior portion of the rectum. The anastomosing ends of the branches of the superior and inferior mesenteric arteries form a marginal artery, “The Artery of Drummond”. The marginal artery of Drummond may not be a functionally efficient channel for its entire course. Two areas of anastomoses are likely to be small or not present: the anastomoses of the middle and left colic artery, proximal to the left colic flexure and the lower sigmoidal artery and the superior hemorrhoidal artery, proximal to the rectum. Lymphatic channels course back along arterial channels to the principle nodes of the superior mesenteric artery and the inferior mesenteric artery. Lymph then flows to the celiac nodes. These three groups of nodes are known as the intestinal lymphatic trunk or pre-aortic nodes. Appendicitis will cause deep pain in the right iliac fossa (Blue Box p. 259). Once the peritoneum is involved in the inflammatory process, the pain becomes severe and localized. The afferent pain fibers from the appendix enter the spinal cord at Tl0; therefore, appendicitis may give rise to referred pain around the umbilicus initially – afferents from the umbilicus also are located in Tl0. Resection of any portion of the large intestine must be done with extreme caution in order to insure that the remaining gut will have an adequate blood supply. |
Occasionally, the ileum will invaginate into the cecum through the ileo-cecal valve. This results in intestinal obstruction. This condition, known as intussusception (Netter 272b), occurs primarily in young children around one year old.
The rectum (from the Latin meaning straight intestine) is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long. At its commencement its caliber is similar to that of the sigmoid colon, but near its termination it is dilated, forming the rectal ampulla.
The rectum acts as a temporary storage facility for feces. As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the rectum is often returned to the colon where more water is absorbed. If defecation is delayed for a prolonged period, constipation and hardened feces results
When the rectum becomes full the increase in intrarectal pressure forces the walls of the anal canal apart, allowing the fecal matter to enter the canal. The rectum shortens as material is forced into the anal canal and peristaltic waves propel the feces out of the rectum. The internal and external sphincter allow the feces to be passed by muscles pulling the anus up over the exiting feces.
Observe the curvature of the rectum and the acute flexion at the rectosigmoid junction. Note the three transverse rectal folds.
The rectum and anal canal are clinically important parts of the intestinal tract because, by either palpation or rectoscope or sigmoidoscope, they can be easily examined in a routine physical. Tumors, hemorrhoids or abscesses are frequent in this part of the GI tract.
The rectum is the continuation of the sigmoid colon and at the point of their junction, the rectum becomes covered by peritoneum only on its anterior surface, and therefore becomes retroperitoneal.
The rectum terminates approximately at the attachment of the levator ani to its borders. Also at this point, is the pectinate line which, anatomically, is the anorectal junction.
The inside of the rectum is thrown into folds called rectal valves. These maintain the fecal material until water is removed and a bowel movement occurs. At that point the rectum elongates and the valves become less prominent.
At the lower end of the rectum, a series of rectal columns encircle the rectum. Between the column are rectal sinuses. Outside of the columns is found the internal rectal plexus of veins. It is here that internal hemorrhoids are found.
At the junction of the rectum and anal canal, the columns and sinuses form a dentate or pectinate appearance. This is called the pectinate line and is the starting point of the anal canal which is about 2.5-4.0 cm long.
The lining of the anal canal is continuous with the skin at the white line of Hilton (or intersphincteric line). This line can be felt with the finger as a small indentation between the internal anal sphincter (circular muscle of the rectal wall) and the subcutaneous external anal sphincter. The external anal sphincter is much stronger to the touch than the internal. Note that the external anal sphincter consists of three parts, the deep, superficial and subcutaneous.
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Blood Supply of the Rectum Netter 378 shows the arteries of the rectum and anal canal. Observe the branches of the right and left divisions of the superior rectal artery obliquely encircling the rectum much as the branches of the dorsal arteries of the penis encircle the penis. The middle rectal arteries (branches of the internal iliac arteries) are usually small; in this specimen the right artery is small but the left one is large and partly replaces the left division of the superior rectal artery. Note that the inferior rectal arteries (branches of the internal pudendal arteries) are largely expended on the anal canal Observe the lateral flexures of the rectum. These flexures and the transverse rectal folds help to support the weight of the feces. The anastomosis of arteries in the wall of the rectum is so extensive that the middle and inferior rectal arteries can supply the entire rectum if the Inferior mesentenc artery (and thereby the superior rectal artery) is clamped. |
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Arterial Supply – Summary There are three sources of arterial supply to the rectum and anus:
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Veins of the Rectum and Anus (Netter 377)
Surrounding the rectum and anus is a very dense rectal plexus of veins. The upper part of the plexus will send tributaries to form the superior rectal vein which then goes into the inferior mesenteric vein.
From the middle part of the plexus, along with tributaries from the bladder, prostate and seminal vesicle pass to the internal iliac vein
From the inferior part of the plexus, drainage is into the internal pudendal vein.
Lymphatics From the Rectum,
Lymphatics pass eventually into the inferior mesenteric group of preaortic lymph nodes.(Netter 296)
From the anal canal, lymphatics pass along the middle rectal artery to end in the internal iliac nodes and from these to the common iliac nodes and then to the lateral aortic group of nodes.
From the anus, below the white line of Hilton (representing the transition point from non-keratinized stratified squamous epithelium to keratinized stratified squamous epithelium in the anus), the lymphatics join those of the perineum and scrotum and pass into the superficial inguinal nodes of the Rectum and Anus (Netter 261)
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Clinical Correlations Internal hemorrhoids are found above the pectinate line and outside the rectal columns. External hemorrhoids are below the pectinate line and are the more common clinically and can be seen when enlarged. Both types of hemorrhoids can be sources of bleeding when abraded. This type of bleeding is bright red compared to bleeding higher up in the GI tract where the blood is occult and must be identified by chemical tests. A rectal examination is used to determine the size and consistency of the prostate gland. It is also used to palpate the bladder, seminal vesicle, and the ampulla of the vas deferens anteriorly; the coccyx and sacrum posteriorly, and the ischiorectal fossa laterally. Diverticulitis is inflammation of diverticula (external evaginations or out-pocketings) of the intestinal wall, commonly found in the colon, especially in the sigmoid colon. The condition of having diverticula is called diverticulosis and diverticula develop as a result of high pressure within the colon. Symptoms are abdominal pain usually in the lower abdomen, chills, fever, nausea and constipation. Risk factors include old age and a low-fiber diet. Complications may onclude bleeding, perforations, peritonitis and stricture or istula formation. Megacolon (Hirschprung’s disease) is caused by the absence of enteric ganglia ini the lower part of the colon, which leads to dilation of the colon proximal to the inactive segment. It is of congential origin, resulting from failure of neural crest cells to form the myenteric plexus and is usually diagnosed during infancy and childhood. Symptoms are constipation or diarrhea, abdominall distention, vomiting and a lack of appetite. However, the primary symptom is not passing meconium, the first bowel movement of the newborn infant. |
Internal Sphincter Muscles and Anal Canal – Netter 371 , 372, 373 Note position of subcutaneous portion of anal sphincter passing between tip of coccyx and the central perineal tendon (surrounds anal opening).
The Internal anal sphincter (Netter 372 – bottom) is circular fibers of gut wall (involuntary) which surround anal canal. It measures about 8 to 10 cm in length, from its anterior to its posterior extremity, and is about 2.5 cm broad opposite the anus.
The external anal sphincter is subdivided into: 1) subcutaneous, 2) superficial and 3) deep portions. The external anal sphincter muscle is under voluntary control. This muscle is, like other muscles, always in a state of tonic contraction, and having no antagonistic muscle it keeps the anal canal and orifice closed. It can be put into a condition of greater contraction under the influence of the will, so as more firmly to occlude the anal aperture, in expiratory efforts unconnected with defecation. The inferior rectal branch of the pudendal nerve provides motor innervation to the external anal sphincter and sensation to the inferior portion of the anal canal.
The Internal Anal Sphincter is a muscular ring which surrounds about 2.5 cm of the anal canal; its inferior border is in contact with, but quite separate from, the external anal sphincter. It is about 5 mm thick, and is formed by an aggregation of the involuntary circular fibers of the intestine. Its lower border is about 6 mm from the orifice of the anus. Its action is entirely involuntary. It helps the External Anal Sphincter to occlude the anal aperture and aids in the expulsion of the feces. It is innervated by parasympathetic nerves from S2, S3, and S4 (Netter 303) . It is not innervated by the pudendal nerve, which is a somatic nerve.
Cross sections of the Abdomen at the T11, T12, L1, L3, L5 levels
The Gastrointestinal System – Foregut, Midgut and Hindgut quiz – click here