Large Intestine

1. Anatomy and Histology of the GIT hollow organs III Large Intestine Dr. NabilKhouri, MD. PH.D

2. Large Intestine • The large intestine extends from the ileum to the anus. It is divided into the cecum, appendix, ascendingcolon, transversecolon, descendingcolon, and sigmoidcolon. • The rectum and analcanal are considered in the sections on the pelvis and perineum. • The primary function of the large intestine is the absorption of water and electrolytes and the storage of undigested material until it can be expelled from the body as feces.

3. Large Intestine • Has three unique features: • Teniae coli – three bands of longitudinal smooth muscle in its muscularis • Haustra – pocketlike sacs caused by the tone of the teniae coli • Epiploic appendages – fat-filled pouches of visceral peritoneum • Is subdivided into the cecum, appendix, colon, rectum, and anal canal • The saclike cecum: • Lies below the ileocecal valve in the right iliac fossa • Contains a wormlike vermiform appendix

6. Ascending Colon Location and Description • The ascending colon lies in the right lower quadrant. • It extends upward from the cecum to the inferior surface of the right lobe of the liver, where it turns to the left, forming the right colic flexure, and becomes continuous with the transverse colon. The peritoneum covers the front and the sides of the ascending colon, binding it to the posterior abdominal wall. Relations • ■■ Anteriorly: Coils of small intestine, the greater omentum, and the anterior abdominal wall. • ■■ Posteriorly: The iliacus, the iliac crest, the quadratuslumborum, the origin of the transversusabdominismuscle, and the lower pole of the right kidney. The iliohypogastricand the ilioinguinal nerves cross behind it

7. CecumLocation and Description • The cecum is that part of the large intestine that lies below the level of the junction of the ileum with the large intestine. • It is a blind-ended pouch that is situated in the right iliac fossa. It is about 6 cm long and is completely covered with peritoneum. • It possesses a considerable amount of mobility, although it does not have a mesentery. • Attached to its posteromedial surface is the appendix. • The presence of peritoneal folds in the vicinity of the cecum creates the superior ileocecal, the inferior ileocecal, and the retrocecal recesses .

8. As in the colon, the longitudinal muscle is restricted to three flat bands, the TENIAE COLI, which converge on the base of the appendix and provide for it a complete longitudinal muscle coat . The terminal part of the ileum enters the large intestine at the junction of the cecum with the ascending colon. This opening is provided with two folds, or lips, which form the so-called ILEOCECAL VALVE. The appendix communicates with the cavity of the cecum through an opening located below and behind the ileocecal opening.

9. Relations ■■ Anteriorly: Coils of small intestine, sometimes part of the greater omentum, and the anterior abdominal wall in the right iliac region ■■ Posteriorly: The psoas and the iliacus muscles, the femoral nerve, and the lateral cutaneous nerve of the thigh. The appendix is commonly found behind the cecum. ■■ Medially: The appendix arises from the cecum on its medial side.

12. Blood Supply • Arteries Anterior and posterior cecal arteries form the ileocolic artery, a branch of the superior mesenteric artery • Veins The veins correspond to the arteries and drain into the superior mesenteric vein.

13. Ileo-cecalValve • The ileocecal valve consists of two horizontal folds of mucous membrane that project around the orifice of the ileum. • The valve plays little or no part in the prevention of reflux of cecal contents into the ileum. • The circular muscle of the lower end of the ileum (CALLED THE ILEOCECAL SPHINCTER BY PHYSIOLOGISTS) • serves as a sphincter and controls the flow of contents from the ileum into the colon.

15. Appendix Location and Description • The appendix is a narrow, muscular tube containing a large amount of lymphoid tissue. • It varies in length from 8 to 13 cm. • The base is attached to the posteromedial surface of the cecum about 1 in. (2.5 cm) below the ileocecaljunction. • The remainder of the appendix is free. • It has a complete peritoneal covering, which is attached to the mesentery of the small intestine by a short mesentery of its own, the mesoappendix. • The line joining the right anterior superior iliac spine to the umbilicus (McBurney’s point).

16. The glands are lined with simple columnar epithelium and a high number of mucin producing goblet cells. The lamina propria typically contains lymphocytes that partly obscure the underlying muscularis Mucosae. The submucosa is almost fully occupied by lymphoid tissue mainly arranged in lymphatic nodules. The center of the lymphoid nodules stain lighter and are termed germinal centers. The germinal center contains the larger dividing lymphoblasts, similar to the arrangement in lymph nodes. The outer portions of the submucosa harbor larger vessels and have less dense infiltrates of immune cells.

17. Transverse Colon Location and Description • The transverse colon is about 38 cm long and extends across the abdomen, occupying the umbilical region. • It begins at the right colic flexure below the right lobe of the liver and hangs downward, suspended by the transverse mesocolon from the pancreas . It then ascends to the left colic flexure below the spleen. • The left colic flexure is higher than the right colic flexure and is suspended from the diaphragm by the phrenicocolicligament. • The transverse mesocolon, or mesentery of the transverse colon, suspends the transverse colon from the anterior border of the pancreas. The mesentery is attached to the superior border of the transverse colon, and the posterior layers of the greater omentum are attached to the inferior border

19. Relations • ■■ Anteriorly: The greater omentum and the anterior abdominal wall (umbilical and hypogastric regions) • ■■ Posteriorly: The second part of the duodenum, the head of the pancreas, and the coils of the jejunum and the ileum

20. Descending Colon Location and Description • The descending colon is about 25 cm long and lies in the left upper and lower quadrants . It extends downward from the left colic flexure, to the pelvic brim, where it becomes continuous with the sigmoid colon.. Relations • ■■ Anteriorly: Coils of small intestine, the greater omentum, and the anterior abdominal wall . • ■■ Posteriorly: The lateral border of the left kidney, the origin of the transversusabdominis muscle, the quadratuslumborum, the iliac crest, the iliacus, and the left psoas. The iliohypogastric and the ilioinguinalnerves, the lateral cutaneous nerve of the thigh, and the femoral nerve also lie posteriorly.

22. Sigmoid Colon Location and Description • The sigmoid colon is 25 to 38 cm long and begins as a continuation of the descending colon in front of the pelvic brim. Below, it becomes continuous with therectum in front of the 3rd sacral vertebra. • The sigmoid colon is mobile and hangs down into the pelvic cavity in the form of a loop. The sigmoid colon is attached to the posterior pelvic wall by the fan-shaped sigmoid mesocolon. Relations • ■■ Anteriorly: In the male, the urinary bladder; in the female, the posterior surface of the uterus and the upper part of the vagina • ■■ Posteriorly: The rectum and the sacrum. The sigmoid colon is also related to the lower coils of the terminal part of the ileum.

24. Location and DescriptionRectum • The rectum is about 13 cm long and begins in front of the third sacral vertebra as a continuation of the sigmoidcolon. It passes downward, following the curve of the sacrum and coccyx, and ends in front of the tip of the coccyx by piercing the pelvic diaphragm and becoming continuous with the anal canal. The lower part of the rectum is dilated to form the rectal ampulla. The rectum deviates to the left, but it quickly returns to the median plane . On lateral view, the rectum follows the anterior concavity of the sacrum before bending downward and backward at its junction with the anal canal The peritoneum covers the anterior and lateral surfaces of the first third of the rectum and only the anterior surface of the middle third, leaving the lower third devoid of peritoneum . The muscular coat of the rectum is arranged in the usual outer longitudinal and inner circular layers of smooth muscle. The three teniae coli of the sigmoid colon, however, come together so that the longitudinal fibers form a broad band on the anterior and posterior surfaces of the rectum.

26. The Rectum • The mucous membrane of the rectum, together with the circular muscle layer, forms two or three semicircular permanent folds called the transverse folds of the rectumthey vary in position. Relations • ■■ Posteriorly: The rectum is in contact with the sacrum and coccyx; the piriformis, coccygeus, and levatoresanimuscles; the sacral plexus; and the sympathetic trunks. • ■■ Anteriorly: In the male, the upper two thirds of the rectum, which is covered by peritoneum, is related to the sigmoid colon and coils of ileum that occupy the rectovesicalpouch. The lower third of the rectum, which is devoid of peritoneum, is related to the posterior surface of the bladder, to the termination of the vas deferens and the seminal vesicles on each side, and to the prostate .In the female, the upper two thirds of the rectum, which is covered by peritoneum, is related to the sigmoid colon and coils of ileum that occupy the rectouterine pouch (pouch of Douglas). The lower third of the rectum, which is devoid of peritoneum, is related to the posterior surface of the vagina .

29. Ano-rectal junction

30. Anal Canal Location and Description • The anal canal is about 4 cm long and passes downward and backward from the rectal ampulla to the anus . Except during defecation, its lateral walls are kept in apposition by the levatoresani muscles and the anal sphincters. Relations • ■■ Posteriorly: The anococcygeal body, which is a mass of fibrous tissue lying between the anal canal and the coccyx. • ■■ Laterally: The fat-filled ischiorectal fossae (Fig. 8.5). • ■■ Anteriorly: In the male, the perineal body, the urogenital diaphragm, the membranous part of the urethra, and the bulb of the penis. In the female, the perinealbody, the urogenital diaphragm, and the lower part of the vagina.

32. Structure • The mucous membrane of the upper half of the anal canal • is derived from hindgut entoderm. It has the following important anatomic features: • ■■ It is lined by columnar epithelium. • ■■ It is thrown into vertical folds called anal columns, which are joined together at their lower ends by small semilunar folds called anal valves (remains of proctodeal membrane). • ■■ The arterial supply is that of the hindgut—namely, the superior rectal artery, a branch of the inferior mesenteric artery. The venous drainage is mainly

33. Valves and Sphincters of the Rectum and Anus • Three valves of the rectum stop feces from being passed with gas • The anus has two sphincters: • Internal anal sphincter composed of smooth muscle • External anal sphincter composed of skeletal muscle • These sphincters are closed except during defecation

34. Histology • Colon mucosa is simple columnar epithelium except in the anal canal • Has numerous deep crypts lined with goblet cells • Anal canal mucosa is stratified squamous epithelium • Anal sinuses exude mucus and compress feces • Superficial venous plexuses are associated with the anal canal • Inflammation of these veins results in itchy varicosities called hemorrhoids

35. Large intestine Mucosa folded No plecae circulares two types of cells Goblet Mucus-secreting Absorptive Glands packed straight tubular Crypts extend to the MM

36. Large intestine

39. Small Vs Large Intestine