Abdominal Wall, Peritoneum and Intestines - LO 8

8. Compare and contrast the small and large intestines, and describe the blood supply to both.

The small intestine consists (proximally to distally) of the duodenum, jejunum, and ileum. The small intestine mesentery (aka ‘THE’ mesentery) anchors the jejunum and ileum to the posterior abdominal wall. The mucosa of the small intestine are characterized by circular folds, except in the first part of the duodenum and terminal portions of the ileum. Functionally, these folds serve to increase surface area for absorption and segmentalize the intestine.

The ileocecal valve is located at the junction of the ileum of the small intestine and cecum of the large intestine. The valve is composed of a localized thickening of muscle, and serves to impede reflux and regulate transit of the small intestine.

The large intestine is comprised of several regions, proximal-to-distal: the cecum & appendix, ascending colon, R. colic (hepatic) flexure, transverse colon, L. colic (splenic) flexure, descending colon, sigmoid colon, and rectum.  

The outer longitudinal layer of the muscularis of the colon condenses into three bands of muscle: the taeniae coli. Deviations of fibers from the free taenia coli form circumferential bands around the large intestine at regular intervals. These bands give the large intestine a sacculated appearance, and these sacs are known as haustra. Appendages of the large intestine are omental appendices, fat-filled outpouchings from the peritoneum. Semilunar folds of colon are caused by the haustra of the colon. Unlike the circular folds of the small intestine, the semilunar folds do not form complete rings.

Small Intestine Blood Supply (excluding duodenum)

• • The superior mesenteric a. (SMA) supplies derivatives of the midgut (distal duodenum through the proximal two-thirds of the transverse colon. Therefore, it is the sole supply to the jejunum and ileum. The distal small intestine is supplied by a series of SMA branches called the jejunal and ileal (intestinal) aa. Intestinal branches exit the SMA linearly, form elaborate anastomotic arcades, and relatively long straight arteries (arteriae rectae) deliver blood to the jejunum and ileum. These intestinal arteries (and their venous counterparts) are sandwiched in the mesentery of the small intestine.The ileocolic a. also supplies a small portion of the distal ileum.

Large Intestine Blood Supply

• • The large intestine is supplied by the superior mesenteric (SMA) and inferior mesenteric (IMA) aa. The IMA supplies blood to the derivatives of the hindgut (the distal third of the transverse colon through the superior portion of rectum).

  • There is an important anastomotic arcade of branches of the SMA and IMA serving the large intestine, the marginal a. This artery runs along the some portions of the border of the inferior large intestine. Relatively short straight aa. (arteriae rectae) bridge the distance from the marginal a. to the large intestine.

  • Branches of the SMA that supply the large intestine:

    • • Ileocolic a. (cecum, appendix through appendicular a. branch)

      • R. colic a. (ascending colon)

      • Middle colic a. (transverse colon)

  • Branches of the IMA that supply the large intestine:

    • • L. colic a. (descending colon)

      • Sigmoid aa. (sigmoid colon)

      • Superior rectal a. (superior rectum)