Lower Abdomen and Mesenteric Vessels

Structures of the lower abdomen are derived from embryonic mid and hind gut therefore receive all the arterial blood supply from branches of the Superior or Inferior mesenteric artery (the 2^nd and 3^rd unpaired branches of the descending aorta) They are drained by the Portal system.

Midgut starts just dorsal to the Major Duodenal Papilla and ends 2/3rds to 3/4ths through the transverse colon before the left colic flexure. Hindgut – last 1/3^rd of the transverse colon Þ rectum

celiac trunk supplies foregut

Superior Mesenteric

1 cm inferior to the celiac trunk (~ L1), gives rise to: (from superior to inferior)

Inferior (anterior and posterior) pancreoduodenal arteries (anastomose with the superior pancreoduodenal arteries)
Middle colic artery – blood to the ascending and transverse colon (1st 2/3rds)
Right colic artery – blood to ascending colon
Ileocloic artery (its 5 branches:

(1) ascending (gives rise to the marginal)
(2) anterior cecum-
(3) posterior cecum-
(4) appendicular branch-
(5) ileal branch- anastomoses with a branch of the ileocolic artery

the marginal artery (from the ascending branch of the superior mesenteric) is a branch of a number of other arteries. The vessels are found between the leaflets of the root of the mesentery. Veins follow arteries in name and position.

Inferior Mesenteric

4 cm superior to the bifurcation of the aorta into the iliacs (~L3), gives rise to:

left colic artery – vascularizes the descending colon

sigmoid branches- vascularizes the sigmoid colon

superior rectal artery- vascularizes the superior rectum (the veins are similar / the superior rectal vein anastomoses with the middle and inferior rectal veins. This can lead to hemorrhoids)

Small Intestine

Jejunum (40% of the small intestine) is in the Upper Left Quadrant (ULQ)

Ileum (60% of small intestine) in the Lower Right Quadrant (LRQ) and dumps into the ascending colon.

Jejunum	Ileum
Long Vasa Recti	Short Vasa Recti
Simple arcades	Complex (tortuous) arcades
Fat up to (but not onto) bowel	Encroaching fat
Circular folds on inside	Little to no circular folds on inside

Meckle’s Diverticulum - incomplete development, a Yolk Stalk remnant on the anti-mesenteric side of ileum Þ near ileo-colic junction

Ileocecal Valve – there is a sphincter between the small and large bowel. It is not as tight as the pyloric sphincter.

Nerves – sympathetics cause vasoconstriction and reduce motility / Parasymphathetics increase motility

Greater splanchnic Þ foregut

Lesser and Least splanchnic Þ midgut

Lumbar splanchnic (L1, L2) Þ sympathetic innervation of hindgut

Sacral splanchnic (S1-S4) Þ parasympathetic innervation of hindgut

Lymph

drains in a pattern similar to the vessels to the Cisterna chyli (bottom of thoracic duct)

Large Intestine

Appendix –The appendix is most likely to be found posterior and medially off of the cecum.

Appendices epiploica are fat bags that hang off the colon

Tenia coli are the 3 bands of longitudinal muscle down the length of the colon to the rectum. They are shorter than the large intestine and create pockets called Haustra

Fluid tends to drain superiorly and collect in the subphrenic spaces where lymphatics remove it. When infected, drainage switches and goes inferiorly. (Perforated duodenal ulcer will cause problems on the right paracolic gutter due to this inferiorly directed drainage)

Diverticulitis – branches of the marginal artery feed the colonic wall. When the colonic wall punches through the space where the vessels are, it forms a diverticulum. If this herniated pouch of colonic wall becomes inflamed it causes Diverticulitis.

The right colic flexure is inferior to the left colic flexure because of the liver

large intestine alternates between retroperitoneal and intraperitoneal:

cecum – intraperitoneal

ascending colon – retroperitoneal

transverse colon – intraperitoneal

descending colon – retrtoperieoneal

sigmoid colon – intraperitoneal

rectum – retroperitoneal

Stuctures

Vessels:

Superior Mesenteric Artery and its branches: Supplies Midgut, 1cm below the Celiac trunk (~L1)

15-18 Jejunal and Ileal Branches- form arcades (see diagram in D3J Anatomy of Lower Abdomen and Mesenteric vessels)

Vasa Recta- short vessels (longer in jejunum than ileum) that connect the arcades to the wall of the intestine
Iliocolic Artery- has 5 branches, 3^rd off the superior mesenteric, supplies cecum and appendix
Right Colic Artery- 2^nd off the superior mesenteric, supplies the ascending colon
Middle Colic Artery- supplies the ascending and 1^st 2/3rds of the transverse colon

Inferior Mesenteric Artery and its branches: supplies Hindgut, 4 cm above the bifurcation of the aorta (~L3)

Left Colic Artery- supplies the descending colon
Sigmoid Arteries- supplies the sigmoid colon
Superior Rectal Artery- supplies the rectum
Marginal Artery- formed by the anastomoses of a number of other arteries
Superior Mesenteric Vein- when joined by the splenic and inferior mesenteric becomes the Portal Vein

Nerves: Superior Mesenteric Plexus- receives its parasympathetic fibers from the celiac division of the posterior vagal trunk, and its sympathetic fibers from the superior mesenteric ganglion. Located above where the superior mesenteric stems from the aorta.

Review

distinguishing between the ileum and the jejunum

Jejunum has longer Vasa Recti, simpler arcades, and has prominent plicae circulares inside, the ileum lacks these features.

arterial supply to the ascending colon

The ascending colon is supplied by the right colic artery and the middle colic artery.

The sigmoid arteries are branches off the Inferior Mesenteric artery.

The superior rectal vein drains into the inferior mesenteric vein (Portal) whereas the inferior/middle rectal veins drain into the pudendal and iliac veins, respectively (Caval)

significance in colonic surgery of the anastomosis between the superior and inferior mesenteric arteries is that blood supply will not be stopped by clamping just one of the arteries. This anastomosis normally occurs in the marginal artery