Small + Large Intestine

small intestine divisions (superior → inferior)

1. duodenum

2. jejunum

3. ileum

duodenum

connected to stomach via pyloric sphincter

• contains superior, descending, horizontal, and ascending parts

• major site of digestion via bile and pancreatic enzymes

• majority is secondarily retroperitoneal

descending duodenum

1. major duodenal papilla

2. minor duodenal papilla

major duodenal papilla

site where the bile duct and main pancreatic duct empty into the small intestine

• marks approximate foregut/midgut boundary

minor duodenal papilla

receives the accessory pancreatic duct alone

jejunum

gradual transition into the ileum

• intraperitoneal, suspended by the mesentery

• have long, folded internal structures to increase surface area for absorption of nutrients

ileum

connected to the cecum of the large intestine at the ileocecal sphincter

• intraperitoneal, suspended by the mesentery

plicae circulares

circular folds in the mucosa and submucosa of jejunum + ileum that aid in the absorption of nutrients → more densely packed in jejunum than ileum

villi

protrusions within the plicae circulares that further increase surface area

soluble metabolite absorption

portal vein for filtration

lipid absorption

lymphatic system → thoracic duct → venous circulation

myenteric plexus (jejunum and ileum)

between muscular layers

• controls intestinal motility

submucuosal plexus (jejunum and ileum)

within submucosa; controls intestinal secretions, blood flow, and absorption

small intestine arterial supply

• majority via branches of superior mesenteric artery

• arterial arcades form large, low complexity loops in jejunum with long vasa recta

• arterial arcades form SMALL, complex loops in ILEUM, with short vasa recta

SI venous drainage

into portal vein via superior mesenteric vein

large intestine

cecum with appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal, anus

appendix

blind tube-like pouch attached to cecum; an immune organ rich in gut-associated lymphoid tissue (GALT) that contributes to immune surveillance

• microbiome reservoir → may serve as safe haven for commensal bacteria to repopulate gut after diarrheal illness

appendicitis

if lump of feces/swelling of lymph tissue causes blocked, bacteria cannot escape → infection

• Sx = referred umbilical pain (T10 level innervation) early in disease with nausea, lower right quadrant pain with advanced disease due to inflammation of peritoneum → fever

• swelling and rupture → sepsis and death if NOT treated promptly

• appendectomies frequently performed to remove infected appendices

large intestine

absorbs water + electrolytes from digested material

• compacts feces

haustra

pouches formed by taeniae coli (3 muscular bands) that facilitate fecal compaction

• churning of digested matter within pouches → water absorption and feces compaction

stool expulsion

enters rectum, exits via anal canal and anal sphincter (all of these structures do NOT have haustra and taeniae coli and are outside of peritoneum)

transverse and sigmoid colon

intraperitoneal organs (LI)

ascending and descending colon

secondarily retroperitoneal organs (LI)

midgut arterial supply (LI)

branches of superior mesenteric artery (LI)

hindgut structures arterial supply

branches of inferior mesenteric artery (LI)

LI venous drainage

into portal vein via superior mesenteric vein and inferior mesenteric vein (LI)