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Lower GI Anatomy Laura McCafferty 2365005ILOs ● Small Intestine ○ Basic S+F ○ Features of the SI (mesentery, plicae circulares, villi, crypts) ○ Jejunum vs Ileum ○ Cell types ○ Blood supply, lymphatics ● Large Intestine ○ Basic S+F ○ Blood supply, lymphatics, and innervation ○ Features ○ MotilitySmall Intestine ● Function ○ Final stages of enzymatic digestion ○ Sole site for amino acid and monosaccharide absorption ○ Most lipids absorbed ● Duodenum ○ ~25cm ○ No mesentery ○ Ends at duodenojejunal flexure ○ 4 parts ■ D1: superior ■ D2: descending ● Contains major duodenal papilla ■ D3: inferior ■ D4: ascending ● Jejunum ○ ~1.5m ○ No clear anatomical distinction between jejunum and ileum ● Ileum ○ ~2.5mSmall Intestine - Features ● Mesentery ○ Fan shaped peritoneal fold ■ Supports SI ○ Attached to posterior abdominal wall ■ Root runs from duodenojejunal flexure to right iliac fossa ● Plicae circulares ○ Mucosal folds ■ More prominent in distal duodenum and jejunum ○ Functions ■ Increase surface area ■ Help chyme to spiral which aids mixing and absorption ● Villi ○ Broad in duodenum, leaf like in jejunum, and shorter ‘finger like’ in ileum ○ Contains a lacteal - modified lymph vessels - for fat absorption ○ Enterocytes have microvilli - increases SA and forms a ‘brush border’ ● Crypts of Lieberkuhn ○ Between villi ○ Fluid secretion and stem cell production ● Brunner's Glands ○ Duodenum ○ Mucus secretion when acidic chyme enters - alkaline to neutraliseSmall Intestine - Jejunum vs Ileum ● Jejunum ○ More plicae circulares ○ Windows of fat free mesentery ○ Vasa recta longer ○ Less complex arcades ● Ileum ○ Less plicae circulares ○ No windows in mesentery ○ Vasa recta shorter ○ More complex arcades ○ Peyer’s patches ■ Large patches of lymph nodules in ileum wallSmall Intestine - Cell types ● Enteroendocrine cells ○ D - somatostatin ■ Reduces gastric hormone secretion ○ S - Secretin ■ Stimulate bile flow ○ N - neurotensin ○ I -CCK ■ Inhibits gastric emptying ■ Stimulates bile flow ○ Enterochromaffin - 5-HT/serotonin ● Goblet - mucus ○ Lubrication ● Paneth - lysozyme ○ Within crypts ○ Antimicrobial activity ● Stem cells - replacementSmall Intestine - HistologySmall Intestine - Blood Supply + Lymphatics ● Proximal duodenum ○ Celiac trunk → hepatic and gastroduodenal arteries ● Distal duodenum → 2/3rds along transverse colon ○ Superior mesenteric ■ Inferior pancreaticoduodenal artery (distal duodenum and pancreas) ■ Jejunal ■ Ilieal ■ Ileocolic (terminal ileum) ● Lymphatics ○ Nodes in mesentery drain to paraaortic nodes ●Large Intestine - Basic Structure and Function ● Functions: ○ Reservoir - storing unabsorbed and unusable food residues ○ Production and absorption of vitamins ○ Forms and propels faeces towards the rectum ● Basic Structure: ○ ~1.2m long ○ Ileocaecal junction ○ Caecum (+appendix) ○ Ascending colon ○ Transverse colon ○ Descending colon ○ Sigmoid colon ○ Rectum ○ Anal canalLarge Intestine - The Beginning ● Ileocaecal junction ○ Valve closes caecal pressure is high ○ → prevents reflux into ileum ● Caecum ○ R. iliac fossa ○ Held in place by ascending colon ● Appendix ○ ~2cm from ileocaecal junction ○ Suspended in mesoappendix ○ 2-20cm in length (ave. 8cm) ○ Position - retrocaecal, pelvic ○ Appendicular artery from ileocolic arteryLarge Intestine - Continued ● Ascending and Descending ○ Retroperitoneal ○ Held in contact with posterior wall by overlying peritoneum ● Transverse ○ Suspended by transverse mesocolon ○ Can lie close to stomach or partly in pelvis ● Sigmoid ○ Extends towards mid sacrum ○ Ends ~S2 ○ Suspended by mesocolon ■ Allows it to be distinguished from the descending colonLarge Intestine - Continued ● Rectum ○ Internal anal sphincter ■ Smooth muscle ■ Involuntary control ○ External anal sphinctyer ■ Striated muscle ■ Voluntary control ● Anal canal ○ Mucosa hangs in long folds called anal columns in the upper 2/3rds ○ Upper 2/3rds = simple columnar ○ Lower 1/3rds = stratified squamousLarge Intestine - Features ● Taenia Coli ○ 3 separate longitudinal bands of thickened smooth muscle on outside of colon ■ Mesocolic, free, omental ■ Join at appendix and rectosigmoid junction ● Haustrations ○ Small pouches caused by sacculation ■ Taenia coli run the length of the colon but are overall shorter → sacculation ○ Give colon segmented appearance ■ Rectum has no haustra ○ Haustra vs plicae in the SI ■ Wider distance between haustra than between plicae ■ Haustra do not reach all the way around ● Appendices epiploicae ○ Tags of fat scattered along surface ■ Largely on transverse and sigmoid ○ Unknown function ■ Can become inflamed - epiploic appendagitis ● Can mimic acute appendicitis ● Painful but benign Large Intestine - Features ● Flexures ○ Hepatic/right ■ Hangs lower than splenic flexure due to liver ○ Splenic/ left ■ ‘Watershed’ area - most likely to lose blood supply here ● Compartments ○ Greater sac can be divided by transverse mesocolon into supracolic and infracolic compartments ■ Infracolic compartment contains SI, ascending and descending colon ● Oblique root of mesentery of SI divides infracolic compartment into left and right ■ Infracolic and supracolic compartments are connected by paracolic gutters ● Between posterolateral abdominal wall and lateral asc. or desc. colon ○ Pus from appendix can track up right gutter into subphrenic spaceLarge Intestine: Histology ● Glands/crypts, no villi ● No paneth cellsLarge Intestine - Blood Supply ● Greater duodenal papilla to 2/3rds along transverse colon ○ Midgut of embryo ○ Superior mesenteric artery ■ Ileocolic, right colic, middle colic ● 2/3rds along transverse colon to rectum ○ Hindgut of embryo ○ Inferior mesenteric artery ■ Left colic, sigmoid, superior rectal (above IAS) ● Below IAS ○ Internal iliac (middle and inferior rectal arteries) ● Arcades ○ Motility without compromising blood supply ○ Joined by marginal artery ● Venous drainage ○ Largely follows arterial supply ■ Drains into superior or inferior mesenteric veins ■ All drain to portal systemLarge Intestine - Nerve Supply by ANS + Enteric NS ● S ○ Caecim → splenic flexure (midgut) ■ Superior mesenteric plexus ○ Splenic flexure onwards ■ Inferior mesenteric plexus ○ Rectum ■ Inferior hypogastric plexus ● PS ○ Caecum → splenic flexure ■ Vagus nerve ○ Splenic flexure onwards ■ Pelvic splanchnic (S2-S4) ● Also: Enteric Nervous System SMG = superior mesenteric ganglion ○ Auerbach plexus IMG = inferior mesenteric ganglion ○ Submucosal plexus of Meissner HP = hypogastric plexus PHP = pelvic hypogastric plexusLarge Intestine - Lymphatics ● 4 general lymph node groups ○ Epicolic ■ Outer surface of intestinal wall ○ Paracolic ■ Intestinal margin ○ Intermediate nodes ■ Along arterial branches of superior and inferior mesenteric arteries ○ Preaortic ■ At branching point of coeliac trunk and superior and inferior mesenteric arteries ● Drainage ○ Midgut ⇒ superior mesenteric nodes ○ Hindgut → inferior mesenteric nodesLarge Intestine - Motility ● Haustral contractions ○ Activated by chyme ■ Moves matter from one haustra to the next ■ Slow, segmental ~every 30 minutes ■ Circular muscle ■ Squeeze and rolls ■ Aids water absorption by mixing chyme ● Mass movements ○ Initiated by distension of stomach and duodenum ■ I.e. gastrocolic and duodenocolic reflex ■ Moves chyme to rectum quickly ● Defecation reflex ○ Activated by distension of rectal wall ■ Every 2-3 hours ■ Stretch reflex → Sacral spinal cord → relaxing of anal sphincter and contraction of rectum