Lower GI Anatomy PDF

Part 2: Lower GI  Learning outcomes  M1.I.GAS.ANA1: Describe the functional anatomy of the digestive tract (oral cavity, oesophagus, stomach, duodenum, jejunum, ileum, caecum and colon), accessory digestive organs (salivary glands, liver, gallbladder, pancreas) and t...

Part 2: Lower GI  Learning outcomes  M1.I.GAS.ANA1: Describe the functional anatomy of the digestive tract (oral cavity, oesophagus, stomach, duodenum, jejunum, ileum, caecum and colon), accessory digestive organs (salivary glands, liver, gallbladder, pancreas) and the spleen.  M1.I.GAS.ANA2: Outline the major structures of the neurovascular supply to the gastrointestinal system.  M1.I.GAS.ANA3: Describe the organisation and clinical significance of the parietal and visceral peritoneum, the greater and lesser sacs, mesenteries and peritoneal ‘ligaments’.  M1.I.GAS.ANA4: Understand specific common clinical examples associated with the gastrointestinal system.  What we will cover…  Functional anatomy of the small intestine (duodenum, jejunum, ileum)  Functional anatomy of the large intestine (Cecum, appendix, colon, rectum and anal canal)  Lower GI Histology  Their neurovascular supply Small and Large intestine  Large intestine  Small intestine Cecum 1. Duodenum Appendix 2. Jejunum Ascending colon Transverse colon 3. Ileum Descending colon Sigmoid colon Rectum Anal canal Blood supply: Duodenum  Dual origin organ: From the 1 st part to the proximal half of 2 nd part are foregut derivative. From the distal half 2 nd part to remainder are midgut derivative. Hence has dual blood Suspensory ligament of duodenum supply – Celiac and SMA. (Ligament of Treitz) marks the duodenojejunal flexure Junction between duodenum and jejunum. Small intestine: Jejunum and Ileum  Location: Jejunum mostly in LUQ. Ileum mostly in RLQ. No clear external delineation of where ileum begins.  Function: Absorbing nutrients – has specialised epithelial lining and mucosal folds.  Intraperitoneal  Highly mobile as it is suspended within abdominal cavity by mesentery proper – double fold peritoneum that attaches small bowels to the posterior abdominal wall. Mesentery proper acts as a conduit for neurovasculature Jejunum and Ileum: Internal features  Circular folds (plicae circulares) are most prominent in the jejunum becomes low and spare in proximal ileum and absent in distal ileum. Increases absorptive properties of the intestine by increasing surface area. Jejunum, unique feature – has the largest area for luminal secretion and absorption Histology: Small intestine  Villi – mucosa forms finger like processes.  Epithelial type: Simple columnar. Made of enterocytes and goblet cells.  Microvilli – enterocytes have apical projections.  Crypts (of Lieberkühn) or intestinal glands – invagination of mucosa. Lined with Paneth cells – antimicrobial (innate immunity). Histology: Small intestine – Duodenum  Duodenum, unique feature: Brunner glands (BG) – Compound tubular mucous gland.  BG are concentrated in the upper duodenum (closer to the stomach) and are larger. Secrets alkaline mucus that acts to neutralises the pH of the gastric chyme Supplement the mucus from goblet cells to lubricate and protect the lining of the small bowel. Ileum wall. Key: (SM) Submucosa; (P) Large masses of aggregated lymphoid nodules (Peyer’s patches) lie in the mucosa Histology: Small intestine –Ileum  Ileum, unique feature: Ratio of goblet cells to enterocytes is greatest Numerous Paneth cells in crypts Has largest amount of MALT or GALT (Mucous ‘gut’ – associated lymphoid tissue) – Peyer’s patches. Large intestine Retroperitoneal vs. Intraperitoneal Large intestine External features:  Haustra – small pouches. The sacs are formed by the inner circular muscle layer that gives the colon its segmented appearance.  Omental appendices – small, fatty tags or omentum-like projections.  Taeniae coli – three distinct smooth muscles longitudinal bands. Are named according to its position and location.  Cecum  First part of the large intestine and is continuous with the ascending colon.  Intraperitoneal organ  Ileocecal junction – junction between small intestine and large intestine when ileum empties content into the cecum via the ileocecal valve. Valve acts to prevent reflux of chyme from the cecum back into the ileum when digested material is acting against gravity to be propelled up the ascending colon. Appendix  Intraperitoneal  Blind intestinal diverticulum which contains masses of lymphoid tissue Significant part of MALT with its lamina propria and submucosa filled with lymphocytes and lymphoid follicles (L). Appendix  Variations in position of the appendix. Rectum & Anal canal  Fixed part of the GI tract. Terminal part of the large intestine.  Follows the sacrococcygeal curve and forms several flexures.  Rectum lies between the sigmoid colon and the anal canal.  Primarily retroperitoneal – Proximal part  Subperitoneal – Distal part  Anorectal junction – indicated by the top ends of the anal columns.  Anal canal – continuous with the rectum at the pelvic diaphragm where it makes a 90 degree posterior bend known as the anorectal flexure. Anal canal  Terminal part of the GI tract. About 3.5 cm  Functions: Fecal continence and defecation.  Internal anal sphincter and external anal sphincter  Pectinate line divides the anal canal into upper half and lower half.  Upper half anal canal – formed by anal columns. Made up of simple columnar epithelium.  Lower half of anal canal – lined by non- keratinised stratified squamous epithelium. Anal canal Transitions occurring at the pectinate line.  Superior to the pectinate line - vessels and nerves are visceral  Inferior to the pectinate line - vessels and nerves are parietal or Blood supply: Small & Large intestine – Superior mesenteric artery  SMA branches: Jejunal and ileal arteries Ileocolic artery Appendicular a. Right colic artery Middle colic artery Blood supply: Small & Large intestine  SMA branches: Jejunal and ileal arteries Jejnum (A) has longer vasa recta (straight a.) while ileum (B) has more prominent complex arterial arcades compared to the jejunum. Blood supply: Large intestine – Inferior mesenteric artery  IMA branches: Left colic Sigmoid arteries (2-4) Superior rectal artery Marginal artery  Marginal artery (of Drummond): Anastomotic connections between the SMA and IMA Clinically, anastomosis provides collateral follow in the event of significant occlusion or stenosis. Left splenic flexure – commonest area of colonic ischemia. Why? Porta-caval anastomosis  These portacaval anastomotic connections become clinically important when the direct drainage route to the liver is blocked. Nerve supply – Autonomic Nervous System (ANS) Parasympathetic innervation – Simulates digestion by increasing motility, secretion and relaxing sphincters. Pathways: Vagus nerve (CN X): Innervates foregut and midgut (Proximal 2/3 of transverse colon) Pelvic splanchnic nerves (S2-S4): Innervate the hindgut (from distal 1/3 of transverse colon to rectum and anal canal). Nerve supply – Autonomic Nervous System (ANS) Sympathetic innervation – Inhibits digestion by decreasing motility, reducing secretions, and contracting sphincters. Pathways: Preganglionic fibres from thoracic and lumbar spinal levels (T5-L2) synapse in preverterbral ganglia: Foregut: T5-T9 (Greater splanchnic n.) → Celiac ganglion Midgut: T10-T12 (Lesser and least splanchnic n.) → Superior mesenteric ganglion Hindgut: L1-L2 (Lumbar splanchnic n.) → Inferior mesenteric ganglion Post ganglionic fibres fallow blood vessels to the GI tract. Effects of ANS in the gut  In general, sympathetic stimulation causes inhibition of gastrointestinal secretion and motor activity, and contraction of gastrointestinal sphincters and blood vessels. Conversely, parasympathetic stimuli typically stimulate these digestive activities GI Sphincters 2) Prevents of reflux 1) Facilitation of digestive and protecting GI tract process: Important for from damage. LES regulation of flow, prevents stomach acid segmentation and timing from regurgitating into helps different segment of oesophagus. Pyloric GI tract to reach optimal sphincter prevents bile conditions for digestion and intestinal content and absorption at each from flowing back into stage. Sphincter of stomach. Ileocecal Hepatopancreatic valve prevents backflow regulates bile and from large intestine into pancreatic juice entry into small intestine. the duodenum. 3) Contributes to continence and voluntary defection – Anal sphincters. GI Sphincters Dysfunction of GI sphincters can lead to conditions like: Gastro-oesophageal reflux disease (GORD) Pyloric stenosis Biliary reflux Fecal incontinence or constipation Additional resource Moore’s Clinically Oriented Anatomy. Chapter 5: Abdomen https://internationalpbs.lwwhealthlibrary.com/content.aspx?sectionid=252 432839&bookid=3187#252433107 Wheater's functional histology : a text and colour atlas https://www.clinicalkey.com/student/content/toc/3-s2.0-C20190030178?ori gin=share&title=Wheater's%20Functional%20Histology&meta=2023%2C %20O%E2%80%99Dowd%2C%20Geraldine%2C%20BSc%20(Hons)%2C%2 0MBChB%20(Hons)%2C%20FRCPath&img=https%3A%2F%2Fcdn.clinicalke y.com%2Fck-thumbnails%2FC20190030178%2Fcov200h.gif