OS 206 Small & Large Intestines, Anorectum PDF UPCM 2029

OS 206: ABDOMEN AND PELVIS SMALL & LARGE INTESTINES, ANORECTUM UPCM 2029 | Dr. Sylvia Alip | LU3 A.Y. 2024-2025 OUTLINE I.​ General Overview of IV.​ Rectum Organs and Suborgans...

OS 206: ABDOMEN AND PELVIS SMALL & LARGE INTESTINES, ANORECTUM UPCM 2029 | Dr. Sylvia Alip | LU3 A.Y. 2024-2025 OUTLINE I.​ General Overview of IV.​ Rectum Organs and Suborgans A.​ Segments, Location, II.​ Small Intestine Position Overview B.​ Vascularity A.​ Segments, Location, V.​ Anal Canal & Anus Position A.​ Components B.​ Embryology B.​ Embryology & C.​ Lymphovascularity Lymphovascularity III.​ Large Intestine VI.​ Innervation of Overview Abdominal Viscera A.​ Related VII.​ References Terms/Concepts/Stru ctures B.​ Embryology C.​ Segments, Location, Position, & Lymphovascularity I.​ GENERAL OVERVIEW OF ORGANS AND SUBORGANS ​ Small intestine ○​ Duodenum ○​ Jejunum ○​ Ileum ​ Ileocecal valve - sphincter that demarcates small intestine and large intestine ​ Large intestine ○​ Colon ​ Cecum ​ Ascending/right colon ​ Transverse colon ​ Descending/left colon Figure 1. Mesentery (top = sagittal; bottom = coronal) ​ Sigmoid colon ○​ Appendix ​ Rectum ​ Anal canal II.​ SMALL INTESTINE OVERVIEW A.​ FUNCTION, SEGMENTS, LOCATION, POSITION FUNCTION ​ Digestion ​ Nutrient absorption ​ Electrolyte regulation ​ Control of peristalsis SEGMENTS ​ Three segments: duodenum, jejunum, ileum ​ 6-7 meters long in total Table 1. DUODENUM, JEJUNUM, ILEUM DUODENUM JEJUNUM ILEUM Remaining 2/5 of Remaining 3/5 of the 6 to 7-meter long the 6 to 7-meter long small intestine small intestine Mostly in the right Mostly in left upper lower quadrant quadrant (LUQ) ​ Shortest (RLQ) ​ Widest Thick and deep red Thin and pale pink ​ Most fixed small (more vascular, less (less vascular, more intestine fat in mesentery) fat in mesentery) ​ Partly retroperitoneal Long vasa recta with Short vasa recta with large arcades short arcades Large and dense Low and sparse plicae circularis plicae circularis Figure 2. Root of the mesentery Fewer lymphoid Many lymphoid nodules nodules B.​ EMBRYOLOGY ​ Derivative of the midgut from the duodenum distal to the opening LOCATION of the bile duct, cecum, appendix, ascending colon, ½ to ⅔ of the transverse ​ Gastric material passes sequentially from the duodenum → ○​ Midgut derivatives are supplied by the superior mesenteric jejunum → ileum → cecum artery (SMA) ​ Held in place by a fan-shaped peritoneum, a mesentery that ​ The midgut forms a loop that herniates into the umbilical cord (6th contains the superior mesenteric vessels, lymph nodes, fat, and week age of gestation [AOG]) before it returns while rotating 270° autonomic nerves counterclockwise into the enlarging abdomen (starting at 10 week ○​ The root of the mesentery is 15 cm long, and 20 cm in diameter and completed by 11 weeks AOG) C.​ LYMPHOVASCULARITY VASCULAR ​ Superior mesenteric artery (SMA): arises from the abdominal aorta at the level of L1 ○​ Sends off 15-18 branches to the jejunum and ileum ○​ Arteries unite to form loops called arterial arcades Trans 6 TG17: Mallari, Mamaril, Mancenido, Manzano, Marcelo, Mariano A, Mariano R TH: Oribello 1 of 7  ○​ Arcades give rise to straight arteries called the vasa recta ​ Cecum - blind intestinal pouch ​ Superior Mesenteric Vein: also in the root of the mesentery to the ○​ 7.5 x 7.5 cm right of the artery ○​ Completely intraperitoneal with no mesentery ○​ Unites with the splenic vein to form the hepatic portal vein ○​ The ileal orifice enters the cecum between the ileocolic lips behind the neck of the pancreas ​ Serve as a valve to prevent reflux back into the ileum when the cecum distends with fecal material (in theory, but rarely observed) ​ Appendix - blind intestinal diverticulum with lymphoid tissue ○​ 6-10 cm length ○​ Mesoappendix: its triangular mesentery, with the lymphoid tissue ○​ SMA → ileocolic artery → appendicular artery ​ Clinical correlate: appendicitis ○​ Inflammation of the appendix from hyperplasia of lymphoid follicles (younger) or obstruction from a fecalith (older) ○​ Symptomatology: ​ Initially, vague pain in the periumbilical area (T10 visceral afferents) - visceral pain ​ As inflammation progresses: somatic pain is felt from irritation of parietal peritoneum in the RLQ or McBurney point ​Pain may be aggravated by hip extension (iliopsoas irritation) ​ Appendiceal rupture causes generalized peritonitis, severe pain and tenderness in the entire abdomen ​ Clinical correlate: appendectomy ○​ Surgical removal of the appendix Figure 3. Small intestine vasculature ○​ Indications: Acute appendicitis, ruptured appendix, appendiceal LYMPH abscess ​ Lacteals: specialized intestinal villi for fat absorption → lymphatic ○​ Types: plexuses in the intestinal walls → lymphatic vessels → three groups ​ Open Appendectomy: Single incision in the lower right of nodes abdomen ○​ Juxtointestinal Lymph Nodes - close to the intestinal wall ​ Laparoscopic Appendectomy: Minimally invasive, small ○​ Mesenteric Lymph Nodes - along the arterial arcades incisions with a camera ○​ Superior Central / Central Superior Lymph Nodes - along the ○​ Complications include Infection, bleeding, bowel obstruction, proximal SMA abscess formation ​ Terminal ileum (adjacent to the cecum) drains into the ileocolic lymph nodes Figure 6. Appendectomy B.​ EMBRYOLOGY ​ Derivative of the hindgut ​ Extends from the left 1/3 to 1/2 of the transverse colon, descending colon, sigmoid colon, rectum, and superior 1/3 of the anal canal ​ Blood Supply: All hindgut derivatives are supplied by the inferior mesenteric artery (IMA) Figure 4. Small intestine lymphatics III.​ LARGE INTESTINE OVERVIEW A.​ RELATED CONCEPTS/TERMS/STRUCTURES ​ Large intestine ○​ Digestion, water absorption ○​ Electrolyte regulation ○​ Control of elimination reflexes ​ The following distinguishes the large intestine from the small intestines: ○​ Omental appendices (appendices epiploicae) - small, fatty omentum-like projections ○​ Teniae coli - longitudinal bands of smooth muscle that originate from the base of the appendix to the rectosigmoid junction ​ Mesocolic tenia - the mesenteries of the transverse and sigmoid colons (mesocolons) attach ​ Omental tenia - omental appendices attach ​ Free tenia (tenia libera) nothing is attached ○​ Haustra - sacculations that form between the tenia because of tonic contractions Figure 7. Embryological origins of the GI organs C.​ SEGMENTS, LOCATION, POSITION, & LYMPHOVASCULARITY ​ Ascending colon ○​ From the cecum to the right colic (hepatic) flexure ○​ Position: 2° Retroperitoneal in most ○​ Blood Supply: SMA → Right colic artery → Marginal artery of Drummond (juxtacolic artery) Figure 5. Omental appendices (top black arrow) and teniae coli (bottom black arrow) OS 206 Small & Large Intestines, Anorectum 2 of 7  Figure 8. Ascending colon of the large intestine ​ Transverse Colon Figure 11. Sigmoid colon of the large intestine ○​ Longest, most mobile segment IV.​ RECTUM ○​ From the right colic (hepatic) flexure to the left colic (splenic) flexure ​ Temporary storage of fecal material ○​ Position: Intraperitoneal; has a mesentery (transverse ​ Control of defecation and elimination mesocolon) ​ Maintenance of continence ○​ Blood Supply: SMA → Middle colic artery → Marginal artery of A.​ SEGMENTS, LOCATION, POSITION Drummond (juxtacolic artery) LOCATION, POSITION ​ Pelvic part of the digestive tract ​ The rectosigmoid junction lies anterior to the S3 vertebra ​ The rectum ends at the level of the tip of the coccyx ​ Peritoneum covers the anterior and lateral surfaces of the upper third, the anterior surface of the middle third, and no surface of the inferior third ○​ The inferior third is subperitoneal ○​ Lowest points of the rectum in anatomic position (standing) ​ Rectovesical pouch in males ​ Rectouterine pouch of Douglas in females ​ When the patient has ascites, it goes to these spaces first Figure 9. Transverse colon of the large intestine ​ Descending Colon ○​ From the left colic (splenic) flexure to the iliac fossa ○​ Position: 2° Retroperitoneal ○​ Blood Supply: IMA → Left colic artery → Marginal artery of Drummond (juxtacolic artery) Figure 12. Rectum SEGMENTS, FOLDS, FLEXURES ​ Flexures of the rectum ○​ Sacral flexure - follows the curve of the sacrum ○​ Anorectal flexure - roughly 80° and is important for continence; maintained by the puborectalis muscle ​ Also maintained by the levators, along with the puborectalis muscle, like a sling ○​ Lateral flexures - formed in relation to 3 internal infoldings called transverse rectal folds (thickened parts of the circular muscle layer) ​ Superior - left side ​ Intermediate - right side ​ Inferior - left side Figure 10. Descending colon of the large intestine ​ Sigmoid Colon ○​ Features: Prominent omental appendices ○​ From the iliac fossa to the rectosigmoid junction (S3) ○​ Position: Intraperitoneal; long mesentery (sigmoid mesocolon) ○​ Blood Supply: IMA → 3–4 sigmoidal arteries → Marginal artery of Drummond (juxtacolic artery) OS 206 Small & Large Intestines, Anorectum 3 of 7  V.​ ANAL CANAL & ANUS ​ Control of defecation and elimination ​ Maintenance of continence ​ Extends from the superior aspect of the pelvic diaphragm (or the levators), beginning at the narrowing of rectal ampulla, to the anus ○​ Rectal ampulla is the sudden opening before anal canal narrows D.​ COMPONENTS EXTERNAL Figure 13. Flexures of the Rectum B.​ VASCULARITY ARTERIES ​ IMA → Superior rectal artery (only 1; after this, everything is bilateral) ○​ Upper rectum ​ Common iliac → right & left Internal iliac artery → middle rectal arteries (2: right and left) ○​ Middle and lower rectum ​ Internal pudendal → inferior rectal arteries (2: right and left) Figure 16. External components of anal canal and anus. ○​ Anorectal junction and anal canal ​ Internal anal sphincter ○​ Thickening of the circular muscle in the upper ⅔ ○​ Receives both autonomic and somatic innervation ​ Contraction (tonus) is stimulated and maintained by sympathetic fibers from the superior rectal (peri-arterial) and hypogastric plexusesMoore ​ Contraction is inhibited by parasympathetic fiber stimulation, both intrinsically in relation to peristalsis and extrinsically by fibers conveyed by the pelvic splanchnic nervesMoore ​ External anal sphincter ○​ Blends with the puborectalis which covers it ○​ It is composed of: Deep, subcutaneous, superficial zones ○​ Innervated by inferior rectal n. (majority coming from S4) INTERNAL Figure 14. Arteries of the Rectum VEINS ​ 2 venous systems ○​ Portal ​ Hepatic portal vein: main venous drainage of portal system ○​ Systemic ​ IVC: main venous drainage of systemic system ​ Veins form plexuses, unlike arteries which are mostly solitary. ○​ That means cutting an artery has worse implications than cutting a vein ​ Superior rectal vein → IMV → splenic vein → hepatic portal vein ​ Middle rectal vein → internal iliac vein → common iliac vein → inferior vena cava ​ Inferior rectal vein → internal pudendal vein → internal iliac vein → common iliac vein → inferior vena cava ​ Rectal venous plexus - collection of veins that communicate freely with the vesical and vaginal venous plexus ○​ Internal rectal venous plexus - deep to the mucosa of the anorectal junction ○​ External rectal venous plexus - subcutaneous and external to the muscular wall Figure 17. External and internal components of anal canal and anus. ​ Anal columns ○​ longitudinal ridges that contain the terminal ends of the superior rectal artery and vein ​ Superiorly: anorectal junction ​ Inferiorly: anal valves ​ Recesses: anal sinuses ​Small spaces that exude mucus when compressed by feces to aid evacuation or defecation Figure 15. Veins of the Rectum OS 206 Small & Large Intestines, Anorectum 4 of 7  E.​ EMBRYOLOGY & LYMPHOVASCULARITY Figure 19. Esophageal varices (left), caput medusae (middle), and hemorrhoids (right) VI.​ INNERVATION OF ABDOMINAL VISCERA ​ Sympathetic Innervation (efferent) 1.​ (Presynaptic) Splanchnic nerves: Intermediolateral nuclei (lateral horns of T5-L2/3) → sympathetic trunks 2.​ Sympathetic ganglia 3.​ (Post-synaptic) Peri-arterial plexuses ○​ Accompanied by visceral afferent (orad to the mid-sigmoid) conveying pain sensations ​ Parasympathetic Innervation (efferent) 1.​ (Presynaptic) Pelvic splanchnic nerves (from anterior rami of S2-4), Vagal trunks 2.​ Peri-arterial plexus Figure 18. Pectinate and white line in the anal canal 3.​ (Post-synaptic) Intrinsic enteric ganglia ​ Superior 2/5 from hindgut (endoderm), inferior 1/3 from anal pit ○​ Accompanied by visceral afferent conveying reflex sensations (ectoderm) and pain sensations (aborad from mid-sigmoid) ○​ Demarcated by pectinate (dentate line) ​ Plexus - nerve networks containing both sympathetic and ​ Aside from midgut-hindgut, there is also endoderm-ectoderm parasympathetic fibers differentiation further down a.​ Extrinsic plexuses - surround the abdominal aorta and ​From the hindgut, that is demarcated by the pectinate line branches or dentate line, that’s all supplied by superior rectal artery, b.​ Intrinsic plexuses = enteric nervous system - consists of which comes from the IMA; all lymph drains to internal iliac the myenteric plexus of Auerbach and submucosal plexus of lymph nodes Meissner ​Below the pectinate line, from the anal pit, it is supplied by ​ Ganglion - nerve relay points, most often where pre- and inferior rectal artery and drains to superficial inguinal post-synaptic fibers converge lymph nodes A.​ SYMPATHETIC ​ White line or anocutaneous line ○​ Before this line is columnar; beyond it’s stratified squamous ​ Responsible for producing vasoconstriction (keratinized in anal skin) ​ Inhibits peristalsis ​ Sphincters: tonic contraction is stimulated F.​ CLINICAL CORRELATE HEMORRHOIDS & THE PORTOCAVAL ANASTOMOSIS ​ Hemorrhoids is the enlargement of the terminal plexuses (e.g., superior rectal artery) ​ Areas of connection between portal and systemic venous systems (i.e., site of anastomoses between portal and systemic/caval) ○​ If portal is obstructed, blood will go to the caval and vice versa ○​ If inferior vena cava is ligated, it’s not so bad, because there is extreme collateralization by the anastomoses ○​ The two venous systems connect at three points aside from other pathologic diseases: esophagus, rectum, and paraumbilical Table 2. ESOPHAGUS, RECTUM, PARAUMBILICAL Figure 20. Sympathetic nerve fibers. TWO VENOUS ESOPHAGUS RECTUM PARAUMBILICAL B.​ PARASYMPATHETIC SYSTEM ​ Responsible for promotion of peristalsis and secretion in tandem Left gastric Superior Paraumbilical with hormonal regulation PORTAL vein rectal vein veins ​ Sphincters: tonic contraction is inhibited Middle & Superior SYSTEMIC/CAVAL Azygos vein inferior rectal epigastric veins veins ​ In patients with liver cirrhosis, obstruction of the liver leads to portal hypertension, causing congestion within the portal venous or portocaval system. ​ As a result, blood is diverted to the systemic circulation through three major portosystemic anastomoses, leading to characteristic clinical manifestations: ○​ Esophageal varices – Engorged veins at the lower esophagus due to shunting between the left gastric vein (portal) and esophageal veins (systemic) ○​ Caput medusae – Dilated umbilical veins due to porto-systemic shunting at the periumbilical region ○​ Hemorrhoids – Dilated rectal veins due to shunting between the Figure 21. Parasympathetic nerve fibers. superior rectal vein (portal) and middle/inferior rectal veins (systemic) VII.​ REFERENCES ​ These three features should always be assessed in cirrhotic Alip, S. (2025). Small & Large Intestine, Anorectum. patients as indicators of portal hypertension and potential Moore’s Clinically Oriented Anatomy, 9th Edition (2023). complications. OS 206 Small & Large Intestines, Anorectum 5 of 7  APPENDIX Figure 13. Flexures of the Rectum ○​ Figure 14. Arteries of the Rectum ○​ Figure 15. Veins of the Rectum OS 206 Small & Large Intestines, Anorectum 6 of 7  Figure 17. External and internal components of anal canal and anus. OS 206 Small & Large Intestines, Anorectum 7 of 7