Large Intestine Anatomy, Histology and Physiology PDF

Large Intestine E-Learning: Anatomy, Histology and Physiology Dr. Maria Shapoval BMS 150 Week 14 Overview Anatomy and histology of the large intestine Physiology of the large intestine Diseases of the large intestine Irritable Bowel Syndrome (IBS) Inflammator...

Large Intestine E-Learning: Anatomy, Histology and Physiology Dr. Maria Shapoval BMS 150 Week 14 Overview Anatomy and histology of the large intestine Physiology of the large intestine Diseases of the large intestine Irritable Bowel Syndrome (IBS) Inflammatory Bowel Disease (IBD) UC Crohn’s Diverticular Disease and Hemorrhoids Learning Objectives Identify the major anatomical components of the large intestine, rectum and anal canal Identify the main vessels supplying the large intestine and their respective structures Identify the sympathetic and parasympathetic nerves innervating the large intestine Compare the histology of large intestine to the typical histological findings in other parts of the digestive tract Relate the role of mucin to the function of the large intestine Compare gastroileal and gastrocolic reflexes Describe the mechanism of defecation and inhibition of defecation Describe the epidemiology, main clinical feature and pathogenesis of the common pathologies of the large intestines, including: Learning Objectives Describe the epidemiology, main clinical feature and pathogenesis of the common pathologies of the large intestines, including: Irritable bowel syndrome Inflammatory bowel disease Diverticular diseases Hemorrhoids Large Intestine – Big Picture Functions to convert undigested material into feces by removing water and adding mucus Store and transport feces Largest microbial presence Produce Vitamin K and Biotin Slower motility to allow for water and solute absorption Large Intestine: Anatomy LI begins at ileocecal valve and ends at the anus Cecum – widest part of colon and most prone to perforation Ascending colon transitions to transverse colon at the hepatic flexure, and the splenic flexure marks the transition to descending colon Sigmoid colon – narrowest part of LI and most mobile Generally located within left lower quadrant, but parts can actually migrate to the right side Vulnerable to volvulus; intestinal loop twists around itself causing bowel obstruction Rectum 12-15 cm long Denonvilliers’ fascia/ rectovaginal fascia runs anterior to rectum and separates it from prostate and seminal vesicles or vagina Rectocele can develop if fascia defective Lateral ligament support the lower portion of the rectum and provide scaffold for blood vessels and nerves Innervated by parasympathetic NS; pelvic splanchnic nerves Supplied by branches of the inferior mesenteric artery/vein (superior rectal a/v.), internal iliac artery/vein (middle rectal a./v) and internal pudendal artery/vein (inferior rectal a./v) Anal Canal 4 cm in length; connects rectum to anal opening Mucosa transitions from simple columnar epithelium to stratified squamous epithelium Submucosa: Hemorrhoidal plexuses Sebaceous glands and apocrine sweat glands Internal anal sphincter – involuntary; circular layer of smooth muscle External anal sphincter – voluntary; skeletal muscle Vasculature of the LI Superior Mesenteric Artery: Ileocolic artery Terminal ileum Proximal ascending colon Right colic artery Ascending colon Middle colic artery Transverse colon Inferior Mesenteric Artery: Left colic artery Descending colon Sigmoid branches Sigmoid colon Superior rectal artery Proximal rectum Venous Supply – same terminology as the arteries Innervation of the LI Sympathetic NS Sympathetic nerves arise from T6-T12 and L1-L3 Parasympathetic NS Vagus nerve Ascending and transverse colon S2-S4 nerve roots – pelvic splanchnic nerves Descending and sigmoid colon Large Intestine: Histology Mucosa No folds (only rectum – rectal columns of Morgagni) or villi Absorptive colonocytes do have irregular microvilli and appear to be dedicated to fluid absorption Simple columnar epithelium with lots of goblet cells Many deep crypts/ intestinal glands Lamina propria with lots of lymphoid cells and nodules (some extend into submucosa) Submucosa Lower rectum – hemorrhoidal plexus of veins; no valves – high likelihood of varicosity (aka hemorrhoids) Large Intestine: Histology Muscularis Externa Teniae coli – 3 thick longitudinal bands of smooth muscle in the outer layer with thin layer of sm between the bands Inner layer is the same as SI Serosa Appendices epiploicae – teardrop-shaped adipose-filled outpockets within the serosa throughout the colon Adventitia – surrounds rectum Digestion, Absorption, and Secretion Digestion accomplished exclusively by microbiota and breakdown products such as SCFA are used to feed the enterocytes Minimal absorption beyond water, Na+ and other minerals Vitamin K, biotin – produced by microbiota Secretes mucin proteins which attract water and create a layer of mucus on epithelial cells Lubricate and prevent intestines from sticking together Support innate immune function – acts as barrier Motility 3 types of movement: segmentation, peristalsis and mass action contraction: Simultaneous contraction of smooth muscle over large areas of LI with the goal of moving feces into rectum Hirschsprung Disease – aganglionic megacolon is a congenital disorder where ganglions are absent in myenteric and submucosal plexuses in distal colon resulting in lack of peristalsis and defecation rate of 1 every 3 weeks Gastroileal reflex Food leaving the stomach allows the cecum to relax which allows the ileocecal valve to relax and chyme to pass from SI to LI Gastrocolic reflex Large waves of peristalsis are stimulated by presence of food in the stomach after a meal Fluid Homeostasis Absorbs up to 5 L of water per day 1-2L of chyme enters LI and after 90% of fluid is absorbed only 200 mL of feces remains Water absorption is primarily driven by absorption of Na+, which differs across ascending and descending colons with greater absorption capacity in the ascending compared to descending and sigmoid colons Fluid Homeostasis There are multiple mechanisms for Na+ absorption: Electrogenic Na+ absorption based on electrochemical gradient for influx, and Na+/K+ ATPase on basolateral side Electroneutral NaCl absorption; Na+ is exchanged with H+ while Cl- is exchanged for HCO3- SCFA coupled Na+ absorption Na+ AND water absorption by colonic crypts Defecation The presence of feces causes distention of the rectum which initiates a reflex contraction of the rectal muscles and communicates to the CNS a desire to defecate Voluntary defecation Internal anal sphincter (smooth muscle) involuntarily relaxes in response to inhibitory signals from sacral parasympathetic NS and distention When not relaxing, sphincter is tonically active (promoted by sympathetic NS) Parasympathetic nerves also relax puborectalis muscle which causes the rectoanal angle to straighten External anal sphincter (skeletal muscle) can voluntarily relax and expulsion of feces may occur unaided if enough pressure has been reached (lots of distention) OR supported by contraction of abdominal muscles (aka straining/ Valsalva maneuver) Innervated by pudendal nerve Defecation The presence of feces causes distention of the rectum which initiates a reflex contraction of the rectal muscles and communicates to the CNS a desire to defecate Inhibition of defecation (aka – maintaining fecal continence) is achieved by the following 3 components: Both sphincters are tonically active Puborectalis muscle maintains contraction Angle between rectum and anus is 90 degrees Large Intestine Diseases of the Large Intestine BMS 150 Week 14 Irritable Bowel Syndrome (IBS) Most common Functional Gastrointestinal disorders (FGID) Epidemiology 9-23% of global population (other sources suggest 10-15%) Onset typically in adolescence (prevalent in