Anatomy and Physiology Part 3: Large Intestine and Defecation Marieb’s Human Anatomy and Physiology Ninth Edition Marieb w Hoehn Chapter 23 Digestive System Lecture 11 Part 3: Large Intestine and Defecation Slides 1-15; 80 min (with review of syllabus and Web sites) [Lecture 1] Slides 16 – 38; 50 min [Lecture 2] 118 min (38 slides plus review of course Web sites and syllabus)

Large Intestine The bowel includes the small and large intestines. The main forms of Inflammatory Bowel Disease are Crohn's disease and ulcerative colitis (UC). Accounting for far fewer cases are other forms of IBD, which are not always classified as typical IBD: Collagenous colitis Lymphocytic colitis Ischaemic colitis Diversion colitis Behçet's disease Indeterminate colitis The main difference between Crohn's disease and UC is the location and nature of the inflammatory changes. Crohn's can affect any part of the gastrointestinal tract, from mouth to anus (skip lesions), although a majority of the cases start in the terminal ileum. Ulcerative colitis, in contrast, is restricted to the colon and the rectum.[4] Pathophysiology in Crohn's disease vs. ulcerative colitis: Crohn's disease Ulcerative colitis Autoimmune disease Widely regarded as an autoimmune disease No consensus Cytokine response Associated with Th17[5] Vaguely associated with Th2 Microscopically, ulcerative colitis is restricted to the mucosa (epithelial lining of the gut), while Crohn's disease affects the whole bowel wall ("transmural lesions"). Finally, Crohn's disease and ulcerative colitis present with extra-intestinal manifestations (such as liver problems, arthritis, skin manifestations and eye problems) in different proportions. Rarely, a definitive diagnosis of neither Crohn's disease nor ulcerative colitis can be made because of idiosyncrasies in the presentation. In this case, a diagnosis of indeterminate colitis may be made. Although a recognised definition, not all centres refer to this. Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 *

Histology of the Large Intestine Walls of large intestine are much thinner than the small intestine, however, the lumen is larger Note lack of villi and presence of numerous goblet cells (mucus) No enzymes produced; any digestion is from previously introduced enzymes or bacteria *

Functions of Large Intestine little or no digestive function absorbs water, bile salts, and electrolytes secretes mucus (lubrication, binding, protection, pH) conversion of bilirubin (uro- and stercobilinogen) houses intestinal flora (~800 species of bacteria) and absorbs vitamins liberated by bacterial action (K, B5, and Biotin); produces intestinal gas (flatus) forms and stores feces carries out defecation Receives about 500 ml of indigestible material per day and reduces the volume to about 150 ml of feces.

The Rectum, Anal Canal, and Anus Temporary storage of fecal material in rectum triggers the urge to defecate Internal anal sphincter is usually contracted but relaxes in response to distension. External sphincter must be tensed reflexively to retain feces Rectal valves Procto- = anus or rectum (Keratinzed strat. squamous epithelium)

Movements of Large Intestine slower and less frequent than those of small intestine mixing movements (haustral churning every 30 min) mass movements - usually follow meals (stimulated by distension of stomach and duodenum) gastrocolic reflex duodenocolic reflex peristaltic wave from transverse colon through rest of large intestine

Parasympathetic Defecation Reflex Note that this reflex opens the internal sphincter and closes the external sphincter Need voluntary relaxation of the external sphincter for defecation

Feces water (75%), solids (25%) electrolytes mucus bacteria (30% of solids) and sloughed epithelial cells bile pigments altered by bacteria provide color (mainly urobilins and stercobilins) odor produced by bacterial compounds (indoles and skatoles, phenols, H2S, ammonia) indigestible materials

Review Liver Two major lobes Blood supply and drainage Functional unit is the lobule Plates of hepatocytes Portal triads Blood/bile flow in lobules Three major categories of function Metabolic regulation Hematological regulation Bile production

Review Gall Bladder (GB) Major function – storage and concentration of bile Stimulated to contract by CCK Path of bile from GB Cystic duct Common bile duct Hepatopancreatic sphincter (Oddi) – CCK relaxes Duodenal ampulla Bile Water, bile salts, bile pigments, electrolytes, cholesterol Function is emulsification of fats for digestion Absorption of fats

Review Small Intestine Three major parts Blood supply and drainage Duodenum – mixing chamber; mucus Jejunum – digestion Ileum – connects to cecum of large intestine Blood supply and drainage Suspended from the posterior abdominal wall by the mesentery Surface area greatly increased by Plicae Villi Microvilli

Review Small Intestine (cont’d) Secretions Control of secretion and absorption Absorption Protein, CHO, electrolytes – facilitated diffusion Fats via chylomicrons and lacteals -> circulation Movements Local via myenteric plexuses Long distance via stomach filling Gastroenteric reflex Gastroileal reflex

Review Large Intestine Three major parts Blood supply and drainage Cecum (veriform appendix) Colon (ascending, transverse, descending) Rectum Blood supply and drainage Haustra (pouches) Longitudinal muscle of external muscularis reduced to long strips (taeniae coli) Wall is thinner than small intestine, less muscle, no villi

Review Large Intestine (cont’d) Rectum Major Functions Temporary storage of feces – urge to defecate Internal sphincter contracted until distended External sphincter is voluntary Major Functions Absorption of water, bile salts, and electrolytes Absorbs vitamins produced by bacteria (K, biotin, B5) Converts bilirubin Movements slower and less frequent than small intestine Mass movements Stimulated by gastric and duodenal distension