1. 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)

2. 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 *

3. 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 *

4. 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.

5. 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)

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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