Large Intestine Anatomy

• The large intestine's primary function is to contain undigested, unabsorbed materials, volume has decreased from small intestine

Anatomy of the Large Intestine

• Length is approximately 1.5 meters
• Diameter is approximately 6.5 cm
• Cecum: Opens into the small intestine and forms a pocket that leads into the large intestine
• Ileocecal sphincter: Valve at the end of the ileum which leads into the cecum wall
• Vermiform appendix: A small connection to the appendix with lymphatic tissue that may aid in microbial repopulation, unknown function
• Colon: Comprises most of the large intestine and has 4 regions
• Rectum: The region that leads into the anal canal
• Rectum's length is 15 cm
• Mucosa: Simple columnar epithelium
• Muscularis: Thicker than other regions of GI tract
• Anal canal: Connects to the external opening
• Anal canal length is 2-3 cm of the GI tract
• Mucosa: Stratified squamous epithelium (protection from abrasive waste)
• Muscularis: Very thickened and forms sphincters
• Anus: External opening for expulsion of feces
• Internal anal sphincters: Thickened smooth muscle walls (involuntary control)
• External anal sphincters: Thickened skeletal muscle walls (voluntary control)
• Anal columns: Ridges in the mucous membrane that run longitudinally to the canal, containing arteries and veins; when inflamed, hemorrhoids are formed

Colon

• Four regions: ascending, transverse, descending, and sigmoid
• Ascending colon: Rises on the right side of the body; build-up from cecum gathers here
• Transverse colon: Lays between the liver and stomach
• Right colic flexure: Sharp turn beneath the liver from the ascending into the transverse colon
• Left colic flexure: Sharp turn beneath the spleen from transverse into descending colon
• Descending colon: Moves downwards on the left side of the body
• Sigmoid colon: S-shaped region which returns colon to the midline of the body
• Ascending and descending colon only contacts peritoneum on some regions of anterior lateral sides; retroperitoneal
• Peritoneum: Visceral peritoneum surrounds the transverse and sigmoid colon; connects to abdominal wall by mesocolon
• Omental appendices: Small lipid-filled pouches of peritoneum that attach onto teniae coli

Histology

• Serosa: Normal. Boring
• Muscularis: Thicker than other regions of GI tract; has internal circular layer, myenteric plexus, and external longitudinal fibers
• Circular muscles: Thickened layer of muscle; teniae coli: 3 bands of thickened longitudinal muscle (longitudinal muscle does not cover the whole tube, just these regions)
• Haustra: Pockets formed by the contraction of teniae coli
• Submucosa: Contains arteries, veins, and lymphatic vessels
• Mucosa: Comprises muscularis mucosae and lamina propria
• Pores: Formed by fingerlike projections in the membrane
• Lymphatic nodules: Formed by lymphatic tissue
• Intestinal glands: Also formed by projections; open into the lumen
• Absorptive cells: Absorb water (as well as some ions and vitamins); have a microvilli surface
• Goblet cells: Secrete mucus to aid the movement of chyme (especially as it turns into feces)

Mechanical Digestion

• Gastroileal reflex: When the stomach is full, gastrin relaxes the ileocecal sphincter
• Gastroileal reflex: Creates space in the small intestine (by emptying into large intestine) so that the stomach can empty into the small intestine
• Ileocecal sphincter: Usually closed to keep chyme in the small intestine to allow full absorption
• Haustral churning: Contraction in haustra to move chyme along
• Haustral churning initiation: Distention of haustra initiates contraction
• Peristaltic waves: 3-12 contractions per minute (slower but similar to what we see in other regions)
• Gastrocolic reflex: When the stomach is full, signals mass peristalsis
• Mass peristalsis: Strong peristaltic wave starting from the middle of the transverse colon, pushes contents into the rectum
• Defecation reflex: Signals relaxation of the internal anal sphincter
• Receptors: Stretch receptors in the rectum signal the sacral spinal cord
• PSNS: Stimulates contraction of the descending and sigmoid colon, and longitudinal muscles of the rectum; the internal anal sphincter relaxes
• External anal sphincter: Skeletal muscle, must be voluntarily opened to allow defecation, else the feces just moves back up to the sigmoid colon
• Local reflexes: Distention can cause contraction of the rectum and colon, and relaxation of the internal anal sphincter

Chemical Digestion

• Does not secrete enzymes (just mucus), not really digesting things for absorption anymore
• Microbes: Bacteria ferment any remaining materials
• Carbs: Broken down into gases (H2 CO2, CH4) which contribute to flatulence
• AAs: Broken down into indoles and skatoles, which give feces its odor
• Bilirubin: Decomposed into stercobilin, which gives feces its color
• Vitamins: Bacteria produce vitamins K and B, which can be absorbed along with water and ions in the large intestine
• Homeostasis: Less microbial diversity is associated with a higher incidence of obesity
• Feces: Remaining dead epithelial cells (sloughed off walls of GI tract), undigested food, bacteria (alive and dead), metabolic byproducts (e.g. stercobilin)
• Dehydration: After 3-10 hours in the large intestine, 90% of remaining water has been removed
• Bacteria: Makes up approximately 30% of dry weight

Phases of Digestion

• Cephalic: Prepares mouth and stomach
• Cephalic phase timing: Before, or just as, the food is eaten
• Cephalic phase action: Stimulates gastric secretion and motility
• Cephalic phase triggers: Sensory information from the environment (sight, smell, thought, taste)
• Cephalic phase neural control: Senses signal the cerebral cortex, hypothalamus, and brain stem, which stimulates salivation (CN VII and IX) and gastric glands (CN X)
• Gastric: Promotes gastric secretions and motility to break bolus into chyme
• Gastric phase timing: Once the bolus reaches the stomach
• Gastric phase action: Stimulates gastric secretion and motility, promotes gastric emptying
• Gastric phase triggers: Increase in stretch on walls or pH (incoming proteins buffering the gastric acids)
• Gastric phase hormonal action: G cells release gastrin > release gastric juices, constrict lower esophageal sphincter (prevent reflux), increases stomach motility (peristaltic waves), and relaxes the pyloric sphincter (allow gastric emptying)
• Gastric phase neural action: Submucosal plexus stimulated > signals sent to PSNS and ENS > stimulates peristaltic waves and gastric juice secretion
• Intestinal: Promotion of digestion by limiting influx into small intestine
• Intestinal phase timing: Once chyme reaches the small intestine
• Intestinal phase action: Slows gastric emptying
• Secretin: Acidic pH > release of pancreatic juice and inhibits gastric secretions
• Cholecystokinin: AA and fats > release pancreatic juices, contract gallbladder, relax hepatopancreatic ampulla; contract pyloric sphincter; stimulate satiety (hypothalamus)
• Intestinal phase neural action: Stretch in duodenum > enterogastric reflex > signals MO > inhibits PSNS, stimulates SNS > inhibits gastric motility, increases contraction of pyloric sphincter