GI Module 3A Flashcards
Total length of small intestine
5-6 meters
Where does SI begin and end?
- Pyloric sphincter
- Ileocecal valve
Regions of the SI
Dudoenum, jejunum, ileum
Where does duodenum begin and end?
- Duodenal bulb
- Ligament of Treitz
What is the function of the hepatopancreatic ampulla?
Allows bile and pancreatic enzyme secretions
What are Brunner’s glands and where are they located?
Submucosal glands
Located in proximal duodenum
What do Brunner’s glands do?
Secrete bicarb rich mucus to:
- Protect against acidic chyme entering duodenum
- Lubricate intestinal wall
Functions of duodenum
- Enzymatic digestion
- Regulate rate of gastric emptying
What does acidic chyme entering the duodenum stimulate?
- Long loop reflexes (duodenum to CNS)
- Short loop reflexes (duodenum to stomach)
- Both cause inhibition of gastric motility/secretion
Acidic chyme entering duodenum stimulates release of which hormones?
- Secretin
- VIP
- CCK
- Inhibit gastric motility/secretion
What are hormones of the duodenum?
Secretin CCK GIP VIP Intestinal gastrin Motilin
What is the function of secretin?
- Regulates pH in duodenum
- Inhibitory to gastric activity
- Facilitates digestion
What stimulates secretin release?
Chyme entering duodenum
What are the target organs of secretin?
- Stomach (inhibits gastrin)
- Pancreas (to secrete watery bicarb)
- Liver (stimulates bile output)
- Brunner’s glands of duodenum (stimulates alkaline rich mucus)
What is the purpose of CCK?
- Promotes bile/pancreatic enzyme release
- Inhibitory to gastric activity
What stimulates CCK?
Chyme entering duodenum
What are the target organs of CCK?
- Stomach
- Pancreas
- Liver
- Gallbladder (release of bile)
What is GIP?
- Gastric inhibitory peptide
- “Relative” of secretin
What stimulates GIP release?
Chyme entering duodenum
What are the target organs of GIP?
- Stomach
- Pancreas (to release insulin)
What is VIP?
Vasoactive intestinal peptide
What is VIP stimulated by?
Chyme entering duodenum
What are the target organs of VIP?
- Stomach
- Intestine (vasodilation, promote motility)
What is the function of intestinal gastrin?
Similar to role of antrum gastrin (stimulates motility/secretions)
What stimulates release of intestinal gastrin?
Chyme entering duodenum
What is motilin and what does it do?
- Housekeeper of intestinal tract
- Initiates MMC (migration motor complex)
What stimulates motilin?
- Fasting
- Periodic release (q few hrs)
Where do the jejunum and ileum begin and end?
- Ligament of Treitz
- Ileocecal valve
Role of jejunum
Major site of digested fat, carb, protein, water, electrolyte absorption
Role of ileum
Absorbs Vit B12, bile salts, remaining digested nutrients and water not absorbed in jejunum
What are plica?
Mucosal folds in jejunum/ileum that physically slow passage of food
What are villi?
- Cover the mucosal folds of jejunum/ileum
- Considered function unit of SI (site of both secretion and absorption)
What are villi composed of?
- Goblet cells (mucus)
- Absorptive columnar cells w/microvilli on end of each
What do microvilli form?
- “Brush border” of mucosal surface
- Thin layer of fluid is found along surface
What is the function of the “brush border” layer of the mucosa in the jejunum/ileum and what forms it?
- Facilitates absorption of all substances except water/electrolytes (they go b/w cells)
- Formed by microvilli
What is the lamina propria?
Layer that extends into each villi in jejunum/ileum to optimize absorption
What does the lamina propria contain?
Lacteal and central arteriole
Function and location of central arteriole?
- Transport substances directly to liver via hepatic portal vein
- Located in lamina propria layer of jejunum/ileum
Function and location of lacteal?
- Transport fats to systemic circulation via thoracic duct
- Located in lamina propria layer of jejunum/ileum
What are Crypts of Lieberkuhn?
- At base of villi in jejunum/ileum
- Optimizes absorption
What are Crypts of Lieberkuhn composed of?
- Precursor cells of SI eipthelium (goblet, columnar)
- Paneth cells (immune)
- Secretory cells
Function of precursor cells in Crypts of Lieberkuhn
- Migrate to tip of villi
- Mature SI epithelial cells slough off and serve as a source of protein
- Complete turnover of SI is approx 4-7 days
Complete turnover of SI epithelium occurs in approx:
4-7 days
Function of paneth cells in Crypts of Lieberkuhn
- Immune function
- Produce/secrete antibiotic peptides
Function of secretory cells in Crypts of Lieberkuhn
Secrete brush border digestive enzymes
Average transit time in the small intestine =
1-3 hours (large variations however)
Which hormones stimulate SI motility?
- Secretin
- CCK
- Intestinal gastrin
3 motility patterns that occur in SI:
- Segmentation
- Peristalsis
- Migrating motor complex (MMC)
What is segmentation?
- Frequent small rhythmic contractions of circular muscle
- 8 to 12 per min
- More frequent than peristalsis
Function of segmentation
Mixes chyme to allow contact with brush border
Which is more frequent - segmentation or peristalsis?
Segmentation
Which motility patterns occur during feeding and which during fasting?
- Feeding: segmentation and peristalsis
- Fasting: MMC
What is peristalsis?
- Coordinated waves of contraction/relaxation of longitudinal muscles
- Short segments of 10 cm and SLOW (1-2 cm/sec)
Function of peristalsis
Move chyme toward large intestine
What is migrating motor complex (MMC)?
- Slow periodic waves of peristalsis that occurs every 1-2 hours
- Originates in stomach and passes through SI
- Motilin helps stimulate this
Function of MMC
“House cleaning”
- Sweeps out stomach/SI
- Pushes along residual chyme, non-digested substances
- Bacteria homeostasis
What are the intestinal motility reflexes?
- Ileogastric reflex
- Intestinointestinal reflex
- Gastroileal reflex
What stimulates the ileogastric reflex? What is its action?
- Ileum distension
- Inhibits gastric motility
- This adds more chyme to SI to allow terminal ileum to empty contents to LI
What stimulates the intestinointestinal reflex? What is its action?
- Section of SI distension
- Relaxes distal SI
- Allows chyme to be moved toward LI
- Inhibitory protective reflex
What stimulates the gastroileal reflex? What is its action?
- Increased gastric motility/secretion
- Promotes terminal ileum motility/ileocecal valve relaxation
- Empties the SI so it can receive more chyme from stomach
Is the ileocecal valve normally open or closed?
Closed
How is opening of the ileocecal valve regulated?
Approaching peristaltic wave in ileum stimulates relaxation of sphincter
*Opens similar to LES/pyloric valve
How is closing of the ileocecal valve regulated?
Distension of cecum/ascending LI stimulates constriction of sphincter
Where does the LI begin and end?
- Ileocecal valve
- Anus
What are the regions of the LI?
- Cecum
- Appendix
- Colon (also has regions)
- Rectum
- Anus
What are the regions of the colon?
Ascending, transverse, descending, sigmoid
What are teniae coli?
3 longitudinal bands of muscle along length of colon
What are haustra?
Pouches formed from circular muscle layer of LI
What is the O’Bierne sphincter?
Controls passage of chyme from sigmoid colon into rectum
What are haustra segment contractions and when do they occur?
-Individual haustra segments push contents back and forth
-During fasting
(occurs 90% of the time)
What are multihaustral segment contractions and when do they occur?
- Several segments contract and relax as a single unit
- Push contents a short distance forward
- During fasting (occurs 10% of the time)
What is the function of mass movement peristalsis?
Promotes emptying of the intestine into the sigmoid colon and rectum
What are the intestinal reflexes of the colon?
- Orthocolic
- Gastrocolic
- Rectal
What is the orthocolic reflex and what stimulates it?
- Peristalsis that propels fecal mass into sigmoid colon and rectum
- Occurs upon awakening
- Standing stimulates
What is the gastrocolic reflex and what stimulates it?
- “Continuation” of gastroileal reflex
- Peristalsis propels fecal mass into sigmoid colon and rectum
- Ingestion of food stimulates this
What is the rectal reflex and what stimulates it?
- Parasympathetic relaxation of internal anal sphincter and the urge to have BM
- Stimulated by stretch of rectum wall
What is cauda equina syndrome?
- Damage to nerve roots of lower spine
- Results in loss of bowel control
Where does carb digestion begin?
Mouth
Which digestive enzymes work to break down carbs?
- Salivary amylase (mouth)
- Pancreatic amylase
- Brush border enzymes (lactase, maltase, sucrase)
How and where does carb absorption occur?
- Brush border of villi
- Monosaccharides are absorbed by villi capillaries and sent directly to liver
- Insulin is NOT required for intestinal uptake of glucose
Where does protein digestion begin?
Stomach
Which enzymes work to break down proteins?
- Pepsin
- Pancreatic enzymes (trypsin)
- Brush border enzymes (peptidases)
How are proteins absorbed and where?
- Villi in small intestine
- Amino acids are absorbed and transported directly to liver
How are fats digested?
- Emulsification
- Small fat particles are surrounded by emulsifying agents and prevent them from re-forming into larger fat droplets
Where does digestion of fats begin and how does it proceed?
- Stomach by gastric lipase
- Then in the SI = emulsification and lypolysis
How are fats absorbed?
- Must be made water soluble
- This is accomplished by formation of micelles
What are micelles and what is their purpose?
- Water soluble molecules
- Makes fats water soluble in order to reach intestinal epithelium for absorption
What are the steps in fat absorption?
- Micelles formed
- Micelles pass through aqueous brush border and contact epithelial cell
- Fat contents then diffuse into epithelial cell
- Fats are resynthesized into TGs and phospholipids (aka chylomicrons)
- Chylomicrons exit epithelial cell into lacteals of villi and are transported into systemic circulation
How do chylomicrons exit the intestinal epithelial cell?
Exocytosis
Total ingested/secreted fluid into GI tract per day =
7.5 to 8 L
What percentage of fluid in the small intestine is absorbed back into the blood stream?
Approx 85-90%
What is the LI’s role in fluid management?
It “fine tunes” fluid and electrolyte homeostasis
In normal circumstances, how much fluid is excreted in stool?
100 mL (1/10 L)
Where does Vit B12 absorption take place and what does it require?
- Terminal ileum
- Requires binding with intrinsic factor
