Summary Cluster Abdomen - The GI Tract

Cases

I. Anatomy

1. Stomach

- Innervation:
- Sympathetic: thoracic splanchnic nerves
- Parasympathetic: vagus nerve
- Vascularization
- Arterial: left and right gastric arteries, left and right gastroomental arteries
- Histology
- Superficial epithelial: bicarbonate secretion => barrier
- Mucous neck: mucus production => barrier
- Parietal: HCl production
- Chief: pepsinogen, lipase production
- Enteroendocrine: chemical messengers into the interstitial fluid of the lamina
propria
- Hormones
- Gastrin:
- Stimulates: ECL cells to release histamine, motility
- Histamine:
- Stimulates: parietal cells to release HCl
- Serotonin:
- Stimulates: stomach muscle contraction
- Somatostatin:
- Decreases: secretions, absorption
- Ghrelin:
- Stimulates: hunger
- Pepsinogen:
- Requires low pH to be activated
- HCl forms it into pepsin -> active proteolytic enzyme
- Other
- Intrinsic factor: binds to vitamin B12

- Acid secretion
- Gastrin -> ECL histamine production -> parietal HCl production
- Parietal cells are also stimulated by ACh
- Works by hydrogen-potassium pump

2. Intermediate Organs: Liver, Gallbladder, Pancreas

- Liver
- Functions in digestion
- Bile production
- Storage of fat-soluble vitamins
- Gallbladder
- Bile secretion
- Fat emulsification: micelle formation

,- Pancreas
- Retroperitoneal, epigastric, left hypochondric regions
- Stomach: anteriorly, superiorly
- Duodenum: anteriorly, medially, curves around
- Spleen: posteriorly, laterally
- Aorta, vena cava: posterior to the head of the pancreas
- Blood supply: branches of celiac and superior mesenteric arteries
- Drainage: portal vein to liver
- Exocrine
- Tubuloacinar glands -> ducts -> main pancreatic duct
- Proteases: break proteins into amino acids
- Amylase: break polysaccharides into small sugar (maltose, glucose)
- Maltase: breaks maltose into monosaccharides
- Lipase: breaks triglycerides into fatty acids and monoglycerides
- Nuclease: breaks RNA/DNA into ribose/deoxyribose and 4
nitrogenous bases
- HCO3- neutralizes H+ arriving from the stomach
- Endocrine
- Islets of Langerhans (in the tail)
- Alpha cells: glucagon
- Beta cells: insulin
- D (delta) cells: somatostatin -> inhibit alpha and beta cells,
exocrine function
- PP (gamma) cells: pancreatic polypeptide -> inhibit exocrine
function

3. Small Intestine

- Hormones
- CCK
- Stimulates: gallbladder muscle contraction, secretion of pancreatic juice
- Decreases: gastric acid secretion
- GIP (gastric inhibitory peptide)
- Stimulates: insulin release
- Motilin
- Stimulates: motor complex, bile release, pancreatic enzyme release
- Secretin:
- Stimulates: HCO3- rich pancreatic juice, CCK action
- Decrease: gastric secretion/motility

- Digestion
- Enzymes
- Intestinal brush border enzymes
- Hydrolyze maltose/sucrose/lactose to constituent
monosaccharides
- Carboxypeptidase: splits off amino acids
- aminopeptidase/dipeptidase: liberate final amino acids
- (Chymo)trypsin:

, - Cleaves proteins into smaller peptides

- Absorption
- Ileum: major absorption site
- Via blood stream or lymphatics
- Vitamin B12 absorbed in the terminal ileum when bound to intrinsic factor
- Bile salt cycling and secondary production

4. Large Intestine

- Ascending and transverse colon
- Water absorption
- Metabolite absorption
- Vitamine absorption: vitamin K, thiamine, riboflavin
- Recycling: fermentation of carbohydrates, short chain fatty acids, urea cycling
- Descending Colon
- Fecal storage
- Sigmoid Colon
- Muscular contraction to increase colon pressure and induce movement of stool
into the rectum
- Rectum
- Holds the feces awaiting elimination

- Hormones
- VIP (vasoactive intestinal peptide)
- Stimulates: secretions, capillary and smooth muscle dilation
- Decrease: gastric acid secretion
- Peptide YYY (postprandially)
- Decrease: appetite in the hypothalamus

5. Digestion and Absorption

- Carbohydrates
- Digestion
- Salivary amylase (stomach): to oligosaccharides
- Pancreatic amylase (duodenum): various oligosaccharides (ie. maltose)
- Intestinal brush border enzymes (intestine)
- Dextrinase, glucoamylase: to monosaccharides
- Maltase, sucrase, lactase: to monosaccharides via hydrolyzation
- Absorption
- Only mono/disaccharides can be absorbed
- Glucose/galactose:
- Into cells: secondary active transport (Na+)
- Exit cells: facilitated diffusion
- Into capillaries: intercellular clefts

- Proteins
- Digestion
- Pepsin (stomach): cleaves proteins bonds

, - Trypsin/chymotrypsin (small intestine): cleaves proteins to peptides
- Intestinal brush border enzymes: splits amino acids from the end of the
chain and liberate final amino acids
- Absorption
- Carriers: coupled to active Na+ transport
- Short chains: active absorption via H+ - dependent cotransport
- Into capillaries: diffusion

- Lipids
- Digestion
- Emulsification: formation of micelles
- Monoglycerides + fatty acids + bile salts + lecithin
- Pancreatic lipases: cleave fatty acid into free fatty acids and
monoglycerides
- Absorption
- Micelles: diffusion between microvilli to apical cell surface -> lipid
dissociation -> pass through plasma membrane via diffusion
- Reconversion to triglycerides
- Chylomicron formation: triglycerides + lecithin + cholesterol +
phospholipids + protein coating
- Enter lacteals -> venous blood (via thoracic duct)

- Iron
- Ferrous (Fe2+) -> binds to transferring -> absorption
- Role of cytochrome B
- Haem: facilitated diffusion

- Calcium
- Mainly active transport ; passive (in high concentrations)
- Basolateral membrane: Ca2+ ATPase, Na+/Ca2+ exchange
- Ca2+ n chyme to binds to brush border proteins
- Ca2+ in cells binds to calbindin

6. Anal Anatomy

- Anal triangle of perineum between right and left ischioanal fossae
- Zones:
- Columnar (upper half): simple columnar epithelium
- Zona hemorrhagic
- Zona cutanea
- Pectinate line
- Boundary between visceral and somatic nerve supply
- Vascular supply
- Upper ⅔: superior rectal arteries (from inf. mes. aa)
- Lower ⅓: inf. rectal artery from internal pudendal artery

II. Dyspepsia and GERD