Gastrointestinal System
Secretion and hormones
Gastrin from antral G cells drives acid secretion; CCK from duodenal I cells drives gallbladder contraction and pancreatic enzyme secretion and relaxes the sphincter of Oddi; secretin from S cells drives pancreatic bicarbonate. Parietal cells secrete acid and intrinsic factor; chief cells secrete pepsinogen. Somatostatin inhibits nearly everything; VIP causes secretory diarrhea; motilin drives migrating motor complexes.
Absorption map
Duodenum absorbs iron; jejunum absorbs folate and most nutrients; terminal ileum absorbs vitamin B12 bound to intrinsic factor and bile salts; colon reclaims water and sodium. Resecting the terminal ileum therefore causes macrocytic anemia plus fat-soluble vitamin deficiency and oxalate kidney stones.
Liver and pancreas
Review bilirubin conjugation and excretion, urea cycle, albumin and clotting factor synthesis, and zonation: zone 3 is most vulnerable to ischemia and toxins. Ascites with a serum-ascites albumin gradient of 1.1 or more suggests portal hypertension. Acute pancreatitis presents with epigastric pain radiating to the back, elevated lipase, and causes of gallstones and alcohol. Chronic pancreatitis causes steatorrhea and diabetes.
Bowel disease
Ulcerative colitis is continuous from the rectum with pseudopolyps and risk of toxic megacolon; Crohn disease has skip lesions, transmural inflammation, granulomas, and fistulas. Celiac disease shows anti-tissue transglutaminase antibodies with villous blunting and improves on a gluten-free diet. Hirschsprung disease causes aganglionic distal bowel; achalasia shows loss of myenteric plexus with a bird-beak on barium swallow.
Traps
Attributing B12 deficiency to diet when ileal disease is present. Confusing the anion gap acidosis of diarrhea with that of vomiting. Assuming all colitis is infectious.
Worked rule
Terminal ileum resection produces B12 deficiency and bile salt malabsorption together.