Gastrointestinal bleeding is classified by location relative to the ligament of Treitz: upper GI bleeding (UGIB) occurs above (esophagus, stomach, duodenum) and lower GI bleeding (LGIB) occurs below (jejunum, ileum, colon, rectum). UGIB is more common and more likely to be hemodynamically significant. Peptic ulcer disease accounts for 35-50% of UGIB (H. pylori and NSAIDs damage the protective mucosal barrier, exposing submucosa to acid-pepsin digestion and eroding into blood vessels). Esophageal varices from portal hypertension cause 10-20% of UGIB and carry 30-50% mortality. Mallory-Weiss tears from forceful vomiting cause 5-10%. In LGIB, diverticular bleeding (painless, massive) and angiodysplasia (AV malformations) are most common in older adults; hemorrhoids and anal fissures are most common overall but rarely hemodynamically significant. Blood in the GI tract is an osmotic and cathartic stimulus, often causing increased peristalsis. Hematemesis (vomiting blood) is always upper GI. Melena (black tarry stool) typically indicates UGIB (requires >50-100 mL blood degraded by gastric acid and bacteria). Hematochezia (bright red rectal bleeding) typically indicates LGIB but can occur with brisk UGIB (>1L rapid hemorrhage). The body compensates for blood loss...