Gastrointestinal hemorrhage is classified as upper (proximal to the ligament of Treitz) or lower (distal), with upper GI bleeds most commonly caused by peptic ulcer disease, esophageal varices, and Mallory-Weiss tears. Peptic ulcers develop when the mucosal defense mechanisms (mucus layer, bicarbonate secretion, prostaglandin-mediated blood flow) are overwhelmed by aggressive factors — Helicobacter pylori infection causes chronic inflammation and disrupts the mucus layer, while NSAIDs inhibit cyclooxygenase-1, reducing protective prostaglandin synthesis. Bowel obstruction causes proximal bowel distention with fluid and gas accumulation, increased intraluminal pressure, compromised mucosal blood flow, bacterial translocation, and eventual bowel wall necrosis and perforation if untreated. The nurse must assess for signs of hemodynamic instability (tachycardia, hypotension, orthostatic changes), monitor hemoglobin and hematocrit trends, and prepare for emergency intervention including fluid resuscitation and potential surgical consultation.