Hepatopulmonary syndrome (HPS) occurs in patients with liver disease when pulmonary capillaries dilate abnormally (from 8-15 micrometres to 15-500 micrometres). The dilation is driven by increased hepatic production and impaired clearance of vasodilators, primarily nitric oxide and endothelin-1. In dilated capillaries, red blood cells passing through the center of the vessel are too far from the alveolar wall for effective oxygen diffusion, creating a functional right-to-left shunt. Hypoxemia worsens in the upright position (orthodeoxia) because gravity increases blood flow to the dilated basilar capillaries. The triad of liver disease, intrapulmonary vascular dilation, and impaired oxygenation defines HPS.