text: The exact pathogenesis of<unk>  syndrome is not fully understood. Pulmonary microvascular dilation and angiogenesis are two central pathogenic features that drive abnormal pulmonary gas exchange in experimental<unk>  syndrome, and thus might underlie<unk>  syndrome in humans. It is thought that<unk>  syndrome is the result of microscopic intrapulmonary arteriovenous dilatations due to either increased liver production or decreased the liver clearance of vasodilators, possibly involving nitric oxide. The progression to<unk>  syndrome is believed that involves the  nitric oxide metabolism. The dilation of these blood vessels causes overperfusion relative to ventilation, leading to ventilation-perfusion mismatch and hypoxemia. There is an increased gradient between the partial pressure of oxygen in the alveoli of the lung and adjacent arteries (alveolar-arterial [A-a] gradient) while breathing room air. Patients with<unk>  syndrome have platypnea-orthodeoxia syndrome (POS); that is, because intrapulmonary vascular dilations (IPVDs) predominate in the bases of the lungs, standing worsens hypoxemia (orthodeoxia)/dyspnea (platypnea) and the supine position improves oxygenation as blood is redistributed from the bases to the apices. Additionally, late in cirrhosis, it is common to develop high output failure, which would lead to less time in capillaries per red blood cell, exacerbating the hypoxemia. As discussed below a variety of angiogenesis-related genes polymorphism has been linked to<unk>  syndrome. Increased levels of endothelin-1 in cirrhotic patients have been correlated with intrapulmonary molecular and gas exchange abnormalities, hypothesizing a probable contribution to the pathogenesis of<unk>  syndrome.
option_0: Hepatopulmonary syndrome pathophysiology
option_1: Acute liver failure pathophysiology
option_2: Hepatotoxicity pathophysiology
option_3: Drug induced liver injury pathophysiology
option_4: Budd-Chiari syndrome pathophysiology
option_5: Alcoholic hepatitis pathophysiology
option_6: Ascites pathophysiology
option_7: Cirrhosis pathophysiology