Esophageal varices are swollen veins in the lining of the lower esophagus near the stomach. Gastric varices are swollen veins in the lining of the stomach. Swollen veins in the esophagus or stomach resemble the varicose veins that some people have in their legs. Because the veins in the esophagus are so close to the surface of the esophagus, swollen veins in this location can rupture and cause dangerous bleeding.
Esophageal varices almost always occur in people who have cirrhosis of the liver. Cirrhosis causes scarring of the liver, which slows the flow of blood through the liver. Scarring causes blood to back up in the portal vein, the main vein that delivers blood from the stomach and intestines to the liver. This "back up" causes high blood pressure in the portal vein and other nearby veins. This is called portal hypertension.
Less common causes of portal hypertension and esophageal varices include blood clots in the veins leading to and from the liver and schistosomiasis. Schistosomiasis is a parasitic infection that can clog up the liver, causing pressure to back up in the portal vein.
The backup of blood forces veins to enlarge in the vicinity of the stomach and esophagus. The veins don't enlarge in a uniform fashion. Esophageal varices usually have enlarged, irregularly shaped bulbous regions (varicosities) that are interrupted by narrower regions. These abnormal dilated veins rupture easily and can bleed profusely because:
The pressure inside the varices is higher than the pressure inside normal veins
The walls of the varices are thin
The varices are close to the surface of the esophagus.