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A portosystemic shunt (PSS), also known as a liver shunt, is a bypass of the liver by the body's circulatory system. It can be either a congenital (present at birth) or acquired condition. Congenital PSS is a hereditary condition in dogs and cats, its frequency varying depending on the breed. The shunts found mainly in small dog breeds such as Shih Tzus, Tibetan Spaniels, Miniature Schnauzers and Yorkshire Terriers, and in cats such as Persians, British Shorthairs, Himalayans, and mixed breeds are usually extrahepatic (outside the liver), while the shunts found in large dog breeds such as Irish Wolfhounds and Labrador Retrievers tend to be intrahepatic (inside the liver). Acquired PSS is uncommon and is found in dogs and cats with liver disease such as cirrhosis causing portal hypertension, which is high blood pressure in the portal vein. ==Pathology== Congenital PSS is caused by the failure of the fetal circulatory system of the liver to change. Normally, the blood from the placenta bypasses the liver and goes into circulation via the ductus venosus, a blood vessel found in the fetus. A failure of the ductus venosus to close causes an intrahepatic shunt, while extrahepatic shunts are usually a developmental abnormality of the vitelline veins, which connect the portal vein to the caudal vena cava. Thus in the juvenile and adult animal with PSS, blood from the intestines only partly goes through the liver, and the rest mixes into general circulation. Toxins such as ammonia are not cleared by the liver. Most commonly, extrahepatic shunts are found connecting the portal vein or left gastric vein to the caudal vena cava. Congenital shunts are usually solitary. Acquired shunts are usually multiple, and are caused by portal hypertension in dogs with liver disease. This is most commonly seen in older dogs with cirrhosis, but may also be seen in younger dogs with liver fibrosis caused by lobular dissecting hepatitis. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Portosystemic shunt」の詳細全文を読む スポンサード リンク
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