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==Description==
==Description==
'''Portsystemic shunts (PSS)''' are anomalous vessels vascular connections between the portal and systemic venous systems. This allows for some portal blood draining from the stomach, intestines, pancreas and spleen to bypass the liver and drains directly into the systemic circulation. PSS may be congenital or acquired secondary to portal hypertension.
'''Portsystemic shunts (PSS)''' are anomalous vessels vascular connections between the portal and systemic venous systems. This allows for some portal blood draining from the stomach, intestines, pancreas and spleen to bypass the liver and drains directly into the systemic circulation. PSS may be congenital or acquired secondary to portal hypertension.
Congenital PSS occurs in approximately 70% of PSS in dogs and majority of PSS in cats. Usually a single extrahepatic or intrahepatic anomolous vessel is present. Extrahepatic PSS accounts for 63% of single shunts in dog. Intrahepatic shunts are usually left-sided and result from persistent foetal ductus venosus. Right-sided or central intrahepatic shunts are recognised and these may have a different embryological origin.
Acquired PSS occurs in approximately 20% of PSS and often consists of multiple shunts. They arise as a result of portal hypertension following increased resistance to portal blood flow. This leads to opening of some of the numerous normal, non-functional microvascular communications. Underlying causes of portal hypertension included