The Portal Venous System

Venous reserve is available to maintain tissue oxygenation if either systemic oxygen demand increases or arterial content falls.

our total blood volume is unevenly distributed amongst the arteries, veins and capillaries. The heart, arteries , pulmonary and systemic circuits contain about 1.5 liters of whole blood. The venous system contains 3.5 liters, which is circulating within the liveer, bone marrow and skin.

The answer is Protosystemis ShuntOrdinarily the blood coming away from the digestive tract is kept in a venous network (the "portal" system) separated from from blood that is returning directly to the heart via the vena cava (the "systemic" flow). This portal blood must first be cleaned and detoxified by the liver before merging with systemic venous blood.

If there is abnormal flow which permits the "dirty" digestive blood to
bypass the liver, it is called a Porto-Systemic "shunt".

systemic circulation - BETWEEN the heart and lungs