Portal Circulation

       In the systemic circulation the venous blood passing through spleen, pancreas, stomach and interstine is not carried back directly to the heart. It passes through the hepatic portal vein to the liver. This vein begins as capilaries from the visceral organs and ends in the liver again as capillaries. These capillaries converge to form the hepatic vein which joins the inferior vena cava, conveying blood to right atrium. This route is the portal circulation. Components of Circulatory system Blood vessels The blood vessels carrying blood away from the heart are the arteries.
The Veins carry blood towards the heart. The arteries and veins are named and classified according to their anatomical position. They can also be classified according to their size and wall structure. Functionally, arteries are subdivided into conducting, distributing and resistance vessels.

1. Conducting vessels :
   These are large arteries from the heart and their main branches. the walls of these vessels are elastic in nature.

2. Distributing vessels : 
   These are smaller arteries reaching individual organs. They branch into the organs. They have muscular walls.

3. Resistance vessels :
 These are mostly arterioles. While these vessels are smaller, their walls are highly muscular. Hence these vessels can reduce pressure of blood due to peripheral resistance.

4. Exchange vessels :
 These are the capillaries. The walls of these vessels allow exchanges between blood and the tissue fluid surrounding the cells. The substances commonly exchanged are oxygen, carbon-di-oxide, nutrients, water, inorganic ions, vitamins, hormones, metabolic products and antibodies.

5. Capacitance or reservoir vessels :
 These are the larger vessels and veins. These are of varying sizes. They collect and convey blood back to the heart. The higher capacitance of these vessels is due to their distensibility. Hence their blood content is more, even at low pressure. The number of such veins is also enormous.. Thus the veins are called as the “blood reservoirs”