Why hepatic portal vein start and end with capillaries

Hepatic portal vein start and end with capillaries because:

The hepatic portal system releases the capillaries of the splenic veins as well as the mes-enteric and helps in completion of the hepatic capillaries. The portal vein helps in transfer of partially oxygenated blood flow to the liver, providing the highly oxygenated blood flow of the hepatic artery to the liver. from the spleen as well as from the gastrointestinal tract it also carries nutrient-rich blood to the liver. The liver can also help in processing of the nutrients from the blood and also helps to filter out any toxic substances present in the blood before the blood goes moves back to general circulation.

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