Final answer:
Portal veins become narrower and turn into capillaries as they progress through the liver, where they participate in the critical function of filtering and processing blood received from the GI tract and spleen.
Step-by-step explanation:
The student's question asks about the hepatic portal system and specifically about the portal veins as they progress through the liver. Portal veins become narrower and subdivide into smaller branches called capillaries when they enter the liver via the porta hepatis. The hepatic portal vein is formed by the confluence of the superior mesenteric and splenic veins, among others, and delivers substances from the GI tract and spleen directly to the liver for detoxification and processing. This is crucial for regulating blood composition and storing nutrients.
Within the liver, the portal vein's blood mixes with oxygenated blood from the hepatic artery. The liver processes this mixture, filtering out toxins and managing nutrients. The processed blood then exits the liver via the hepatic veins, entering the systemic circulation through the inferior vena cava. As the portal vein passes through the liver, it branches into an intricate network of capillaries in the liver lobules, facilitating this essential metabolic activity.