Final answer:
The splenic, gastric, superior mesenteric, and inferior mesenteric veins drain into the hepatic portal vein, which delivers blood from the digestive organs and spleen to the liver for processing.
Step-by-step explanation:
The splenic, gastric, superior mesenteric, and inferior mesenteric veins all drain into the hepatic portal vein. This vein is a key component of the hepatic portal system, which delivers blood containing nutrients absorbed from the small intestine, as well as products filtered by the spleen, to the liver. Once in the liver, this blood - enriched with nutrients and other substances - is processed before being sent to the systemic circulation. After the liver has processed these substances, nutrients needed by other cells are released into the blood, which then drains into the central vein and subsequently through the hepatic vein to the inferior vena cava.
This system differs from the systemic circulation, where arteries deliver oxygenated blood to the body's tissues and veins return deoxygenated blood back to the heart. The hepatic portal circulation is unique in that it provides a direct route for blood from the digestive tract and spleen to reach the liver, facilitating essential processing of nutrients and toxins.
Therefore, the correct answer to the student's question is: a. hepatic portal vein.