Final answer:
The mesenteric veins are part of the hepatic portal system, which transports nutrient-rich blood from the small intestine to the liver for processing before it returns to the heart via the hepatic vein and inferior vena cava.
Step-by-step explanation:
The mesenteric veins do not connect directly to the vena cava because they are part of the hepatic portal system, which carries nutrient-rich blood from the small intestine to the liver for processing. After nutrients are absorbed in the small intestine, the mesenteric veins transport this blood to the superior mesenteric vein, which then empties into the hepatic portal vein. The hepatic portal vein conveys this blood to the liver, where it is processed; excess nutrients are stored, and toxins are removed. Eventually, the liver releases the processed blood to the heart through the hepatic vein which empties into the inferior vena cava.
This process is critical, as it ensures that all nutrients and substances absorbed in the gut can be immediately regulated and detoxified by the liver before entering the systemic circulation. This is also significant for preventing hyperglycemia by enabling the liver to regulate blood glucose levels efficiently. This intricate system also provides an explanation as to why the liver is the most common site for metastasis of cancers that originate in the alimentary canal due to the direct blood flow.