Final answer:
The superior rectal vein, part of the portal vein circulation, meets the systemic circulation at the rectal venous plexus through the middle and inferior rectal veins. The portal vein delivers nutrient-rich blood to the liver, where it is processed before entering the systemic circulation.
Step-by-step explanation:
The area where the superior rectal vein, which arises from the inferior mesenteric vein and is part of the portal vein circulation, meets the systemic circulation through the middle and inferior rectal veins, is known as the rectal venous plexus. This plexus represents an important site of anastomosis between the portal and systemic venous systems, with clinical significance in conditions like hemorrhoids, where these veins can become varicose.
The hepatic portal vein receives nutrient-rich blood from the digestive organs, such as the small intestine and portions of the large intestine, and delivers it directly to the liver for detoxification and nutrient processing. The liver then filters this blood, which exits through the hepatic veins and enters the systemic circulation via the inferior vena cava.