Final answer:
The unique portal system in question is the hepatic portal system, which channels blood from the gastrointestinal tract and spleen directly to the liver for detoxification and metabolism before returning it to the systemic circulation.
Step-by-step explanation:
The portal system with unique and unusual circulation is the hepatic portal system. This system includes the hepatic portal vein and the veins that feed into it, such as the superior mesenteric vein, the splenic vein, and the inferior mesenteric vein. The hepatic portal vein is essential because it collects blood from the gastrointestinal tract and spleen and delivers it directly to the liver. This blood is rich in nutrients and also contains waste products and toxins. At the liver, these substances are processed before the blood is returned to the heart via the hepatic vein and into the systemic circulation through the inferior vena cava.
The hepatic portal system is unique because it is one of the only two portal systems in the human body, the other being the hypothalamic-hypophyseal portal vessel found in the endocrine system. This system is critical because all blood from the alimentary canal passes through the liver via the hepatic portal vein, making it an essential site for the metabolism and detoxification of substances absorbed from the digestive tract.