A portal system is one in which
A. A vein starts from an organ and ends up in heart
B. A vein starts from an organ and ends up in another organ
C. A vein starts from heart and ends up in lungs
D. None of the above

Hint: A portal system helps in the transport of deoxygenated blood throughout the different organs. It includes the series of veins that carries deoxygenated blood. A portal system is of two types: hepatic portal system and renal portal system.

Complete answer:
Human body mainly consists of three types of the blood vessels: Arteries, veins, and capillaries. Arteries carry oxygenated blood while the veins carry deoxygenated blood. The portal system can be two types: hepatic portal system and renal portal system.
The hepatic portal system includes the series of veins. The veins carry blood from the capillaries of the stomach, intestine, spleen, and pancreas to the capillaries in the liver. Liver is the organ which is the part of the body's filtration system. The liver detoxifies the blood, taken from hepatic portal veins before it returns to the heart. It consists of the hepatic portal vein which is the main vein that connects to the liver. The inferior and superior mesenteric veins connection forms this type of veins. The veins that take blood from the colon and rectum and connect with the portal vein are inferior mesenteric vein. The superior mesenteric vein takes the blood from the small intestine and connects with the hepatic portal vein.
In the renal portal system, the blood from renal glomeruli is collected by vein and passes it to the peritubular plexus. From the above information we have found that a portal system is one in which a vein starts from an organ and ends up in another organ.

Hence, the correct answer is option (B).

Note: Animals such as fishes, amphibians, and reptiles have hepatic and renal both portal systems. Mammals have only hepatic portal systems. Gastrosplenic vein is also a part of the hepatic portal system. It is formed by the combination of the vein of the spleen and the gastric vein of the stomach. A portal system of the body is mainly designed to get rid from the body’s toxins.