Final answer:
A portal system is the term used for a blood vessel arrangement in which blood flows through two capillary beds, such as the hepatic portal system that transports blood from the gastrointestinal tract and spleen to the liver.
Step-by-step explanation:
The term used to describe a blood vessel arrangement where blood flows through two capillary beds is called a portal system. In the human body, the most well-known example of a portal system is the hepatic portal system, which carries blood from the capillaries of the gastrointestinal tract and spleen to the capillaries (sinusoids) in the liver. This system is distinct from general circulation, where blood is pumped from the heart to the capillaries and then returns to the heart.
Blood vessels form a complex network responsible for the transport of materials throughout the body, with arteries, veins, and capillaries being the major types of blood vessels. In a portal system, after blood has gone through the initial capillaries and collected substances for transport, instead of returning directly to the heart, it is channeled through a second set of capillaries where further exchange takes place.