What structure opens and closes to allow fluid to flow in one direction?

The structure that opens and closes to allow fluid to flow in one direction is known as a valve. Valves are essential components in various systems, including the circulatory system and lymphatic system. They ensure that blood or lymph fluid flows in a single direction, preventing backflow. In the cardiovascular system, for example, heart valves open to allow blood to flow from one chamber to another or into the arteries, and they close to prevent the blood from returning to the previous chamber.

In contrast, while arteries carry blood away from the heart, they do not have the one-way flow mechanism that is characteristic of valves. Capillaries are tiny blood vessels where the exchange of gases, nutrients, and waste occurs but do not possess the ability to control the direction of fluid flow like valves do. The term "portal" typically refers to specific systems like the portal vein in the liver, which is involved in directing blood from the gastrointestinal tract, but it does not describe a structure that opens and closes to regulate flow.

Therefore, the functionality of a valve in ensuring unidirectional flow makes it the correct answer to the question regarding the structure that opens and closes for fluid direction.