A Level Biology Practice Exam

Which feature characterizes an open circulatory system?

1. A complex network of vessels

2. The presence of a heart and limited vessels

3. A closed loop of blood flow

4. The absence of a circulatory fluid

The correct answer is: The presence of a heart and limited vessels

The feature that characterizes an open circulatory system is the presence of a heart and limited vessels. In organisms with an open circulatory system, such as many arthropods and some mollusks, the heart pumps hemolymph (the circulatory fluid) into cavities called sinuses, where it bathes the organs directly. This system lacks a complete set of vessels to confine the fluid, allowing it to flow freely and come into direct contact with tissues. The other options illustrate characteristics of different circulatory systems. The complex network of vessels pertains to a closed circulatory system, where the blood is contained within vessels and is circulated in a continuous loop. A closed loop of blood flow further emphasizes that blood is always contained, again indicative of a closed circulatory system. Lastly, the absence of a circulatory fluid is nonsensical within the context of circulatory systems and does not accurately describe any known system. Thus, the presence of a heart and limited vessels is the defining feature of an open circulatory system.