What distinguishes an open circulatory system from a closed circulatory system?

An open circulatory system is characterized by the fact that blood is not always contained within blood vessels. Instead, it is released into the body cavity, where it bathes the organs directly. This means that the blood mixes with interstitial fluid and is in direct contact with the cells and tissues, allowing for the exchange of nutrients and waste products. This mixing provides an efficient means of delivering oxygen and nutrients to tissues, albeit less efficiently than in a closed circulatory system where blood is delivered through a network of vessels.

In contrast, a closed circulatory system maintains blood within a network of blood vessels, allowing for more efficient transport of materials, as the blood can be directed specifically to tissues that need it most. The other options relate to incorrect descriptions or attributes that do not apply distinctly to open circulatory systems. For example, while the heart is important in both systems, it is not the only organ involved, and oxygen delivery is coupled with effective waste removal in both types.