Which circulatory system would primarily describe organisms with gills?

Organisms with gills, such as fish, typically possess a single circulatory system. In this type of system, blood flows in one continuous loop, moving from the heart to the gills, where it is oxygenated. After passing through the gills, the oxygen-rich blood then travels directly to the rest of the body to supply tissues with oxygen before returning to the heart to complete the cycle.

This single circuit design is efficient for aquatic organisms, as gills are specialized for extracting oxygen from water, allowing for a steady and direct flow of blood that supports a constant supply of oxygen throughout the organism. Essentially, the simplicity of the single circulatory system matches the metabolic needs of these organisms living in water, where oxygen availability is different compared to terrestrial environments.

In contrast, a double circulatory system, open circulatory system, and closed circulatory system each have distinct characteristics that do not align with the structure and function of gills, making them less suitable descriptors for organisms with these respiratory systems. For example, a double circulatory system is found in mammals, where blood passes through the heart twice in one complete circuit, while an open circulatory system lacks the defined blood vessels and is common in arthropods and some mollusks