Which type of circulation system involves two loops?

The correct answer is double circulation, which refers to a cardiovascular system that consists of two distinct loops: the systemic circulation and the pulmonary circulation. In this system, blood is pumped from the heart to the lungs where it is oxygenated, and then it returns to the heart before being pumped to the rest of the body. This arrangement allows for efficient oxygen delivery and carbon dioxide removal, optimizing the overall efficiency of the circulatory system.

In contrast, single circulation involves one continuous circuit, commonly found in fish, where blood flows from the heart to the gills for oxygenation and then directly to the rest of the body without returning to the heart in between. The reference to fish circulation is specific to organisms with single circulation, while cardiac circulation is not a recognized type of circulation comparable to the loops of double circulation. Thus, the key characteristic of double circulation is the presence of these two separate pathways that enhance respiratory efficiency and support higher metabolic rates, particularly in mammals and birds.