Ace A Level Biology 2025 – Dive Deep and Conquer Science!

You are correct that the double circulatory system is the appropriate answer. This type of circulatory system ensures that blood travels through the heart twice for each complete circuit around the body.

In a double circulatory system, blood circulation is divided into two separate pathways: one for oxygenated blood that travels to the body (systemic circulation) and another for deoxygenated blood that goes to the lungs (pulmonary circulation). After the oxygen-poor blood returns from the body to the right side of the heart, it's pumped to the lungs to receive oxygen. Then, the oxygen-rich blood returns to the left side of the heart, from where it is pumped to the rest of the body. This separation of the circulatory routes optimizes oxygen delivery and enhances metabolic efficiency.

Conversely, in a single circulatory system, which is typically found in fish, blood travels only once through the heart for each complete circulation, meaning it goes from the heart to the gills, then to the rest of the body, and back to the heart, all in one continuous loop.

The terms systemic and pulmonary circulatory systems refer more specifically to parts of the double circulatory system itself rather than being distinct types of circulatory systems. Thus, stating that