Explain why mammals have a double circulatory system rather than a single circulatory system.

In a double circulatory system the blood is pumped twice per circuit – once from the heart to the lungs and back (pulmonary loop) and again from the heart to the body and back (systemic loop). This arrangement keeps the blood pressure high when it is delivered to the body because the heart can maintain a high pressure in the systemic circulation after the blood has been oxygen‑rich and low‑pressure in the pulmonary circulation. Mammals are larger, more active and warm‑blooded, so they need a greater supply of oxygen and energy to maintain body temperature. A double system allows a high‑pressure systemic circulation to meet this demand efficiently.