Explain why the human circulatory system is described as a 'double circulatory system' and outline the advantage this gives to mammals compared to a single circulatory system.

A double circulatory system means that blood passes through the heart twice in one complete circuit of the body. One circuit is the pulmonary circuit, which carries de‑oxygenated blood from the heart to the lungs and back to the heart. The second circuit is the systemic circuit, which carries oxygenated blood from the heart to the rest of the body and back to the heart.
The heart has four chambers, which keep oxygenated and de‑oxygenated blood separate.
Because the two circuits are separate, the systemic circuit can operate at a higher blood pressure. This allows mammals to supply oxygen to cells more efficiently, meeting their greater oxygen demand for respiration.