Explain why the mammalian circulatory system is described as a double circulatory system, and outline the advantage this arrangement provides for the delivery of oxygen to respiring tissues.

The mammalian circulatory system is a double circulatory system because blood passes through the heart twice in one complete circuit of the body.
1. The pulmonary circuit carries de‑oxygenated blood from the body to the lungs and returns oxygenated blood to the heart.
2. The systemic circuit carries oxygenated blood from the heart to the tissues and returns de‑oxygenated blood to the heart.
This separation means oxygenated and de‑oxygenated blood are kept separate and do not mix.
The left side of the heart receives the oxygenated blood from the lungs and repressurises it before it enters the systemic circuit, creating a high pressure.
The high pressure maintains a steep concentration gradient, allowing oxygen to be delivered rapidly to respiring tissues.