Explain why the human circulatory system is described as a double circulatory system and explain the advantage this gives over a single circulatory system.

The human circulatory system is a double circulatory system because the blood passes through the heart twice in one complete circuit of the body. The first circuit is the pulmonary circulation, which carries de‑oxygenated blood from the right ventricle to the lungs and back to the left ventricle. The second circuit is the systemic circulation, which carries oxygenated blood from the left ventricle to the rest of the body and back to the right ventricle.

The advantage of this arrangement is that the heart re‑pressurises the blood after it returns from the lungs. Consequently, oxygen‑rich blood is delivered to the body tissues at a high pressure, allowing oxygen and nutrients to be delivered more quickly and efficiently than would be possible with a single circulatory system.