The human circulation is described as a double circulation because
- A. the heart has two sides
- B. arteries and veins run side by side
- C. blood passes through the heart twice in one complete circuit of the body
- D. there are two types of blood cell
Explanation
Blood goes heart to lungs to heart in the pulmonary circuit and then heart to body to heart in the systemic circuit, so it passes through the pump twice for each full journey. The advantage is that pressure lost in the lung capillaries is restored before the blood is sent to the body, so the systemic circuit can be supplied at high pressure. A fish has a single circulation and cannot do this.