Which of the following characterizes human circulatory system ?

Circulatory systems are classified along two independent axes: whether blood stays confined within vessels throughout its path (a closed system) or spills directly into body spaces among the tissues (an open system), and whether blood is pumped through the heart once (single circulation) or twice (double circulation) to complete one full round of the body.

In the human body, blood always travels through a continuous network of arteries, veins and capillaries and never pools directly among the tissues, so the system is closed. The heart also drives two separate, sequential circuits: the pulmonary circuit, which carries blood from the right side of the heart to the lungs and back to the left atrium, and the systemic circuit, which carries blood from the left side of the heart to the rest of the body and back to the right atrium. Because blood passes through the heart between these two circuits, one complete round of the body sends blood through the heart twice, which is double circulation.

Checking the other pairings that are offered:

• open, single circulation describes simple invertebrate designs such as insects, where fluid is not confined to a closed vessel network and moves through open body spaces, driven by a basic tubular heart in one loop; neither trait matches the human layout.

• open, double circulation keeps a two-circuit heart routing but wrongly assumes blood leaves the vessel network, which does not happen in the human body.

• closed, single circulation keeps the vessel-confined blood flow correct but wrongly assumes only one loop through the heart, missing the separate pulmonary and systemic circuits.

Only the closed, double circulation pairing satisfies both requirements together, which is why it is the correct description of the human circulatory system.