Mammalian double circulation separates pulmonary and systemic paths

Steps:

• Recall that circulation types differ by how blood flows relative to the heart and body.
• Identify double circulation as blood passing through the heart twice per full body circuit: once for lungs, once for body.
• Eliminate options unrelated to heart passages or circuit paths.
• Match the definition to the choice describing two heart transits per circuit.

Why D is correct:

• Double circulation is defined as blood flowing through the heart twice during one complete circuit of the body, separating oxygen uptake in lungs from nutrient delivery to tissues.

Why the others are wrong:

• A: A cardiac cycle is one heartbeat; blood flows once through the heart per cycle, not twice.
• B: This describes a closed circulatory system where blood stays in vessels, not the double aspect.
• C: Gas transport is a function of blood but unrelated to the double circulation structure.

Final answer: D