8.1.1—Closed double circulation

Mammals have a closed double circulatory system: blood stays inside blood vessels, and one complete journey takes it through the heart twice. The two linked circuits are pulmonary circulation and systemic circulation, not one single loop.

1. Pulmonary circuit: The right side of the heart pumps deoxygenated blood to the lungs. Blood passes through lung capillaries for gas exchange, so this loop links the heart to the lungs.
2. Return and reset: Blood returns from the lungs to the left side of the heart after oxygen loading. The heart therefore receives the blood again before it is sent around the body.
3. Systemic circuit: The left side pumps oxygenated blood at relatively high pressure through the systemic vessels to body tissues, where substances are exchanged.
4. Complete path: heart right side → lungs → heart left side → body tissues → heart right side.

Why double helps: Separating the lung and body circuits lets the heart send blood to the body at high pressure after the lung circuit, while keeping the gas-exchange step as a distinct loop.

“Double” means two circuits in one closed vessel network, not two hearts and not two blood systems that never meet. Arteries are defined by carrying blood away from the heart and veins by returning to it; oxygen content alone does not define those names. Staff-only visual brief: two linked loops with right/left heart sides, pulmonary/systemic labels and arrows; do not generate or bind an image.