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How did separate pulmonary and systemic circuits improve oxygen transport in birds and mammals?

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Silhouette diagram of the human body highlighting the heart and major blood vessels, with labeled arteries in red and veins in blue to show the structure of the cardiovascular system. — Circulatory system Human circulatory system.

The evolution of a four-chambered heart in birds and mammals, which facilitates separate pulmonary and systemic circuits, significantly improved the efficiency of oxygen transport to support higher metabolic demands.1

By completely dividing the heart into right and left chambers, these animals effectively prevent the mixing of oxygenated and deoxygenated blood. This separation allows the heart to function as a double pump with distinct pressure requirements:123

Pulmonary Circuit: The right side of the heart pumps deoxygenated blood to the lungs at lower pressures, which protects the delicate tissues of the lungs while ensuring efficient gas exchange.13
Systemic Circuit: The left side of the heart, equipped with a thicker muscular wall, pumps oxygen-rich blood at higher pressures to meet the extensive demands of the body's tissues.34

This dual-circuit system ensures that tissues receive a continuous supply of highly oxygenated blood, a critical adaptation that enabled the high-energy lifestyles characteristic of birds and mammals.12