How does the lungfish heart keep oxygen-rich blood partly separate from oxygen-poor blood?

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lungfish African lungfish (*Protopterus annectens*).

The lungfish heart achieves partial separation of oxygen-rich and oxygen-poor blood through structural subdivisions that foreshadow the circulatory systems of land vertebrates.1

Instead of entering the sinus venosus with deoxygenated blood, oxygenated blood from the lungs enters the left side of the atrium. The atrium itself is partially divided into two, and the ventricle contains a partial septum.1

Further separation occurs within the large conus, which features a complex system of valves arranged in a spiral pattern known as the spiral valve. Additionally, the ventral aorta is internally subdivided. This arrangement directs oxygenated blood from the left side of the ventricle into the ventral division of the ventral aorta toward the anterior arterial arches, while deoxygenated blood from the right side is directed into the two most posterior arterial arches, primarily toward the lungs.1