Lungfishes are exposed periodically to water with low oxygen content or to situations in which their aquatic environment dries up. The adaptation that deals with these conditions is an outpocketing of the gut that serves as a lung. The lung contains many thin-walled blood vessels, so blood flowing through those vessels can pick up oxygen from air gulped into the lung.

How does the lungfish circulatory system take advantage of this new organ? In fishes, the gills are arranged on supportive gill arches (see Figure 48.5A and B). Blood flows into the gill arch in an afferent arteriole and leaves in an efferent arteriole. In lungfishes, the blood vessels in the posterior pair of gill arteries have been modified into a low-resistance conduit for blood to the lung, and a separate pulmonary vessel carries oxygenated blood from the lung back to the heart. In addition, blood from the heart flows directly into the dorsal aorta through two anterior gill arches that have lost their gill filaments. A few of the gill arches retain gill filaments, so the African lungfishes can exchange respiratory gases with either air or water.

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The lungfish heart partially separates its flow of blood into pulmonary and systemic circuits; it has a partially divided atrium. The left side receives oxygenated blood from the lung, and the right side receives deoxygenated blood from the body via the sinus venosus. These two bloodstreams stay mostly separate as they flow through the ventricle and the bulbus arteriosus. As a result, oxygenated blood goes mostly to the anterior gill arteries leading to the dorsal aorta, and deoxygenated blood goes mostly to the other gill arches that have functional gill filaments. The blood leaving those posterior gill arches flows to the dorsal aorta as well as to the lung. Thus the dorsal aorta can receive oxygenated blood either from the lung (via the anterior gill arches) or from the gill filaments of the posterior gill arches.

We can conclude that the lungfish lung evolved as a means of supplementing oxygen uptake from the gills. When the water is oxygenated, the lungfish can obtain oxygen through its gills; but in oxygen-depleted water, it can depend on getting oxygen from its lung. Associated modifications of the lungfish vascular system set the stage for the evolution of separate pulmonary and systemic circulations in higher vertebrates.