For aquatic air-breathing fishes, the conservation and delivery of aerially obtained O2 is confounded by use of an in-series circulation. In-series blood flow necessarily unsaturates oxygenated blood from the air-breathing organ (ABO) if it mixes with deoxygenated blood from other venous sites. In addition, the presence of gills, which are needed for branchial CO2 release to water, can also cause transbranchial, diffusive loss of aerially obtained O2 before it even reaches the systemic tissues Adaptations to limit these inefficiencies exist among air-breathing fishes. They include partial separation of the ABO and systemic circulations and the use of alternate circulatory routes allowing blood shunting (diversion from the original destination) from the gills or the ABO in phase with the air-breathing cycle. Effective shunting requires cardiorespiratory integration to achieve temporal matching between the requisite shifts in peripheral vascular resistance, in cardiac output, and in both gill perfusion and ventilation. Neural and hormonal controls govern integration; however, there are marked phylogenetic differences in these mechanisms. Adrenergic cardiac innervation is not present in lungfish, weakly expressed in primitive actinopterygians and fully developed in most teleosts. While lungfishes have several structural features that aid shunting, these are less apparent in teleosts, which may be more dependent upon subtle cardiorespiratory controls to conserve and effectively distribute aerially acquired O2.