Aquatic animals are faced with the challenge of extracting oxygen from water, a medium that is metabolically expensive to ventilate and that contains just a fraction of the oxygen concentration relative to air, yet the physiologies of fishes have evolved to support a wide range of activity levels in nature. Oxygen delivery components, including gill surface area (oxygen uptake), blood chemistry (oxygen transport), and the heart (system pump), have been positively correlated to activity level in teleost fishes, yet relatively little is known about how these components are related to activity in elasmobranches. The current study addresses this question by examining heart mass, hemoglobin concentration, hematocrit level, and gill surface area in wild-caught representatives of the benthic Atlantic stingray (Dasyatis sabina) and active cownose ray (Rhinoptera bonasus). Allometric scaling exponents are similar for all four measures between the study species. Heart mass, gill surfaces areas, and hemoglobin concentrations were 2.1 times, approximately 7.1 times, and 2.0 times higher, respectively, in active cownose rays, when compared to benthic Atlantic stingrays, after correcting for differences in body mass. When considered in the context of functional plasticity within the oxygen delivery systems of benthic and active species, data from the current study indicate that higher activity levels in cownose rays are supported by modifications that, at least in part, are likely to enhance oxygen uptake.