Circulatory impairment induced by exercise in the lizard Iguana iguana

Mechanical integration of the cardiac, muscular and ventilatory pumps enabl es mammals to vary cardiac output over a wide range to match metabolic dema nds. We have found this integration lacking in a lizard (Iguana iguana) tha t differs from mammals because blood how from the caudal body and ventilati on are maximal after, rather than during, exercise, Because Iguana iguana a re constrained from ventilation during intense locomotion, they appear to b e unable to recruit the abdomen and thorax as a pump for venous return. Thi s constraint on simultaneous running and costal breathing arises from their musculoskeletal design, which is similar to that of basal tetrapods, and s o a constraint on venous return during exercise may be ancestral for tetrap ods, We suggest that mechanical coupling of the pulmonary and cardiac pumps may have been important for the evolution of high-speed locomotor stamina in terrestrial vertebrates.