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The mechanical and morphological properties of systemic and pulmonary arteries differ in the earth boa, a snake without ventricular pressure separation

The walls of the mammalian aorta and pulmonary artery are characterized by diverging morphologies and mechanical properties, which has been correlated with high systemic and low pulmonary blood pressures, as a result of intraventricular pressure separation in the mammalian ventricle. However, the relation between intraventricular pressure separation and diverging aortic and pulmonary artery wall morphologies and mechanical characteristics is not understood. The snake cardiovascular system poses a unique model for the study of this question, since representatives both with and without intraventricular pressure separation exist. In this study we perform uniaxial tensile testing on vessel samples taken from the aortas and pulmonary arteries of the earth boa, Acrantophis madagascariensis, a species without intraventricular pressure separation. We then compare these morphological and mechanical characteristics with samples from the ball python, Python regius, and the yellow anaconda, Eunectes notaeus, species with and without intraventricular pressure separation, respectively. Strikingly, we find that although the aortas and pulmonary arteries of A. madagascariensis respond similarly to the same intramural blood pressures, they diverge strongly in morphology, and that this is a common attribute among species without intraventricular pressure separation in this study. In contrast, P. regius aortas and pulmonary arteries diverge both morphologically and in terms of their mechanical properties. Altogether our data indicate that intraventricular pressure separation does not explain diverging aortic and pulmonary artery morphologies. Following the Law of Laplace, we propose that thin pulmonary arteries represent a mechanism to protect the fragile pulmonary vascular bed by reducing the blood volume that passes through, to which genetic factors may contribute more strongly than physiological parameters.