Circulatory adaptations of snakes to gravity

Abstract

Comparative investigations of diverse taxa of snakes demonstrate numerous adaptations for counteracting effects of gravity on the circulation, including morphological, physiological and behavioral specializations. Arboreal and terrestrial snakes that are normally subjected to stresses from gravity are characterized by relatively high arterial pressures and ability to regulate pressure by physiological adjustments of flow and flow resistance. The heart occupies an anterior position, and the arterial blood column between the heart and head is comparatively short. Terrestrial snakes characteristically possess short vascular lungs which eliminate risks of pulmonary edema due to gravity effects during vertical posture. Problems of blood pooling in peripheral systemic vasculature are counteracted by relatively non-compliant body tissue, vasomotor adjustments, and specific movements that facilitate the venous cardiac return. Anatomical valves appear to be absent from major venous channels, but gravity, acting in concert withspecific features of venous morphology, can create valving actions that impede shifts of blood volume to dependent segments of these vessels. Nearly all of these characteristics are absent or deficient in several independent lineages of aquatic snakes that are far less subject to gravitational disturbance of hydrostatic pressures. Thus, snakes provide diverse and particularly useful models for examining cardiovascular adaptations to gravity, including mechanisms of function and the evolution of cardiovascular design. © 1987 by the American Society of Zoologists.