GRAVITY, BLOOD-CIRCULATION, AND THE ADAPTATION OF FORM AND FUNCTION IN LOWER-VERTEBRATES

Gravitational force influences musculoskeletal systems, fluid distribu tion, and hydrodynamics of the circulation, especially in larger terre strial vertebrates. The disturbance to hydrodynamics and distribution of body fluids relates largely to the effects of hydrostatic pressure gradients acting in vertical blood columns. These, in turn, are linked to the evolution of adaptive countermeasures involving modifications of structure and function. Comparative studies of snakes suggest there are four generalizations concerning adaptive countermeasures to gravi ty stress that seem relevant to lower vertebrates generally. First, in creasing levels of regulated arterial blood pressure are expected to e volve with some relation to gravitational stresses incurred by the eff ects of height and posture on vertical blood columns above the heart. Second, aspects of gross anatomical organization are expected to evolv e in relation to gravitational influence incurred by habitat and behav ior. Third, natural selection coupled to gravitational stresses has fa vored morphological features that reduce the compliance of perivascula r tissues and provide an anatomical ''antigravity suit.'' Fourth, natu ral selection has produced gradients or regional differences of vascul ar characteristics in tall or elongated vertebrates that are active in high gravity stress environments. Consideration or awareness of these principles should be incorporated into interpretations of structure a nd function in lower vertebrates. (C) 1996 Wiley-Liss, Inc.