Circulatory Adaptations of Snakes to Gravity

Comparative investigations of diverse taxa of snakes demonstratenumerous adaptations for counteracting effects of gravity onthe circulation, including morphological, physiological andbehavioral specializations. Arboreal and terrestrial snakesthat are normally subjected to stresses from gravity are characterizedby relatively high arterial pressures and ability to regulatepressure by physiological adjustments of flow and flow resistance.The heart occupies an anterior position, and the arterial bloodcolumn between the heart and head is comparatively short. Terrestrialsnakes characteristically possess short vascular lungs whicheliminate risks of pulmonary edema due to gravity effects duringvertical posture. Problems of blood pooling in peripheral systemicvasculature are counteracted by relatively non-compliant bodytissue, vasomotor adjustments, and specific movements that facilitatethe venous cardiac return. Anatomical valves appear to be absentfrom major venous channels, but gravity, acting in concert withspecificfeatures of venous morphology, can create valving actions thatimpede shifts of blood volume to dependent segments of thesevessels. Nearly all of these characteristics are absent or deficientin several independent lineages of aquatic snakes that are farless subject to gravitational disturbance of hydrostatic pressures.Thus, snakes provide diverse and particularly useful modelsfor examining cardiovascular adaptations to gravity, includingmechanisms of function and the evolution of cardiovascular design.     

The Structure and Permeability of Integument

SYNOPSIS. The skin is a heterogeneous, multimembrane systemwith multiple diffusion pathways and potentially numerous rate-limitingbarriers for specific molecules that are exchanged with theenvironment. A broad survey of animals indicates that integumentarycoverings are morphologically, biochemically and embryologicallydiverse but with common themes of adaptation. The evolutionaryproliferation of intercellular junctions, fibrous or mineralizedprotective barriers, and lipoid waterproofing barriers emphasizethe generality of diffusion limitations. However, the regulationof specific exchange processes such as the flux rates of respiratorygases entails a complex interaction of multiple factors affectingboth diffusion and perfusion limitations. Consideration of specificpathways and rate limitations for diffusion of various substancessuggests that the permeation of skin by specific molecules canbe partially independent of other exchange processes. Our understandingof regulated permeability is, however, lacking in mechanisticand integrated analysis in most cases. Comprehensive understandingof the integument as a regulatory pathway of communication withthe environment will require comparative studies of specifictransport pathways, their rate-limiting resistances, and theinteractions as well as individual regulation of transportedmolecules.