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.

Circulatory adaptations of snakes to gravity

Most vertebrates show respiratory and circulatory reflexes which can be traced to stimulation of various extero- or interoceptors. Widely distributed groups of exteroceptors, with a variety of stimulus modalities, are associated with defence reflexes which protect the respiratory passages and gas exchange surfaces. Other exteroceptors are associated only with the gas exchange surfaces. These are either mechanoreceptors or chemoreceptors and have a range of different dynamic characteristics, but are none-the-less amazingly uniform in their role in control of the normal breathing pattern. Intravascular receptors stimulated by the arterial blood pressure are located on all the gills in fishes, yet appear to be restricted to the pulmocutaneous arteries in anuran amphibians and to the truncal region or aortic root in reptiles and birds. On the other hand, the distribution of glomus-sustentacular-nerve cell complexes, associated with intravascular chemoreception, appears to be much more diffuse in higher than lower vertebrates. The wide distribution of these cell complexes may be attributed to their embryological origin from neural crest cells. Even so, the presence of these complexes does not appear to be essential for respiratory responses to environmental hypoxia since they have not been located in teleost fishes. The role of peripheral receptors in controlling cardiovascular and respiratory functions, under steady state conditions in non-mammalian vertebrates, is not well defined and it may be that they are only modulators of circulatory and respiratory patterns arising from hormonal or humoral effects on the central nervous system.

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Peripheral Receptors Affecting Breathing and Cardiovascular Function in Non-Mammalian Vertebrates

On the basis of changes in afferent impulsation emerging from the heart of the tortoise A. horsfieldi under various conditions of cardiac haemodynamics, it was shown that bursting activity of the superior cardiac nerves belongs to mechanoreceptors which are located in the ventricle and the adjacent arterial vessels. The discharge in these mechano-receptors coincides with ventricular systole. The second, predominant, type of afferentation is presented by irregular continued activity in both the superior and inferior vagal branches. The origin of this activity remains uncertain.

Udel'nov Mg

Papers

Characteristics of the afferent activity of the cardiac nerves of Agrionemys horsfieldi turtles