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

Synopsis. The elongate body form of snakes and the wide diversity of habitats into which they have radiated have affected the form and function of the cardiovascular system. Heart position is strongly correlated with habitat. The heart is located 15-25% of the body length from the head in terrestrial and arboreal species, but 25-45% in totally aquatic species. Semi-aquatic and fossorial species are intermediate. The viperids are exceptional, with generally more posterior hearts but arboreal species have hearts closer to the head. An anterior heart is favored when snakes climb because it reduces the hydrostatic pressure of the blood column above the heart and tends to stabilize cephalic blood pressure. In water, where hydrostatic blood pressure is not a problem, a more centrally located heart is favored because the heart does less work perfusing the body. In terrestrial species, head-heart distanee increases linearly with body length and the increased hydrostatic pressure is matched by higher resting arterial blood pressure in longer animals. Unlike mammals and birds, snakes have blood pressures that increase with body mass. The added stress on the ventricle wall in larger snakes is correlated with ventricles that are larger than predicted by other reptiles. Heart mass scales with body mass to the 0.95 power in snakes but only 0.77-0.91 in other reptiles that are not as subject to the hydro? static effects of gravity. The spongy hearts of reptiles do not conform well to the Principle of Laplace.

Scaling of cardiovascular physiology in snakes

Systemic arterial pressures of snakes vary interspecifically in relation to gravitational demands imposed by environment and behavior. Terrestrial scansorial and arboreal species are characterized by comparatively high pressures thought to reflect generally greater levels of total peripheral resistance. Pulmonary arterial pressures exhibit less interspecfic variation and are generally lower than systemic pressures, such differences being greater in arboreal species. Both systemic and pulmonary arterial pressures are regulated by neurogenic reflexes, with arboreal species demonstrating superior capability for regulating blood pressure during posture change. Baroreceptors have been identified in the truncus arteriosus and central arteries of snakes, and other mechanoreceptive sites are also likely. Efferent mechanisms affecting vascular muscle tone are correlated with dense adrenergic innervation of vessels that varies interspecifically as well as regionally within species. The evolution of dense but variab...

Neural Regulation of Arterial Blood Pressure in Snakes

Electrophysiological aspects of reflexogenic area in the chelonian, Geoclemmys reevesii.

Cardiovascular responses to head-up tilt, acutely graded hemorrhage, and pharmacologic stimulation by principal autonomic drugs were studied in four species of marine snakes, principally Aipysurus laevis (family Hydrophiidae). Arterial pressure varied inversely with tilt angle and blood volume deficit in conscious snakes outside of water, indicating that physiological regulation was poor or lacking. Calculated arterial pressures at head level typically diminished to zero in A. laevis tilted to angles greater than or equal to 30 degrees. Arterial pressure (corrected for external water pressure) did not change when these snakes were tilted in seawater. Changes of arterial pressure induced by tilt, blood loss, or autonomic drugs elicited reflex adjustments in heart activity, but the magnitude of these responses was less than that observed in terrestrial species of snake. It is concluded that baroreflexes are present but comparatively ineffective in sea snakes. Snakes tolerated large losses of blood volume, and extravascular fluids were absorbed into the circulation during hemorrhage; both hemorrhage and estimated hemodilution volumes exceeded 100% of the initial blood volume in Acalyptophis peronii. Thus, in marine snakes major fluid shifts between nonvascular and vascular compartments significantly compensate hypovolemia but, because of minor autonomic adjustments, do not result in a well-regulated arterial pressure.

Control of arterial pressure in aquatic sea snakes

This study was undertaken to characterize cardiovascular receptors in the turtle, Pseudemys scripta, with particular attention being given to neural activity changes associated with alterations in blood pressure. Vagal afferent nerve traffic, synchronous with heart contractions, was recorded in anesthetized artificially ventilated turtles. Action potentials, from receptors that fired regularly during each heart cycle, occurred during ventricular systole. Mechanical probing and vascular occlusion indicated that these receptors were located in the proximal common pulmonary artery including the bulbus cordis region. Bolus injections of saline into the ventricle or the common pulmonary artery caused immediate but transient increases in cardiac synchronous traffic. Prolonged elevation of arterial and ventricular blood pressure, by either saline injection or arterial occlusion, caused increases in receptor discharge of the same duration as the pressure increases. Although these receptors could participate in the regulation of the systemic and the pulmonary circulation, the physiological role for them is presently unknown.

J. W. Trank

Papers

Circulatory mechanoreceptors in the pond turtle Pseudemys scripta