| Line | In terrestrial environments, gravity places |
| | special demands on the cardiovascular systems of |
| | animals. Gravitational pressure can cause blood to |
| | pool in the lower regions of the body, making it |
| (5) | difficult to circulate blood to critical organs such as |
| | the brain. Terrestrial snakes, in particular, exhibit |
| | adaptations that aid in circulating blood against the |
| | force of gravity. |
| | The problem confronting terrestrial snakes is best |
| (10) | illustrated by what happens to sea snakes when |
| | removed from their supportive medium. Because the |
| | vertical pressure gradients within the blood vessels |
| | are counteracted by similar pressure gradients in |
| | the surrounding water, the distribution of blood |
| (15) | throughout the body of sea snakes remains about |
| | the same regardless of their orientation in space, |
| | provided they remain in the ocean. When removed |
| | from the water and tilted at various angles with the |
| | head up, however, blood pressure at their midpoint |
| (20) | drops significantly, and at brain level falls to zero. |
| | That many terrestrial snakes in similar spatial |
| | orientations do not experience this kind of circulatory |
| | failure suggests that certain adaptations enable them |
| | to regulate blood pressure more effectively in those |
| (25) | orientations. |
| | One such adaptation is the closer proximity of |
| | the terrestrial snakes heart to its head, which helps |
| | to ensure circulation to the brain, regardless of the |
| | snakes orientation in space. The heart of sea snakes |
| (30) | can be located near the middle of the body, a |
| | position that minimizes the work entailed in |
| | circulating blood to both extremities. In arboreal |
| | snakes, however, which dwell in trees and often |
| | assume a vertical posture, the average distance |
| (35) | from the heart to the head can be as little as |
| | 15 percent of overall body length. Such a location |
| | requires that blood circulated to the tail of the |
| | snake travel a greater distance back to the heart, |
| | a problem solved by another adaptation. When |
| (40) | climbing, arboreal snakes often pause |
| | momentarily to wiggle their bodies, causing waves |
| | of muscle contraction that advance from the lower |
| | torso to the head. By compressing the veins and |
| | forcing blood forward, these contractions |
| (45) | apparently improve the flow of venous blood |
| | returning to the heart. |