文/  北京新东方学校 北美部GMAT项目组 李昊 周帆

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　　文章及题目：

　　(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

　　(10) best 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

　　(25) those orientations.

　　One such adaptation is the closer proximity of

　　he terrestrial snake’s heart to its head, which helps

　　to ensure circulation to the brain, regardless of the

　　snake’s orientation in space. The heart of sea

　　(30) snakes 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 apparently improve the

　　(45) flow of venous blood returning to the heart.