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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 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 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 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 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 orientations.
One such adaptation is the closer proximity of the 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 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 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 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 flow of venous blood returning to the heart.
【OG18-P383-443题】 According to the passage, one reason that the distribution of blood in the sea snake changes little while the creature remains in the ocean is that
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从文中第二段第二句不难看出：水中的海蛇血管内的垂直方向的压力变化被四周海水的压力变化抵消，这是海蛇血液分布变化不大的原因所在
A错误，这不是水中海蛇血液分布变化甚微的原因，而是海蛇到陆地上遇到问题的原因。
B正确，该选项将文中的counteract变为counter the effects
C错误，文中未作此项对比
D错误，文中未提及海蛇依靠肌肉收缩来帮助血液循环，这是树栖蛇的特点
E错误，exceed不符合文章内容
文中原句：the vertical pressure gradients within the blood vessels are counteracted by similar pressure gradients in the surrounding water 直接对应B选项。counter和counteract应该表达的是“中和，抵消”的意思，所以E选项中的“exceeds”不对（话说要是超过了的话，海蛇血管要被压爆了）。
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2022-10-01 19:56:26
呃...题干不完整啊...这题完整的题干应该是“According to the passage, one reason that the distribution of blood in the sea snake changes little while the creature remains in the ocean is that" 还麻烦工作人员看到后帮忙更新为完整的题干，完善用户的做题体验呐！
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2018-06-11 21:27:15
akayo回复UniSun
谢谢你，现在好了
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2020-08-22 07:55:05
Because the vertical pressure gradients within the blood vessels are counteracted by similar pressure gradients in the surrounding water, the distribution of blood throughout the body of sea snakes remains about the same regardless of their orientation in space, provided they remain in the ocean.
因为血管内的垂直压力倾斜度被四周水的压力倾斜度中和（抵消）了，所以海蛇身体内血液的分布保持不变，无论他们的空间朝向是什么，只要他们在水里。
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2017-06-30 15:41:55
第二段第二句具体谈到了海蛇在水中时血液的分布情况 A错误，这不是水中海蛇血液分布变化甚微的原因，而是海蛇到陆地上遇到问题的原因。 B正确，该选项将文中的counteract变为counter the effects C错误，文中未作此项对比 D错误，文中未提及海蛇依靠肌肉收缩来帮助血液特点，这是树栖蛇的特点 E错误，exceed不符合文章内容
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2015-10-29 17:31:30
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2015-10-29 17:28:25
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