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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-P384-445题】
The author describes the behavior of the circulatory system of sea snakes when they are removed from the ocean (highlight text) primarily in order to
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暂无雷哥网文字解析
将定位句看完并结合第二段的段意可以推测出，作者的目的在于通过讨论海蛇离开海水后的血液循环来说明陆地蛇应对重力的一个特点，即adaptation
A正确，陈述了作者在第二段论述海蛇的血液循环的用意：通过海蛇来说明陆地蛇如何应对重力对其心血管的影响
B错误，与树栖蛇无关
C错误，与树栖蛇无关
D错误，跟superiority无关
E错误，with hearts located in relatively close proximity to their heads不是海蛇的特点，而且也未能说明作者描述海蛇的血液循环的真正意图
BC选项都是在说arboreal snakes，是第二段的东西，无关。D选项中的等级比较并没有出现在文中。E选项的确说对了一些事情，但不完全对。
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2022-10-01 19:39:00
从做题的角度解析本题： 看完高亮句和第二段，能知道这一段主要是拿海蛇来说明陆地蛇的problem，全段未提到树栖蛇，因而BC肯定是错的。 E也是无关内容（实际在第三段才出现） 然后看到D，定位句并未阐述陆地蛇的优越性。相比而言，A更能体现段中的逻辑：因为海蛇出了水会遇到这个问题—>但是陆地蛇没有遇到—>说明陆地蛇有某种适应机制。反过来看，如果陆地蛇没有这种机制—>会遇到海蛇遇到的问题。
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2021-04-04 20:43:39
整篇文章的核心是讨论重力对陆地蛇的影响，第二段是通过说明海蛇被挪到陆地上可能会出现的问题，解释说明重力对陆地蛇的影响，因此不能选择E，提海蛇不是为了解释海蛇而是为了解释陆蛇。
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2020-09-30 15:12:03
原文：certain adaptations enable them to regulate blood pressure more effectively in those orientations. 选项A：without adaptations that enable them to regulate their blood pressure in vertical orientations 有一个without 就直接把A选项排除了........
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2018-09-30 18:07:49
The problem confronting terrestrial snakes is best illustrated by what happens to sea snakes when removed from their supportive medium. 第二段第一句就表明了目的
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2018-04-07 16:27:28
默默回复Jingsa
看该部分最后一句话
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2018-05-17 04:57:14
默默回复Jingsa
讲适应的东西，血液循环本身没有优越感
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2018-05-17 05:00:40
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