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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-P385-449题】
In the passage, the author is primarily concerned with doing which of the following?
我有更好的解析 
暂无雷哥网文字解析
本文作者主要说明了陆地蛇是如何应对重力对其心血管的影响的
A正确，符合文章主旨
B错误，海蛇与陆地蛇的对比只是其中一个细节
C错误，对陆地蛇与海蛇的循环系统的不同指出的分析仍是文章的部分内容
D错误，文章未提及
E错误，对海蛇的论述只是文章的部分内容
海蛇是被用来解释说明陆地蛇的adaptation的，海蛇不是重点，陆地蛇的adaptation才是。
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2022-10-01 20:23:00
海蛇的循环系统只在第二段一半里讲到，且是为了引出adapatation of 陆地蛇的作用，不是重点 而且文章开头也表明了目的：为了介绍terrestrial snakes的adaptations that aid in circulating blood against the force of gravity.
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2019-05-21 04:07:22
看一下最前面讲的主体
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2018-05-17 04:44:44
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