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 head. By compressing the veins and forcing blood forward, these contractions apparently improve the flow of venous blood returning to the heart.
文章第二段拿sea snakes和第一段的 Terrestrial snakes进行类比,来说明 Terrestrial snakes确实有能力使血液循环不受重力影响!
文章第三段解释了一下 Terrestrial snakes这种蛇具有这种特殊能力的原因,这个原因和这种动物心脏的位置有关系.后面举了一些例子,涉及到了sea snake和arboreal snakes.
Question #25. 147-01 (22404-!-item-!-188;#058&000147-01)
The passage provides information in support of which of the following assertions?
(A) The disadvantages of an adaptation to a particular feature of an environment often outweigh the advantages of such an adaptation.
(B) An organism's reaction to being placed in an environment to which it is not well adapted can sometimes illustrate the problems that have been solved by the adaptations of organisms indigenous to that environment.
(C) The effectiveness of an organism's adaptation to a particular feature of its environment can only be evaluated by examining the effectiveness with which organisms of other species have adapted to a similar feature of a different environment.
(D) Organisms of the same species that inhabit strikingly different environments will often adapt in remarkably similar ways to the few features of those environments that are common.
(E) Different species of organisms living in the same environment will seldom adapt to features of that environment in the same way.
Question #26. 147-02 (22450-!-item-!-188;#058&000147-02)
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
(A) the heart of the sea snake tends to be located near the center of its body
(B) pressure gradients in the water surrounding the sea snake counter the effects of vertical pressure gradients within its blood vessels
(C) the sea snake assumes a vertical posture less frequently than do the terrestrial and the arboreal snake
(D) the sea snake often relies on waves of muscle contractions to help move blood from the torso to the head
the force of pressure gradients in the water surrounding the sea snake exceeds that of vertical pressure gradients within its circulatory system
这个问题提示作用实在是太强了,问为什么sea snake放进水里 distribution of blood没变化的原因,答案要想正确至少得围绕什么谈过?必须跟水有点关系啊!我们来看选项
B说的是压力对血管的影响,E说两种不同的压力哪个大,你问的是压力对distribution of blood的影响,选哪个?显然是B.这就是问题的作用,对问题理解的比较精确,对原文的依赖就少.
Question #27. 147-07 (22496-!-item-!-188;#058&000147-07)
The author suggests that which of the following is a disadvantage that results from the location of a snake's heart in close proximity to its head?
(A) A decrease in the efficiency with which the snake regulates the flow of blood to the brain
(B) A decrease in the number of orientations in space that a snake can assume without loss of blood flow to the brain
(C) A decrease in blood pressure at the snake's midpoint when it is tilted at various angles with its head up
(D) An increase in the tendency of blood to pool at the snake's head when the snake is tilted at various angles with its head down
An increase in the amount of effort required to distribute blood to and from the snake's tail
问的是个细节题,蛇的心脏接近脑袋有什么缺点,考的肯定是文章最后一段的内容,但这段开头讲的是心脏接近脑袋有什么好处,你问的是缺点,在这段however这句方向开始发生变化,我们来读一下这句.说有一种 arboreal snakes心脏离脑袋近,但是这个特点使得血液从尾巴流到脑子走的距离更长,我们来看选项
Question #28. 147-09 (22542-!-item-!-188;#058&000147-09)
In the passage, the author is primarily concerned with doing which of the following?
(A) Explaining adaptations that enable the terrestrial snake to cope with the effects of gravitational pressure on its circulatory system
(B) Comparing the circulatory system of the sea snake with that of the terrestrial snake
(C) Explaining why the circulatory system of the terrestrial snake is different from that of the sea snake
(D) Pointing out features of the terrestrial snake's cardiovascular system that make it superior to that of the sea snake
(E) Explaining how the sea snake is able to neutralize the effects of gravitational pressure on its circulatory system
A 说 terrestrial snake是如何适应重力的,有点关系,先留着
B 那两种蛇进行对比,文章确实拿这两种蛇进行了对比,但是这只是个细节,不是main idea,在GMAT大多数阅读的错误答案都是原文提过但是不能回答问题导致的
E 解释 sea snake如果能中和重力的影响,主体错了,文章讲的是 terrestrial snake如何解决重力影响的,sea snake只是中间的一个类比的例子,杀!