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LineIn 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 best
(10) 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 those
(25) 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
(30) 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
(45) apparently improve the flow of venous blood
returning to the heart.
文章结构：
一、claim :terrestrial snake有抵御gravity的adaptions.
二、没有adaption的话，terrestrial snake 将面临的问题（adaption的重要性）
三、terrestrial snake的adaption机制:心脏离脑袋近。
问题是，第三段提arboreal snake的another adaption有什么用？arboreal snake 心脏离尾巴太远了，所以用another adaption.难道terrestrial也存在这种another adaption? |
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发表于 2011-5-28 11:16:25
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