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.
18. 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
19. 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
(E) the force of pressure gradients in the water
surrounding the sea snake exceeds that of
vertical pressure gradients within its circulatory
system
20. It can be inferred from the passage that which of the
following is true of species of terrestrial snakes that
often need to assume a vertical posture?
(A) They are more likely to be susceptible to
circulatory failure in vertical postures than are
sea snakes.
(B) Their hearts are less likely to be located at the
midpoint of their bodies than is the case with
sea snakes.
(C) They cannot counteract the pooling of blood in
lower regions of their bodies as effectively as
sea snakes can.
(D) The blood pressure at their midpoint decreases
significantly when they are tilted with their
heads up.
(E) They are unable to rely on muscle contractions
to move venous blood from the lower torso to
the head.
21. The author describes the behavior of the circulatory
system of sea snakes when they are removed from
the ocean ) primarily in order to
(A) illustrate what would occur in the circulatory
system of terrestrial snakes without adaptations
that enable them to regulate their blood
pressure in vertical orientations
(B) explain why arboreal snakes in vertical
orientations must rely on muscle contractions to
restore blood pressure to the brain
(C) illustrate the effects of circulatory failure on the
behavior of arboreal snakes
(D) illustrate the superiority of the circulatory
system of the terrestrial snake to that of the sea
snake
(E) explain how changes in spatial orientation can
adversely affect the circulatory system of
snakes with hearts located in relatively close
proximity to their heads
22. It can be inferred from the passage that which of the
following is a true statement about sea snakes?
(A) They frequently rely on waves of muscle
contractions from the lower torso to the head to
supplement the work of the heart.
(B) They cannot effectively regulate their blood
pressure when placed in seawater and tilted at
an angle with the head pointed downward.
(C) They are more likely to have a heart located in
close proximity to their heads than are arboreal
snakes.
(D) They become acutely vulnerable to the effects
of gravitational pressure on their circulatory
system when they are placed in a terrestrial
environment.
(E) Their cardiovascular system is not as
complicated as that of arboreal snakes.
23. 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
(E) An increase in the amount of effort required to
distribute blood to and from the snake's tail
24. The primary purpose of the third paragraph is to
(A) introduce a topic that is not discussed earlier in
the passage
(B) describe a more efficient method of achieving
an effect discussed in the previous paragraph
(C) draw a conclusion based on information
elaborated in the previous paragraph
(D) discuss two specific examples of phenomena
mentioned at the end of the previous paragraph
(E) introduce evidence that undermines a view
reported earlier in the passage
25. 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
No comments:
Post a Comment