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  3. Studies of the Weddell seal in the laboratory have described the physiologic

Studies of the Weddell seal in the laboratory have described the physiologic

游客2024-01-13  75
问题     Studies of the Weddell seal in the laboratory have described the physiological mechanisms that allow the seal to cope with the extreme oxygen depriva- tion that occurs during its longest dives, which can extend 500 meters below the ocean’ s surface and last for over 70 minutes. Recent field studies, however, suggest that during more typical dives in the wild, this seal’ s physiological behavior is different.
    In the laboratory, when the seal dives below the surface of the water and stops breathing, its heart beats more slowly, requiring less oxygen, and its arteries become constricted, ensuring that the seal’ s blood remains concentrated near those organs most crucial to its ability to navigate underwater. The seal essentially shuts off the flow of blood to other organs, which either stop functioning until the seal surfaces or switch to an anaerobic(oxygen-independent)metabolism. The latter results in the production of large amounts of lactic acid which can adversely affect the PH of the seal’ s blood but since the anaerobic metabolism occurs only in those tissues which have been isolated from the seal’s blood supply, the lactic acid is released into the seal’ s blood only after the seal surfaces, when the lungs, liver, and other organs quickly clear the acid from the seal’s blood stream. Recent field studies, however, reveal that on dives in the wild, the seal usually heads directly for its prey and returns to the surface in less than twenty minutes. The absence of high levels of lactic acid in the seal’ s blood after such dives suggests that during them, the seal’ s organs do not resort to the anaerobic metabolism observed in the laboratory, but are supplied with oxy- gen from the blood. The seal’ s longer excursions underwater, during which it appears to be either exploring distant routes or evading a predator, do evoke the diving response seen in the laborato- ry. But why do the seal’ s laboratory dives always evoke this response, regardless of their length or depth? Some biologists speculate that because in laboratory dives the seal is forcibly sub- merged, it does not know how long it will remain underwater and so prepares for the worst. [br] It can be inferred from the passage that by describing the Weddell seal as preparing "for the worst" in line 57, biologists mean that it
选项 A、prepares to remain underwater for no longer than twenty minutes.
B、exhibits physiological behavior similar to that which characterizes dives in which it heads directly for its prey.
C、exhibits physiological behavior similar to that which characterizes its longest dives in the wild.
D、begins to exhibit predatory behavior.
E、cleans the lactic acid from its blood before attempting to dive.
答案 C
解析 L57指出海豹“做好最坏的准备”，是指：A．潜水不超过二十分钟。说反了。B．和直接扑向猎物行为相近。说反了。C．正确。表现出和野外最长潜水时间相似的生理行为。见原文第三段论述。L45—50。D．“predatory behavior”无。E．“cleans the lactic acid”文中未提。
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