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  3. Studies of the Weddell seal in the laboratory have described the physiological mechanisms that allow the seal to cope with the e

Studies of the Weddell seal in the laboratory have described the physiological mechanisms that allow the seal to cope with the e

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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.
The passage suggests that during laboratory dives, the PH of the Weddell seal’s blood is not adversely affected by the production of lactic acid because
选项 A、only those organs that are essential to the seal’ s ability to navigate underwater revert to an anaerobic mechanism.
B、the seal typically reverts to an anaerobic metabolism only at the very end of the dive.
C、organs that revert to an anaerobic metabolism are temporarily isolated from the seal’s bloodstream.
D、oxygen continues to be supplied to organs that clear lactic acid from the seal’s bloodstream.
E、the seal remains submerged for only short periods of time.
答案C
解析 实验室中，海豹血液里PH值为什么并没有因为乳酸产生而受到不好影响?A．只有对潜水活动必要的器官才参与无氧代谢。说反了。见L19—24。B．“only at the very end of the dive”文中无。C．正确。进行无氧代谢的器官暂时和海豹血循环隔开。见原文L19—24。D．“clear lactic acid”文中未提。E．“only short periods of time”和文中所说正相反。
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