XDF-29

ypngzhang

PASSAGE 29
*Studies of the Weddell seal in the laboratory have described the physiological mechanisms that allow the seal to cope with the extreme oxygen deprivation that occurs during its longest dives, which can extend 500
5) 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 physio- logical behavior is different.
* In the laboratory, when the seal dives below the
10) 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
15)
        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 acidwhich can adversely affect the pH of the seal's blood
20) 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-
25) 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
30) 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 oxygen from the blood. The seal's longer excursions underwater,during which it appears to be either exploring distant
35) routes or evading a predator, do evoke the diving response seen in the laboratory. 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
40)
        submerged, it does not know how long it will remain underwater and so prepares for the worst.

有一个问题：

第20行那段蓝色的，“when ”前面的能理解，但是“when ”后面的是什么意思？和这个因果关系有什么联系么？