This type of exchange between blood centers is a common practice and provides a national pool of blood when necessary.
不同血液中心之间的这种交换是一种惯常的做法，在必要时提供了一个全国性的血库。
That is especially true in the heat, when blood vessels in the legs are distended and blood tends to pool in the feet, making the flow of blood up to the head even more difficult.
在高温下这更明显，因为腿部血管膨胀大量血液流向足部，使得向头部输送血液变得更难。
It is essential that every nation has a stable pool of healthy donors who donate blood regularly.
每个国家都具有一个稳定的、可经常献血的健康献血者队伍，这一点至关重要。
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