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编辑叔叔:昨天我上体育课时不小心将腿磕碰了一下,当时皮肤并没有碰破,可过了一会,被碰破的地方漫漫地起了一个包,而且在周围还出现了一片青紫。请编辑叔叔帮我解答一下人皮肤磕碰后为什么会发青?李刚
Edit Uncle: Yesterday when I was in PE class, I accidentally bumped my leg. When the skin did not break, it took a while. The place where it was broken was a long bag and there was a purple in the surroundings. Please edit the uncle to help me explain why the human skin is dark after the collision?