In a countercurrent exchange system, the blood vessels carrying cooled blood from the flippers run close enough to the blood vessels carrying warm blood from the body to pick up some heat from the warmer blood vessels; thus, the heat is transferred from the outgoing to the ingoing vessels before it reaches the flipper itself. (TPO15, 55)

(In a countercurrent exchange system), the blood vessels (carrying cooled blood from the flippers) run close enough to the blood vessels (carrying warm blood from the body) (to pick up some heat from the warmer blood vessels); thus, the heat is transferred (from the outgoing to the ingoing vessels) (before it reaches the flipper itself). (TPO15, 55)

分析：

修饰一：(In a countercurrent exchange system)，介词短语

中文：在逆流交换系统中

修饰二：(carrying cooled blood from the flippers)，非谓语动词，修饰the blood vessels

中文：携带来自鳍部的冷血托福学习

修饰三：(carrying warm blood from the body) ，非谓语动词，修饰the blood vessels

中文：携带来自身体的温血

修饰四：(to pick up some heat from the warmer blood vessels) ，非谓语动词

中文：以便从带有温血的血管中获取热量

修饰五：(from the outgoing to the ingoing vessels) ，介词短语

中文：由流出的血管到流进的血管

修饰六：(before it reaches the flipper itself)，从句

中文：在到达鳍部前

参考翻译：

在逆流交换系统中，携带来自鳍部的冷血的血管非常接近携带来自身体的温血的血管以便从带有温血的血管中获取热量;因此，在到达鳍部前，热量通过由流出的血管到流进的血管完成了热量转移。

这个句子的主要修饰成分就是从句、非谓语动词、介词短语，大家务必想清楚，每一个修饰成分修饰的是什么，才能看清楚这个句子的结构。