for和on through怎么理解？

Much of a bird’s body volume consists of air sacs, which can be inflated or deflated by the muscles of the body cavities. The air sacs are crucial to a bird’s breathing, but also have numerous other functions such as reducing body weight for flying and for voice production by the passage of air through the syrinx. Avian lungs make up a much smaller proportion of a bird’s body than in mammals, and are almost rigid, being firmly fixed to the ribs. Rather than the tidal breathing used by mammals, in birds the various air sacs are used to pass gas through the lungs, in the same direction, during both inspiration and expiration, so the lungs are, in effect, simply acting as a passive gas exchanger.This is achieved with two main groups of air sacs, which vary widely between bird species, but may be approximately divided into caudal and cephalad groups (Fig. 26.6).During inspiration, inspired air passes through the larynx and syrinx into each primary intrapulmonary bronchus, from which some air enters the dorsal secondary bronchus and the remainder inflates the caudal air sacs. From the secondary bronchus air passes through the numerous parabronchi, where gas exchange occurs, and on through the ventral secondary bronchus into the cephalad air sacs. On expiration, the caudal air sacs empty into the dorsal secondary bronchus, the air passing through the lung and being expired through the primary bronchus, while the cephalad air sacs empty and the gas is expired. As a result of this pattern of breathing, there is an almost continuous flow of inspired air through the lung in a caudad to cephalad direction, whereas pulmonary blood flow is in a cephalad to caudad direction, providing an efficient cross-current gas-exchange system (Fig. 26.2).

鸟类身体的大部分是气囊，可通过体腔的肌肉来对气囊进行充放气。气囊对鸟类的呼吸至关重要，同时也有许多其他功能，如减轻鸟飞行时的身体比重，以及提供空气在通过鸣管时发声。与哺乳动物相比，鸟肺在身体中所占的比例要小得多，且大小几乎不变，牢牢地固定在肋骨上。不同于哺乳动物的潮式呼吸，鸟类无论是吸气还是呼气，气流均同向通过的各种气囊，所以肺实际上只是一个被动的气体交换器。气体交换通过两个主气囊群实现，不同鸟类间的气囊群差异很大，但均大致分为尾侧气囊和头侧气囊（图26.6）。吸气时，气体通过喉和鸣管进入肺内初级支气管，部分初级支气管的气体进入背侧的次级支气管，其余的气体则进入尾侧气囊使其膨胀。空气从次级支气管进入众多的副支气管，在此处行气体交换，并通过腹侧的次级支气管进入头侧气囊。呼气时，尾侧气囊排空，将其中的气体释放入背侧次级支气管，气体通过肺并再经初级支气管呼出。基于这种呼吸方式，吸入的空气几乎连续地从尾端流向头端，而肺部血流则是由头端流向尾端，提供了一个高效的逆流气体交换系统（图26.2）。

for和on through怎么理解？谢谢 两个for 不并列吧？ 第2个前面的on啥意思？谢谢