The respiratory system, for example,

例如呼吸系统
the renal system, the digestive system,

泌尿系统消化系统
these are examples - three examples of organ systems

这些系统这三个系统
that contact the external environment.

与外环境相联系
We've talked about the interesting way

我们会讲到
in which the digestive system contacts the external environment.

消化系统与外环境之间十分有趣的联系
Depending on what you call external and internal environment,

基于你们所谓的外环境和内环境
this path that I'm tracing here, deep within your digestive system,

我在这里我所指的消化系统内部通路
is really directly connected to the outside world through both ends.

它的两端确实与外界环境直接相通
Of course when molecules get absorbed through the intestinal tract

当然分子在被肠道吸收后
they become part of your internal environment.

就成为身体内环境的一部分
We've talked about one the main concepts in physiology being homeostasis,

我们讲到了生理学的主要概念内稳态