Lesson 2Survival depends on maintaining homeostasis, a stable internal environment in which the cells can live and carry out their functions. At this point we are going to depict briefly the major organ systems and at least some of the ways in which they contribute to homeostasis. There are really two reasons for doing this: one is to give you an idea of the basic physiological needs of the body and the other is to give you an idea of at least some of the functions of the major internal organs. An overall idea of what goes on in the body should make it easier to learn detailed information about the various organ systems.生存有赖于人体内环境的恒定，在这种恒定的内环境中，细胞才能生存并执行其功能。
这里，我们将简述主要的器官系统，并尽可能地谈谈器官系统促进人体内环境恒定的一些方式。
这样做，实际上有两个原因：一是让你了解人体生理上的基本需要，二是让你至少了解主要内脏的某些功能。
对体内所进行的活动有个总的概念会有助于了解各个器官系统的详细情况。
Survival is dependent on obtaining from the outside world the raw materials that the cells require to carry out their functions. With the exception of oxygen, all of the substances taken into the body from the external world enter through the digestive system. The digestive system must then decompose the food (digest it) into particles small enough to enter the bloodstream. Those substances in the food that for one reason or another cannot be absorbed into the blood are eliminated from the body as feces.生存还有赖于从外界获取细胞进行活动所需要的各种原料。
所有从外界摄入人体的物质，除氧气外，都是经由消化系统进入的，而消化系统则必须把食物分解（消化）成小到足以进入血流的微粒。
由于这样或那样的原因而未被吸入血流的食物残渣以粪便的形式从体内排出。
The only substance needed from the outside world which does not enter the body through the digestive system is oxygen, a gas used in the process by which the energy in food is released for use by the cells. The exchange of gases between the external environment and the internal environment is accomplished by the respiratory system. The respiratory system brings oxygen into the body and eliminates carbon dioxide, a potentially poisonous gas produced by the combination of oxygen and food.从外界摄取但不经由消化系统而进入人体的唯一物质是氧气，即食物中的能量被释放出来供细胞使用的过程中所用的一种气体。
外环境和内环境之间的气体交换是靠呼吸系统完成的，它把氧气输入体内并排出二氧化碳，即由氧与食物化合所产生的一种潜在性的有毒气体。
Food from the digestive tract and oxygen from the respiratory tract must be transported to cells throughout the body. The circulatory system functions to convey material from one part of the body to another in much the same way as a subway system .The circulatory system comprises the blood, which is the transporting fluid that moves throughout the body; the blood vessels, which are tubes through which the blood flows; and the heart, which pumps the blood throughout the body.来自消化道的食物和来自呼吸道的氧气必须运往全身各处的细胞。
循环系统的功能就是把物质运送到全身各处，其作用方式和地铁系统极其相似。
循环系统由血液、血管和心脏组成，血液是运行于全身各处的运输液：血管是血液运行所经过的管道；而心脏的作用则是把血液压送到全身。
The function of the endocrine system and the nervous system is to regulate the other organ system .The former regulates body function through the action of chemicals called hormones. The hormones are manufactured and secreted into the blood by the endocrine system .The nervoussystem regulates body functions through the action of neurons, a network of cells that extends all over the body.With all the organ systems at work, the human body as a whole functions most properly.内分泌系统和神经系统的功能是调节其他器官系统。
前者是通过激素这种化学物质的作用调节人体功能的；激素是由内分泌系统制造并分泌到血液中去的，而神经系统则是通过神经元的作用，也就是通过分布于全身的神经细胞网状结构的作用，而调节人体功能的。
只要各器官系统各自发挥作用，整个人体也就活动正常了。
Lesson 31Human life on earth is dependent upon (among other things) our ability to utilize one gas, oxygen, and to eliminate another gas, carbon dioxide. 2Oxygen, present in the amount of about 21% in the air, must enter the blood and reach every body cell so that foods may be burned – more precisely, oxidized – and so that the cells, therefore, may continue to live. 3Many cells, notably those of the central nervous system and the heart, are extremely sensitive to low blood levels of oxygen, or hypoxia, and course to complete lack of oxygen, anoxia. 4Without oxygen, man could not live long.1人类在地球上的生存，出其他因素外，还取决于人们对氧气这一气体的利用及对另一气体二氧化碳予以排除的能力。
2氧气约占空气含量的21%，只有当它进入血液，并到达全身各处的细胞后，食物才会被燃烧，更确切的说，才能被氧化，因而细胞的生命也就能延续下去。
3许多细胞，特别是中枢神经系统和心脏的细胞，对低血氧（或称低氧）和完全缺氧（或称缺氧）极其敏感。
4没有氧气，人就活不长。
1Oxygen is transported to the body cells, and carbon dioxide is eliminated from these cells, all by a complicated physiological process called respiration, or breathing. 2It consists of two processes: internal respiration and external respiration. 3The former signifies the exchange of gases between the blood and tissue, whereas the latter denotes that between the blood and air. 4Internal respiration is also termed tissue breathing because it takes place in the tissues; external respiration is also known as lung breathing because it occurs in the lungs. 5External respiration is further subdivided into two phases: inspiration, or drawing of air into the lungs, and expiration, or expulsion of air from the lungs.1体细胞摄取氧气及排出二氧化碳的工作都是通过一个叫做呼吸的复杂生理过程来完成的。