第一单元 To understand the human body it is necessary to understand how its parts are put together and how they function. The study of the body's structure is called anatomy; the study of the body's function is known as physiology. Other studies of human body include biology, cytology, embryology, histology, endocrinology, hematology, immunology, psychology etc.了解人体各部分的组成及其功能，对于认识人体是必需的。研究人体结构的科学叫解剖学；研究人体功能的科学叫生理学。其他研究人体的科学包括生物学、细胞学、胚胎学、组织学、内分泌学、血液学、遗传学、免疫学、心理学等等。Anatomists find it useful to divide the human body into ten systems, that is, the skeletal system, the muscular system, the circulatory system, the respiratory system, the digestive system, the urinary system, the endocrine system, the nervous system, the reproductive system and the skin. The principal parts of each of these systems are described in this article.解剖学家发现把整个人体分成骨骼、肌肉、循环、呼吸、消化、泌尿、内分泌、神经、生殖系统以及感觉器官的做法是很有帮助的。本文描绘并阐述了各系统的主要部分。The skeletal system is made of bones, joints between bones, and cartilage. Its function is to provide support and protection for the soft tissues and the organs of the body and to provide points of attachment for the muscles that move the body. There are 206 bones in the human skeleton. They have various shapes - long, short, cube - shaped, flat, and irregular. Many of the long bones have an interior space that is filled with bone marrow, where blood cells are made.骨骼系统由骨、关节以及软骨组成。它对软组织及人体器官起到支持和保护作用，并牵动骨胳肌，引起各种运动。人体有206根骨头。骨形态不一，有长的、短、立方的、扁的及不规则的。许多长骨里有一个内层间隙，里面充填着骨髓，这即是血细胞的制造场所。A joint is where bones are joined together. The connection can be so close that no movement is possible, as is the case in the skull. Other kinds of joints permit movement: either back and forth in one plane - as with the hinge joint of the elbow - or movement around a single axis - as with the pivot joint that permits the head to rotate. A wide range of movement is possible when the ball - shaped end of one bone fits into a socket at the end of another bone, as they do in the shoulder and hip joints. 关节把骨与骨连接起来。颅骨不能运动，是由于骨与骨之间的连接太紧密。但其它的关节可允许活动，如一个平面上的前后屈伸运动，如肘关节；或是绕轴心旋转运动，如枢轴点允许头部转动。如果一根骨的球形末端插入另一根骨的臼槽里，大辐度的运动（如肩关节、髋关节）即成为可能。Cartilage is a more flex
ible material than bone. It serves as a protective, cushioning layer where bones come together. It also connects the ribs to the breastbone and provides a structural base for the nose and the external ear. An infant's skeleton is made of cartilage that is gradually replaced by bone as the infant grows into an adult.软骨是一种比一般骨更具韧性的物质。它是骨连结的保护、缓冲层。它把肋骨与胸骨连结起来，也是鼻腔与内耳的结构基础。一个婴儿的骨骼就是由软骨组成，然后不断生长、
骨化，使婴儿长大成人。The muscular system allows the body to move, and its contractions produce heat, which helps maintain a constant body temperature. Striated muscles can be consciously controlled. The ends of these muscles are attached to different bones by connective tissue bands so that when the muscle contracts, one bone moves in relation to the other. This makes it possible to move the whole body, as when walking, or to move just one part of the body, as when bending a finger. Contractions of the heart and smooth muscles are not under conscious control. Smooth muscles are found in the walls of organs such as the stomach and the intestines and serve to move the contents of these organs through the body.肌肉系统使躯体运动，肌肉收缩产生的热有助于维持一个恒定的体温。人体能够有意识地控制条纹肌。结缔组织使肌肉末端附着于不同的骨面上，所以当肌肉收缩时，两骨彼此靠近而产生运动。这也就使整个人体可以运动起来，如走路，运动躯体某个部位，如弯曲手指。心脏收缩和平滑肌收缩就不是被意识所控制的。器官壁内层的平滑肌，如胃肠壁的平滑肌把胃肠中的物质运送到全身。The circulatory system. All parts of the body must have nourishment and oxygen in order to function and grow, and their waste products must be removed before they accumulate and poison the body. The circulatory system distributes needed materials and removes unneeded ones. It is made up of the heart, blood vessels, and blood, which together make up the cardiovascular system. The blood is also part of the body's defense system. It has antibodies and white blood cells that protect the body against foreign invaders. 循环系统：机体的所有部分需要营养物质和氧气来使之发挥功能和生长，也需要在这些器官所产生的废物积聚而危害生命之前将其排除。循环系统运送有用物质，排泄废物。心血管系统是循环系统的组成之一；心血管系统包括心脏、血管及血液。血液也是机体防御系统的一个部分，血液中有抗体及白细胞来防止机体受到外来的侵袭。 The heart is a muscle that is divided into two nearly identical halves: one half receives blood from the lungs and sends it to the rest of the body, the other half sends blood that has traveled through the body back to the lungs. When the heart musc
le contracts, the blood is forced out into arteries and enters small capillaries. Blood returns to the heart through veins. 心脏是一块被分为几乎对等两半的肌肉。一半吸收来自肺部的血液，并将血液运送到机体的其余部位，另一半使流经全身的血液回流人肺。心脏收缩时，动脉把全身血液输送到毛细血管。静脉输送血液返回心脏。 Also functioning in circulation is the lymphatic system. Some of the fluid that surrounds cells does not reenter the blood vessels directly. This fluid, called lymph, returns to the heart by way of another system of channels - the lymph vessels. Lymph nodes along these vessels filter the fluid before it reenters the blood. The spleen is a large lymphatic organ that filters the blood. 淋巴系统也是循环系统的一个组成部分。一些细胞周围的体液不是直接回流入血管通道，这种体液叫淋巴液，它是流经另一个管道系统——淋巴管而回流人心脏。沿淋巴管的淋巴结将淋巴液过滤，过滤后再回流人血液。脾是一个过滤血液的大淋巴器官。 The respiratory system takes in oxygen from the air and expels carbon dioxide and water vapor. Air enters the nose and mouth and travels through the larynx, and trachea. The trachea divides to enter each of the two lungs and then divides more than 20 times to form a very large number of small
air spaces. Oxygen from the air enters the blood through capillaries in the walls of these air spaces, and the blood release carbon dioxide into the air spaces to be exhaled. 呼吸系统从空气中摄取氧气，并将二氧化碳、水蒸气排出体外。空气经鼻腔、口腔人喉管、气管。气管分成左右支气管，各连结左右肺，左右支气管再分枝20多次，在终端形成大量微小的肺泡。从空气摄取的氧气流经这些肺泡壁内的毛细血管流入血液，血液再经肺泡把释放出的二氧化碳排出体外。 The digestive system consists of a tube extending from the mouth to the anus. In it, food and fluids are taken in, moved through the body, and broken down into small molecules that are absorbed into the circulatory system. This breakdown, known as digestion, is both a mechanical and a chemical process. Food enters through the mouth, where chewing and saliva start to break it up and make it easier to swallow. Next, the food travels down through the esophagus to the stomach. Contractions of the stomach's muscular wall continue to break down the food mechanically, and chemical digestion continues when acid and enzymes are secreted into the stomach cavity. 消化系统是一个从口腔直到肛门的管道。食物和液体在消化道里被吸收，在肠道里移动时，被分解成小分子物质后再进入循环系统。这种分解，即消化，是一个机械过程，也是一个化学过程。食物进入口腔里，咀嚼和唾液开始将食物粉碎，使之便于吞咽。接着，食物
经食管人胃。胃肌壁的收缩继续机械化地分解食物，而当酸和酶分泌入胃腔时，化学性消化开始。 The liquified food gradually passes into the small intestine. In the first part of the small intestine, called the duodenum, enzymes from the pancreas are added. These enzymes complete the chemical breakdown of the food. The digestion of fat is aided by bile, which is made in the liver and stored in the gall bladder. The small intestine of an adult is about 21 feet (6.4 meters) long. Most of its length is devoted to absorbing the nutrients released during these digestive activities. 液体化食物逐渐进入小肠。小肠的起始部分叫十二指肠，胰腺分泌的酶辅助食物消化。这些酶完成食物的化学分解。肝脏分泌的胆汁贮存在胆囊内，胆汁有助于脂肪消化。一个成年人的小肠有21英尺（6.4米）长。小肠的大部分肠段用来吸收消化过程中释放的营养物质。 The liquid remainder of the food enters the large intestine, or colon, which is about 12 feet (3.7 meters) long. It is more than twice as wide as the small intestine. In the large intestine most of the fluid is absorbed, and the relatively dry residues are expelled. 液状的剩余食物进入大肠，或结肠，它大约有12英尺（3.7米）长。大肠是小肠的两倍多宽。大部分液体在大肠内被吸收，相对干化的残余物被排出体外。 The urinary system maintains normal levels of water and of certain small molecules such as sodium and potassium in the body. It does this by passing blood through the kidneys, two efficient filtering organs that get rid of any excess of various molecules and conserve those molecules that are in short supply. 泌尿系统维持水分及体内某些小分子物质，如钠、钾的正常水平。身体是通过让’肾过滤血液来做到这一点的。肾是两个有效的过滤器官，它滤出各种多余的小分子物质，保留那些供应不足的小分子物质。 The fluid that leaves the kidneys, known as urine, travels through a tube called the ureter to the bladder. The bladder holds the urine until it is voided from the body through another tube, the urethra. 从肾流出的液体，即尿，通过输尿管