Cells share certain common characteristics, but most of the body's cells develop specialized features and abilities. The cells that form bone collect calcium salts, these cells are locked together in solid chunks, immobile. By contrast, the white cells of the blood, which fight off invading bacteria, roam freely about the body, Other cells make special chemicals for the body's use -the hormones produced in the endocrine glands, or the digestive enzymes poured into the intestine from the pancreas. Still other cells form the incredibly thin membranes in the lung or kidney that permit the filtering or exchange of dissolved body fuels and wastes.
细胞具有某些共同特征，但身体的大多数细胞都会发展出特殊的特征和能力。形成骨骼的细胞收集钙盐，这些细胞被锁在一起，形成固体块，不动。相比之下，抵抗入侵细菌的血液白细胞在体内自由漫游，其他细胞制造供身体使用的特殊化学物质 - 内分泌腺产生的激素，或从胰腺注入肠道的消化酶。还有一些细胞在肺或肾中形成令人难以置信的薄膜，允许过滤或交换溶解的身体燃料和废物。
The most versatile cells are those of the various kinds of epithelium. Forming the body's external coating - the skin - epithelial cells protect things inside from things outside, They also form the lining ofthe mouth, stomach and bowel, the inner surface of blood vessels, and themembranes that permit the lungs to breathe and the kidneys to excrete. Over the cornea of the eye they become a sort of transparent windshield, to permit the free entry of light to the retina. Other epithelial cells secrete a protective mucus to keep intestines, lungs and nasal passages from drying out. Still others manufacture powerful hormones that regulate the body's chemical reactions
用途最广泛的细胞是各种上皮细胞的细胞。形成身体的外层 - 皮肤 - 上皮细胞保护内在物品免受外界物品的侵害，它们还形成口腔、胃和肠道的内壁、血管的内表面以及允许肺部呼吸和肾脏排泄的膜。在眼睛的角膜上，它们变成了一种透明的挡风玻璃，允许光线自由进入视网膜。其他上皮细胞分泌保护性粘液，以防止肠道、肺和鼻道干燥。还有一些人制造强大的激素来调节身体的化学反应.
Muscles are special fibrous tissues found throughout the body. They control movement and many organic functions by contracting in response to nerve signals. Skeletal muscles are called voluntary because they can be consciously controlled. They are attached to bones by tough fibrous tissues called tendons.Other muscles, such as the stomach muscles and the heart, are involuntary and are operated automatically by the central nervous system.
肌肉是遍布全身的特殊纤维组织。它们通过响应神经信号收缩来控制运动和许多有机功能。骨骼肌被称为自主肌，因为它们可以被有意识地控制。它们通过称为肌腱的坚韧纤维组织附着在骨骼上其他肌肉，如胃部肌肉和心脏，是非自愿的，由中枢神经系统自动操作。
The human heart consists of four chambers, two atria and two ventricles.Each is made up of several layers of cardiac muscles arranged in circles and spirals.During the contraction phase, called the systole, oxygenated blood is pumped out of the left ventricle into the aorta and from there through the arteries to all organs of the body, Carbon dioxide, a waste product of this process, is collected in the blood. The blood is passed back to the right atrium through the veins and the vena cava during the diastole (or relaxation) period of the heart. From there, it is pumped into the right ventricle and to the pulmonary artery to be sent to the lungs, where carbon dioxide is removed and oxygen is added.
人类心脏由四个腔室、两个心房和两个心室组成，每个腔室由几层呈圆圈和螺旋排列的心肌组成在收缩阶段，称为收缩期，含氧血液从左心室泵出进入主动脉，然后从那里通过动脉到达身体的所有器官， 二氧化碳是该过程的废物，被收集在血液中。在心脏舒张期（或舒张期）期间，血液通过静脉和腔静脉流回右心房。从那里，它被泵入右心室和肺动脉，然后被送到肺部，在那里二氧化碳被去除并增加氧气。
If the infection is localized, pus formation, redness, swelling, and irritation often occur. These symptoms indicate that the invading organisms are being fought systematically. Another indication is the development of a fever. (Normal body temperature is 98.6 F.) Fever is often caused by toxins secreted by the pathogens that interfere with the control of body temperature. Although this elevated temperature is often harmful to the body, it is also believed to act as a form of protection, Elevations of body temperature by even l or 2 degrees provide an environment that destroys some types of disease-causing organisms. Also, as body temperature rises, the body is stimulated to produce more white blood cells, which destroy more invaders.
如果感染是局部的，经常会出现脓液形成、发红、肿胀和刺激。这些症状表明入侵的生物体正在被系统地战斗。另一个迹象是发烧。（诺玛l 体温为 98.6 华氏度） 发烧通常是由干扰体温控制的病原体分泌的毒素引起的。虽然这种高温通常对身体有害，但它也被认为是一种保护形式，即使体温升高 l 或 2 度也会提供一种破坏某些类型的致病生物的环境。 此外，随着体温的升高，身体会受到刺激产生更多的白细胞，从而消灭更多的入侵者。
In some cases, however, the risks of contracting a disease are so severe that you may not be able to wait the days or weeks that are necessary for your own body to produce antibodies. Also, in the situation where a person's resistance is terribly weakened as a result of cancer chemotherapy or for other reasons,the body may be unable to produce its own antibodies. When either of these situations occurs and a victim is in need of immediate aid, antibodies formed in another person or animal are often given. Termed passive immunity, this type of immunity is often short-lived but provides the necessary boost for a person to get through a potentially critical period. Antibodies utilized for passive immunity are taken from gamma globulins, proteins synthesized from a donor's blood, A mother also confers passive immunity on her newborn baby through breast-feeding.
然而，在某些情况下，感染疾病的风险非常严重，以至于您可能无法等待自己的身体产生抗体所需的几天或几周。此外，如果一个人的抵抗力因癌症化疗或其他原因而严重减弱，身体可能无法产生自己的抗体。当这些情况中的任何一种发生并且受害者需要立即帮助时，通常会给予在另一个人或动物身上形成的抗体。这种类型的免疫力被称为被动免疫力，通常是短暂的，但可以为一个人度过潜在的关键时期提供必要的推动力。用于被动免疫的抗体取自γ球蛋白，即从供体血液中合成的蛋白质，一位母亲还通过母乳喂养赋予她新生的宝宝被动免疫力。
he most common cardiovascular disease is atherosclerosis (hardening of the arteries). Atherosclerosis of the coronary arteries may cause the development of a coronary thrombus (blood clot), which blocks the flow of blood to the heart muscle. lf, as a result, part of the heart muscle dies, the condition is called myocardial infarction (a heart attack). Some symptoms and signs of a heart attack are pain in the chest (and sometimes also in the jaws and arms), shortness of breath, irregular pulse, nausea, and perspiration. Prompt cardiopulmonary resuscitation can save victims from sudden death.
最常见的心血管疾病是动脉粥样硬化（动脉硬化）。冠状动脉粥样硬化可能导致冠状动脉血栓（血凝块）的发展，从而阻止血液流向心肌。结果，部分心肌坏死，这种情况称为心肌梗塞（心脏病发作）。心脏病发作的一些症状和体征是胸部疼痛（有时也包括下巴和手臂疼痛）、呼吸急促、脉搏不规则、恶心和出汗。及时的心肺复苏可以挽救受害者免于猝死。
Many diseases are transmitted by sexual contact. Commonly called sexually transmitted diseases (STD), these include gonorrhea, syphilis, genital herpes, and others. Some of these can be very destructive to the body if not treated, but all can be either cured or controlled by medication. By far the most frightening of the sexually transmitted disease is Acquired Immune Deficiency Syndrome(AIDS), This fatal disease is spread by direct sexual contact or exchange of blood (for example, by use of a contaminated hypodermic needle). AIDS destroys its victim's immune system, leaving the patient unprotected against infections that healthy people could fight off. “An ounce of prevention is worth a pound of cure" is an expression that especially applies to AIDS, not only because the use of condoms and sterile needles can protect
许多疾病是通过性接触传播的。通常称为性传播疾病 （STD），包括淋病、梅毒、生殖器疱疹等。如果不治疗，其中一些可能对身体造成很大的破坏性，但所有情况都可以通过药物治愈或控制。到目前为止，性传播疾病中最可怕的是获得性免疫缺陷综合症 （AIDS），这种致命的疾病通过直接的性接触或血液交换（例如，通过使用受污染的皮下注射针头）传播。艾滋病会破坏受害者的免疫系统，使患者无法抵御健康人可以抵抗的感染。“一盎司的预防胜过一磅的治疗”这句话特别适用于艾滋病，不仅因为使用避孕套和无菌针头可以保护人们免受疾病侵害，还因为目前没有治愈方法。