Part 2

What is blood made of? Blood contains three types of cells—red blood cells, white blood cells, and platelets. These float around in a yellowish liquid called plasma. Blood is 55 percent plasma and 44 percent red blood cells. The remaining 1 percent is made up of white blood cells and platelets. Blood Blood travels to every part of your body through a network of tubes called blood vessels. It delivers oxygen to the body to keep it working, and removes carbon dioxide by carrying it to the lungs to be breathed out. Blood also fights infections. Plasma Plasma is mostly made of water. The rest is a mix of substances, including antibodies—which fight infections—as well as protein and waste matter. White blood cells and platelets White blood cells patrol the body to kill germs. Platelets, found in plasma, help to heal cuts and wounds. Red blood cells Red blood cells contain a protein called hemoglobin, which carries oxygen. Hemoglobin also gives blood its red color. Some animals, such as crabs and lobsters, have blue blood! BLOOD TYPES There are four main blood groups: A, B, O, and AB. The antigen, or protein, on the red blood cell determines which type you are. Type A The A antigen lives on the surface of the red blood cell. Type B The B antigen lives on the surface of the red blood cell. Type AB Both A and B antigens are found on the cell's surface. Type O There are no antigens on the cell's surface.

血液是由什么组成的呢？ 血液含有三种细胞——红细胞、白细胞和血小板。 它们都悬浮在一种叫做血浆的淡黄色液体中。 血液中55%是血浆，44%是红细胞。 剩下的1%由白细胞和血小板组成。 血液 血液通过名为血管的管道网流到你身体的各个部位。 它给身体输送氧气维持运转，还会带走二氧化碳，将它运到肺部排出体外。 血液还能对抗感染。 血浆 血浆大部分是由水组成的。 剩下的部分是多种物质的混合物，包括对抗感染的抗体，还有蛋白质和代谢废物。 白细胞和血小板 白细胞会在身体里巡逻，杀死病菌。 存在于血浆中的血小板能帮助伤口愈合。 红细胞 红细胞含有一种叫做血红蛋白的蛋白质，它负责运输氧气。 血红蛋白也让血液呈现出红色。 有些动物，比如螃蟹和龙虾，血液是蓝色的哦！ 血型 主要有四种血型：A型、B型、O型和AB型。 红细胞表面的抗原（也就是蛋白质）决定了你的血型。 A型血 A型抗原存在于红细胞的表面。 B型血 B型抗原存在于红细胞的表面。 AB型血 红细胞表面同时存在A型和B型抗原。 O型血 红细胞表面没有任何抗原。

The story of blood People's knowledge of blood and how it circulates around the body has changed greatly since ancient times. Over the years, careful study of the human body has helped our understanding of how blood works. Ancient Egyptians In Ancient Egypt, it was believed that different vessels carried fluids, such as blood, snot, and tears, from organs in the body to the heart. Four humors Ancient Greek doctors believed the body had four "humors": blood, phlegm, black bile, and yellow bile. A transfusion is when blood is transferred from one animal to another. Knowing the different blood groups made blood transfusions much safer. White blood cells In 1843, white particles were seen in blood along with red ones— this was the discovery of white blood cells. Blood groups The different blood groups were discovered by Austrian scientist Karl Landsteiner in 1901. Blood transfusions Scientists successfully carried out blood transfusions in dogs and sheep before the first successful human-to-human blood transfusion in 1818. Platelets Scientists had seen oddly shaped granules in blood, which they had thought were damaged blood cells. In 1882, it was discovered that they were in fact platelets. Blood makes up around 8 percent of your body weight. Ancient India Around 600BCE, Indian doctors believed that by feeling the pulse, they could tell if the humors were balanced correctly. Blood vessels Ancient Romans were the first to suggest that blood was carried in blood vessels. However, they also thought that the liver made blood, and from there it flowed to the body. Plugging blood At the start of the 20th century, experiments showed that platelets in blood flock to the site of an injury, creating a blood clot to stop the bleeding. William Harvey This British doctor discovered the connection between the heart and blood vessels, and showed how blood was transported to and from the rest of the body. Today When blood is donated today, the white blood cells are removed. The red blood cells and platelets may be separated so they can be used where needed.

血液研究史 从古至今，人们对血液以及血液循环的认知发生了很大的改变。 多年来，对人体的细致研究帮助我们弄清了血液的运作方式。 古埃及人 在古埃及，人们认为不同的血管会把血液、鼻涕、眼泪等体液从身体器官输送到心脏。 四种体液 古希腊医生认为人体内有四种“体液”：血液、黏液、黑胆汁和黄胆汁。 输血就是将血液从一个个体输送到另一个个体体内。 了解不同的血型后，输血变得安全多了。 白细胞 1843年，人们在血液的红细胞之外发现了白色颗粒——这就是白细胞。 血型 不同的血型是奥地利科学家卡尔·兰德施泰纳在1901年发现的。 输血 在1818年第一例成功的人对人输血之前，科学家就已经在狗和羊身上成功完成了输血实验。 血小板 科学家早年在血液中发现过形状怪异的颗粒，曾以为它们是受损的血细胞。 1882年，人们发现它们其实就是血小板。 血液大约占人体体重的8%。 古印度 公元前600年左右，印度医生认为通过搭脉就能判断体内的体液是否平衡。 血管 古罗马人最早提出血液是在血管中运输的。 不过他们还认为血液由肝脏制造，之后从肝脏流向全身。 止血 20世纪初，实验证明血液中的血小板会聚集在伤口处，形成血凝块止血。 威廉·哈维 这位英国医生发现了心脏和血管的联系，揭示了血液如何在全身循环运输。 现如今 如今人们捐献血液后，会去除其中的白细胞。 红细胞和血小板可以被分离出来，用到需要它们的地方。

Inside the lungs Air travels to the lungs through the nose and mouth, and down the trachea (windpipe). The trachea splits into two branches, called bronchi, one going to each lung. The bronchi split into smaller branches—the smallest of which are called bronchioles. These end in small air sacs called alveoli. You breathe around 20–25 times per minute. Breathe in When you breathe in, your lungs fill with fresh oxygen gas, which is needed to make your body's cells work. Carbon dioxide, a waste gas, is removed from your body when you breathe out. A HELPING HAND The "iron lung" was a machine used to help people with breathing difficulties. The patient laid inside a large metal tube, and air was pumped in and out to keep their lungs working. Exchanging gases A lot of tiny blood vessels called capillaries wrap around each alveolus, allowing the gases to move in and out. Oxygen passes through the walls of the alveoli into the blood, whilst carbon dioxide moves into the alveoli to be breathed out. Arteries (red) carry oxygen-rich blood away Veins (blue) bring oxygen- poor blood in Oxygen goes in Carbon dioxide comes out Blood cells change from being oxygen-poor (blue) to oxygen-rich (red) In and out The diaphragm is a large sheet of muscle under your ribcage. It contracts and relaxes when you breathe to pull air into your lungs and push it out. Muscles attached to the ribcage move it up and out when you breathe in, to fit your expanding lungs. Breathing in The diaphragm flattens, and the lungs and ribcage become larger. Breathing out The lungs and ribcage shrink, and the diaphragm relaxes upward. Your lungs are like balloons that inflate when you breathe in and deflate when you breathe out.

肺的内部 空气经鼻子和嘴巴进入肺部，再向下进入气管（呼吸管道）。 气管分成两个叫做支气管的分支，分别通向两侧的肺。 支气管继续分出更小的分支，其中最小的分支叫做细支气管。 这些细支气管的末端是名为肺泡的微小气囊。 你每分钟大约会呼吸20-25次。 吸气时 当你吸气，肺部会充满新鲜氧气，氧气是身体细胞工作必不可少的物质。 二氧化碳是一种废气，会在你呼气的时候排出体外。 一个好帮手 “铁肺”是过去用来帮助呼吸困难人群呼吸的机器。 病人躺在一根粗大的金属管里，空气被泵进抽出，以此维持肺部运作。 气体交换 每个肺泡外都缠绕着许多叫做毛细血管的微小血管，方便气体进出交换。 氧气穿过肺泡壁进入血液，同时二氧化碳进入肺泡等待被排出。 动脉（红色）运出富氧血液 静脉（蓝色）输入缺氧血液 氧气进入 二氧化碳排出 血细胞从缺氧（蓝色）变成富氧（红色） 一进一出 横膈膜是胸腔下方一块大大的肌肉膜。 呼吸时它会收缩放松，帮你把空气吸进肺部再排出去。 吸气时，连接肋骨的肌肉带动胸廓向上向外扩张，给膨胀的肺部腾出空间。 吸气时 横膈膜变平，肺部和胸廓的体积变大。 呼气时 肺部和胸廓收缩，横膈膜放松向上回弹。 你的肺就像气球，吸气时膨胀，呼气时缩小。

Your brilliant brain Everything you see, hear, do, feel, and think is controlled by different parts of your brain. It contains billions of cells, called neurons, that communicate with the rest of the body through a large network of nerves. Personality The frontal lobe is home to your personality. This affects how you behave, and what you like and dislike. Cerebrum The largest part of the brain is the cerebrum. Its outer layer is split into four different "lobes." Hearing The temporal lobe makes sense of the sounds picked up by the ears. It helps you to hear and talk. Awareness External information, such as temperature, pain, and pressure, are sent to the parietal lobe. This makes you more aware of your surroundings. Seeing Everything the eye sees is sent as signals to the occipital lobe. It then processes the information to form an image. One-fifth of your blood supply goes to the brain. Cerebellum The cerebellum helps the muscles of your body work together to make smooth movements. LEFT AND RIGHT SIDE The cerebrum is made up of two halves, called hemispheres. The left hemisphere tells the right side of the body what to do and the right hemisphere tells the left side of the body what to do. Control center Just under the cerebellum is the brain stem. It joins the brain to the spinal cord and controls important functions, such as breathing and heart rate.

聪明的大脑 你看到、听到、做到、感受到和想到的一切，都由大脑的不同区域控制。 大脑里有数十亿个叫做神经元的细胞，它们通过庞大的神经网络和身体其他部位传递信息。 性格 额叶是你性格的控制中心。 它影响你的行为方式，还有你的喜好和厌恶。 大脑 脑部最大的部分就是端脑（大脑）。 它的外层分成四个不同的"脑叶"。 听觉 颞叶会处理耳朵收集到的声音信息。 它帮你听见声音，也帮你正常说话。 感知 外界信息比如温度、疼痛、压力，都会被传送到顶叶。 它让你感知到周围环境的变化。 视觉 眼睛看到的所有东西都会以信号的形式发送到枕叶。 枕叶接着处理这些信息，形成我们看到的图像。 你体内五分之一的血液会供给大脑。 小脑 小脑能让全身肌肉互相配合，做出流畅平稳的动作。 左右半脑 端脑由两个部分组成，叫做脑半球。 左半脑控制身体右侧，右半脑控制身体左侧。 控制中枢 脑干就在小脑的下方。 它连接大脑和脊髓，控制着呼吸、心率这些重要的生理功能。

Network of nerves Every part of the body needs a nerve supply in order to work properly. Nerves are like electrical wires, carrying information in the form of signals all around the body. The nervous system controls everything you do. Your brain has around 86 billion neurons! Neurons Nerves are made up of bundles of cells called neurons. Electrical signals travel along the neuron's length—from the dendrites to the nucleus, and then along the axon. A chemical is released at the synapse, passing the signal onto the next neuron. Axon The electrical impulse travels down the axon, a long, thread-like fiber, to the end of the neuron. Myelin sheath Many axons have a protective covering, which helps information travel faster Nucleus The nucleus is found in the cell body. It controls the neuron. Spinal cord This large bundle of nerve tissues links the brain to the rest of the body. Signals travel from nerves in the body along the spinal cord to the brain, where the information is processed. Synapse Information is passed along at the gap where two neurons meet. Dendrites These thread-like branches coming off the cell body gather information from connecting neurons. Ouch! If you touch something hot, nerves in your hand send a signal to the brain along the spinal cord. The brain processes the information and you feel pain. WHERE DO NERVES GO? Together, the brain and spinal cord form the central nervous system (CNS). All other nerves make up the peripheral nervous system (PNS), which links the CNS to your organs and muscles. Sense organs Sensory nerves take signals from your sense organs, such as the eyes, to your brain. Muscles Motor nerves carry information from the brain to your muscles, to tell them when to move. Internal organs Nerves also control your internal organs—for example, they tell your heart to keep beating.

神经网 身体每个部位都需要神经供给，才能正常工作。 神经就像电线，以信号的形式把信息传遍全身各处。 神经系统控制着你做的所有活动。 你的大脑里约有860亿个神经元！ 神经元 神经是由一束束名为神经元的细胞组成的。 电信号沿着神经元全程传导——从树突到细胞核，再沿着轴传出。 突触会释放一种化学物质，把信号传递给下一个神经元。 轴突 电脉冲沿着轴突（一根细长的线状纤维）传到神经元的末端。 髓鞘 许多轴突都有一层保护外壳，能帮助信息更快传递 细胞核 细胞核位于神经元的细胞体中。 它控制着神经元的活动。 脊髓 这一大束神经组织连接着大脑和身体的其他部位。 信号沿着脊髓从身体神经传到大脑，再由大脑处理信息。 突触 信息在两个神经元相接的缝隙处完成传递。 树突 这些从细胞体延伸出的线状分支，负责收集相连神经元传来的信息。 哎哟！ 如果你摸到滚烫的东西，手上的神经就会沿着脊髓向大脑发送信号。 大脑处理完信息，你就感觉到疼痛了。 神经都分布在哪里？ 大脑和脊髓共同组成了中枢神经系统（CNS）。 其余所有神经组成了周围神经系统（PNS），它将中枢神经系统连接到你的器官和肌肉。 感觉器官 感觉神经会把眼睛这类感觉器官接收的信号传递给大脑。 肌肉 运动神经会把大脑的指令传递给肌肉，告诉它们什么时候活动。 内脏器官 神经也会控制你的内脏——比如说，它们会让心脏一直保持跳动。

The secrets of skin Skin is the largest organ in the body. It keeps your body wrapped up and protects your insides. Sensory receptors in the skin detect what is going on around you and keep you safe from danger. Skin cells only live for 30 days. Under the skin Skin is made up of two layers. The thin outer layer is the epidermis. Below this is the dermis, a thick layer full of nerves, blood vessels, and glands. Pores Tiny holes on the skin allow sweat to leave the body. Epidermis This layer is waterproof to keep you dry. The epidermis grows new skin cells every month. Blood vessels These get bigger when the body is too warm, allowing heat to escape from the blood through the skin. Dermis Elastic fibers located here make the skin stretchy. Hair follicle New hairs grow from tubes called hair follicles. Sweat glands These glands make salty sweat to help cool you down. Nerves These send signals from receptors to the brain. LEECHES Leeches are worms that have been used in medicine since the late 1700s. They suck on blood and release substances that numb the skin, stop blood clotting, and make blood vessels get bigger. Fresh blood then flows to the skin, helping it heal. Soggy skin Glands in the skin make an oily substance called sebum to keep it soft. When you spend too long in the water, the sebum is washed away. Without it, water is absorbed into the skin, making it wrinkly. Your skin is thickest on the soles of your feet. AMAZING SKIN Skin tones Skin contains a chemical called melanin—the more melanin you have, the darker your skin is. Sometimes, your skin makes extra melanin in the sun to protect you from its harmful rays. This is why you get a suntan. Healing Tiny cells in the skin called fibroblasts help to heal cuts and wounds. They arrive at the injury and start to make new skin. This includes scar tissue, which is 80 percent as strong as the original skin. Growing old As we age, our skin becomes less elastic. We produce fewer elastic fibers and less sebum, and we also lose water from our bodies. This makes skin stiffer and wrinkly.

皮肤的秘密 皮肤是人体最大的器官。 它包裹着你的身体，保护着你的体内组织。 皮肤里的感觉感受器能感知周围变化，保护你远离危险。 皮肤细胞的寿命只有30天。 皮下结构 皮肤由两层结构组成。 薄薄的外层叫做表皮。 表皮下方是真皮，这层厚厚的组织布满了神经、血管和腺体。 毛孔 皮肤上的小孔能让汗液排出体外。 表皮 这一层可以防水，让你保持干爽。 表皮每个月都会长出新的皮肤细胞。 血管 当身体过热时，血管会扩张，让热量随着血液透过皮肤散出去。 真皮 这里的弹性纤维让皮肤富有弹性。 毛囊 新毛发从叫做毛囊的管道中长出来。 汗腺 这些腺体会分泌带盐分的汗液，帮你降温。 神经 它们将信号从感受器传送到大脑。 水蛭 早在18世纪末，水蛭这种蠕虫就被用于医学领域了。 它们吸血时会释放物质，能麻痹皮肤、阻止血液凝固、还能扩张血管。 新鲜血液随后流到皮肤处，帮助伤口愈合。 皮肤发皱 皮肤里的腺体会分泌一种叫做皮脂的油脂，让皮肤保持柔软。 当你在水里待太久，皮脂就会被冲洗掉。 没有皮脂保护后，水分会渗入皮肤，让皮肤变皱。 你脚底的皮肤是全身最厚的。 神奇皮肤 肤色 皮肤中含有一种叫做黑色素的物质——黑色素越多，你的肤色就越深。 有时皮肤会在阳光下生成额外的黑色素，保护你不受有害光线伤害。 这就是你会晒黑的原因。 伤口愈合 皮肤里有种叫做成纤维细胞的微小细胞，能帮助愈合伤口。 它们抵达伤口后，就会开始生成新的皮肤。 生成的皮肤包括疤痕组织，它的强度能达到原皮肤的80%。 皮肤衰老 随着年龄增长，我们的皮肤弹性会越来越差。 我们生成的弹性纤维和皮脂都会减少，身体的水分也会流失。 这就让皮肤变得更僵硬，还长出了皱纹。