📚 IB Science: Human Body Key Concepts | IB 科学：人体 考点精讲
The human body is a complex, multicellular organism in which specialized cells, tissues, organs and systems work together to maintain life. This article covers essential IB Science concepts relating to human physiology, from the hierarchy of organisation to the major organ systems. You will encounter cell differentiation, enzyme action, gas exchange, blood circulation, nerve signalling and homeostasis, all of which are central to IB assessments.
人体是一个复杂的多细胞生物体，其中特化的细胞、组织、器官和系统协同工作以维持生命。本文梳理了IB科学中人体生理学的核心概念，从组织层次到各大系统。你将遇到细胞分化、酶作用、气体交换、血液循环、神经信号传导和稳态维持等内容，这些都是IB考试的重点。
1. Levels of Organisation | 组织层次
Cells are the basic structural and functional units of living organisms. In humans, stem cells differentiate into specialised cells such as muscle cells, nerve cells (neurons) and red blood cells. Each specialised cell has a unique shape and subcellular structures adapted to its function. For example, red blood cells have a biconcave shape and lack a nucleus to maximise space for haemoglobin, while neurons have long axons to transmit electrical impulses over distance.
细胞是生物体结构和功能的基本单位。在人体中，干细胞分化为特化的细胞，如肌细胞、神经细胞（神经元）和红细胞。每种特化细胞都有独特的形状和亚细胞结构，以适应其功能。例如，红细胞呈双凹圆盘形，无细胞核，以最大化空间容纳血红蛋白；而神经元有长长的轴突，用于远距离传递电冲动。
Tissues are groups of similar cells working together to perform a specific function. The four main types of animal tissues are epithelial tissue (covering and lining), connective tissue (support and binding, e.g. bone, blood, cartilage), muscle tissue (movement) and nervous tissue (signalling). Organs are structures composed of two or more tissue types that perform integrated functions; for instance, the stomach is made of epithelial, muscle and connective tissues. Organs are organised into organ systems, such as the digestive system, which processes food and absorbs nutrients.
组织是由相似细胞聚集在一起行使特定功能的结构。动物的四种基本组织类型是：上皮组织（覆盖和衬里）、结缔组织（支持和连接，如骨、血液、软骨）、肌肉组织（运动）和神经组织（信号传导）。器官是由两种或更多组织构成、能够执行整合功能的结构；例如，胃由上皮组织、肌肉组织和结缔组织组成。器官进一步组成器官系统，如消化系统负责处理食物和吸收营养。
2. Digestive System and Enzymes | 消化系统与酶
The digestive system breaks down large, insoluble food molecules into small, soluble ones that can be absorbed into the blood. Mechanical digestion (e.g. chewing in the mouth, churning in the stomach) increases surface area, while chemical digestion uses enzymes. Amylase in saliva and pancreatic juice hydrolyses starch into maltose. In the stomach, pepsin (a protease) begins protein digestion in acidic conditions (pH ~2, provided by hydrochloric acid). The small intestine is the main site of digestion and absorption, where pancreatic lipase breaks down fats into fatty acids and glycerol, and trypsin continues protein digestion.
消化系统将大的、不溶的食物分子分解为小的、可溶的分子，以便吸收进入血液。物理消化（如口腔咀嚼、胃的搅拌）增加表面积，而化学消化依赖酶。唾液和胰液中的淀粉酶将淀粉水解为麦芽糖。在胃中，胃蛋白酶（一种蛋白酶）在酸性条件下（pH~2，由盐酸提供）开始蛋白质的消化。小肠是消化和吸收的主要部位，胰脂肪酶将脂肪分解为脂肪酸和甘油，胰蛋白酶继续消化蛋白质。
Bile, produced by the liver and stored in the gallbladder, emulsifies fats into smaller droplets, increasing the surface area for lipase action. The products of digestion (glucose, amino acids, fatty acids and glycerol) are absorbed through the villi in the small intestine. Villi have a large surface area, thin epithelium (one cell thick), and a dense capillary network, which facilitates efficient absorption by diffusion and active transport.
胆汁由肝脏产生并储存在胆囊中，能将脂肪乳化成小油滴，增加脂肪酶作用的表面积。消化的产物（葡萄糖、氨基酸、脂肪酸和甘油）通过小肠绒毛吸收。绒毛具有很大的表面积、薄的上皮（单层细胞）和丰富的毛细血管网，通过扩散和主动运输高效吸收。
starch → maltose → glucose (by amylase and maltase)
淀粉 → 麦芽糖 → 葡萄糖（由淀粉酶和麦芽糖酶催化）
3. Circulatory System and Blood | 循环系统与血液
The human circulatory system consists of the heart, blood vessels and blood. It is a double circulation: the pulmonary circulation carries blood to the lungs for gas exchange, while the systemic circulation delivers oxygenated blood to the body. The heart is a muscular organ with four chambers – right atrium, right ventricle, left atrium and left ventricle. Valves prevent backflow of blood. The sinoatrial node (pacemaker) initiates the heartbeat and controls the rate.
人体循环系统由心脏、血管和血液组成，属于双循环：肺循环将血液送到肺部进行气体交换，体循环将含氧血输送到全身。心脏是一个有四个腔室的肌性器官——右心房、右心室、左心房和左心室。瓣膜防止血液倒流。窦房结（起搏器）启动心跳并控制心率。
Blood is composed of plasma, red blood cells (erythrocytes), white blood cells (leucocytes) and platelets. Red blood cells contain haemoglobin, which binds oxygen in the lungs and releases it in tissues. White blood cells are part of the immune system; phagocytes engulf pathogens, while lymphocytes produce antibodies. Platelets are involved in blood clotting. Plasma transports carbon dioxide, nutrients, hormones and waste products.
血液由血浆、红细胞、白细胞和血小板组成。红细胞含血红蛋白，在肺部结合氧，在组织释放氧。白细胞参与免疫；吞噬细胞吞噬病原体，淋巴细胞产生抗体。血小板参与凝血。血浆运输二氧化碳、营养物质、激素和代谢废物。
Arteries carry blood away from the heart and have thick, muscular, elastic walls to withstand high pressure. Veins return blood to the heart; they have thinner walls and contain valves. Capillaries are tiny vessels with one-cell-thick walls, allowing exchange of materials by diffusion.
动脉将血液带离心脏，管壁厚、肌肉发达且富有弹性，以承受高压。静脉将血液送回心脏，管壁较薄且含有瓣膜。毛细血管是极细的血管，管壁只有一层细胞，通过扩散进行物质交换。
4. Respiratory System and Gas Exchange | 呼吸系统与气体交换
The respiratory system provides oxygen for aerobic respiration and removes carbon dioxide. Air enters through the nose or mouth, passes through the trachea, bronchi and bronchioles, and finally reaches the alveoli – tiny air sacs where gas exchange occurs. The trachea and bronchi are supported by rings of cartilage to keep the airways open. The walls of the trachea and bronchi are lined with ciliated epithelial cells and mucus-secreting goblet cells, which trap and remove dust and pathogens.
呼吸系统为有氧呼吸提供氧气，并排出二氧化碳。空气通过鼻或口进入，经气管、支气管和细支气管，最终到达肺泡——气体交换的微小气室。气管和支气管由软骨环支撑以保持气道畅通。气管和支气管壁衬有纤毛上皮细胞和分泌黏液的杯状细胞，能够黏附并清除灰尘和病原体。
Alveoli are surrounded by a dense network of capillaries. They have very thin walls (one cell thick) and are moist, allowing rapid diffusion of gases. Oxygen diffuses from the alveoli into the blood, where it binds to haemoglobin. Carbon dioxide diffuses from the blood into the alveoli to be exhaled. The process of breathing involves the diaphragm and intercostal muscles. During inhalation, the diaphragm contracts and flattens, and the intercostal muscles raise the ribcage, increasing the volume of the thoracic cavity and decreasing pressure, so air rushes in. Exhalation is mostly passive, with relaxation of these muscles.
肺泡被丰富的毛细血管网包裹。它们的壁极薄（单层细胞），湿润，有利于气体快速扩散。氧从肺泡扩散到血液中，与血红蛋白结合。二氧化碳从血液扩散到肺泡，被呼出。呼吸过程涉及膈肌和肋间肌。吸气时，膈肌收缩变平，肋间肌提升胸廓，增大胸腔容积，降低压力，空气进入。呼气主要是被动的，这些肌肉放松。
O₂ + haemoglobin ⇌ oxyhaemoglobin
氧气 + 血红蛋白 ⇌ 氧合血红蛋白
5. Nervous System and Reflex Arcs | 神经系统与反射弧
The nervous system allows rapid communication between receptors, the central nervous system (CNS: brain and spinal cord) and effectors. Neurons are specialised cells that transmit electrical impulses. A typical motor neuron has a cell body, dendrites (receive signals) and a long axon (transmits signals away). The axon is covered by a myelin sheath, which insulates the axon and speeds up impulse transmission via saltatory conduction. Synapses are junctions between neurons where neurotransmitters carry signals across the gap.
神经系统使感受器、中枢神经系统（CNS：脑和脊髓）和效应器之间能够快速沟通。神经元是传递电冲动的特化细胞。典型的运动神经元具有细胞体、树突（接收信号）和长轴突（将信号传出）。轴突由髓鞘包裹，髓鞘起绝缘作用，并通过跳跃传导加快冲动的传递。突触是神经元之间的连接点，神经递质在此将信号传递过间隙。
nd). A classic example is the withdrawal reflex when touching a hot object: the hand is pulled away before the brain consciously perceives pain because the spinal cord processes the reflex.
反射弧是对刺激快速、自动的反应，通常只涉及脊髓，提供迅速的保护性反应。其通路为：刺激 → 感受器 → 感觉神经元 → 脊髓中的联结神经元 → 运动神经元 → 效应器（肌肉或腺体）。一个经典例子是碰到热物体时的缩手反射：手在脑有意识感知疼痛之前就缩回，因为脊髓处理了该反射。
6. Endocrine System and Hormonal Control | 内分泌系统与激素调控
The endocrine system uses chemical messengers called hormones, which are secreted by glands and transported in the blood to target organs. Compared to the nervous system, hormonal responses are slower but longer-lasting. Key glands include the pituitary gland (master gland), thyroid gland (thyroxine, regulates metabolic rate), pancreas (insulin and glucagon, regulate blood glucose), adrenal glands (adrenaline, 'fight or flight' response) and ovaries/testes (sex hormones).
内分泌系统使用称为激素的化学信使，由腺体分泌，通过血液运输至靶器官。与神经系统相比，激素反应较慢但持续时间更长。重要的腺体包括垂体（主腺）、甲状腺（甲状腺素，调节代谢率）、胰腺（胰岛素和胰高血糖素，调节血糖）、肾上腺（肾上腺素，“战斗或逃跑”反应）以及卵巢/睾丸（性激素）。
A key concept in IB is negative feedback, which maintains homeostasis. For example, blood glucose regulation: after a meal, high blood glucose stimulates the pancreas to secrete insulin, which promotes glucose uptake by cells and conversion to glycogen in the liver, lowering blood glucose. When blood glucose is low, the pancreas secretes glucagon, stimulating glycogen breakdown into glucose. Insulin and glucagon work antagonistically to keep glucose levels within a narrow range.
IB中的核心概念是负反馈，它维持稳态。例如，血糖调节：餐后高血糖刺激胰腺分泌胰岛素，促进细胞摄取葡萄糖并在肝脏转化为糖原，降低血糖。当血糖低时，胰腺分泌胰高血糖素，刺激糖原分解为葡萄糖。胰岛素和胰高血糖素拮抗作用，使血糖保持在较窄范围内。
7. Musculoskeletal System and Movement | 肌肉骨骼系统与运动
The human skeleton provides support, protection, and a framework for muscle attachment to enable movement. Bones are connected at joints. Synovial joints, such as the knee and elbow, are freely movable and contain synovial fluid that lubricates the joint and cartilage that cushions the ends of bones. Ligaments connect bone to bone, while tendons attach muscle to bone.
人体骨骼提供支撑、保护，并为肌肉附着以产生运动提供了框架。骨在关节处连接。滑液关节，如膝关节和肘关节，可自由活动，内含滑液以润滑关节，软骨缓冲骨端。韧带连接骨与骨，肌腱连接肌与骨。
Muscles work in antagonistic pairs because they can only contract (pull), not push. For example, the biceps and triceps in the upper arm: when the biceps contracts, the arm flexes (bends); when the triceps contracts, the arm extends (straightens). Muscle contraction is explained by the sliding filament theory, where myosin filaments pull actin filaments, shortening the sarcomere. This process requires energy from ATP and involves calcium ions and the proteins tropomyosin and troponin.
肌肉成对拮抗工作，因为它们只能收缩（拉），不能推。例如上臂的肱二头肌和肱三头肌：当肱二头肌收缩时，手臂屈曲；当肱三头肌收缩时，手臂伸直。肌肉收缩可用肌丝滑动学说解释，即肌球蛋白丝拉动肌动蛋白丝，使肌节缩短。这个过程需要ATP提供能量，并涉及钙离子以及原肌球蛋白和肌钙蛋白。
8. Excretory System and Kidney Function | 排泄系统与肾脏功能
The excretory system removes metabolic wastes, primarily urea, excess water and salts, from the blood. The kidneys are the main organs. Each kidney contains about one million nephrons – the functional units. A nephron consists of the Bowman's capsule, glomerulus, proximal convoluted tubule, loop of Henlé, distal convoluted tubule and collecting duct.
排泄系统将代谢废物，主要是尿素、多余的水和盐，从血液中清除。肾脏是主要器官。每个肾含有约一百万个肾单位——功能性单位。一个肾单位包括鲍曼囊、肾小球、近曲小管、亨勒袢、远曲小管和集合管。
Filtration occurs in the glomerulus, where high pressure forces water, urea, glucose, salts and other small molecules out of the blood into the Bowman's capsule, forming the filtrate. Large molecules like proteins and red blood cells are retained. Selective reabsorption takes place mainly in the proximal convoluted tubule: all glucose, most water and useful ions are reabsorbed back into the blood by active transport and diffusion. The loop of Henlé creates a concentration gradient in the medulla, allowing water reabsorption in the collecting duct. Final urine contains water, urea and excess ions, and is transported via the ureters to the bladder for storage before excretion.
滤过发生在肾小球，高压将水、尿素、葡萄糖、盐和其他小分子从血液中压出，进入鲍曼囊，形成滤液。蛋白质和红细胞等大分子被截留。选择性重吸收主要在近曲小管进行：所有的葡萄糖、大部分水和有用的离子通过主动运输和扩散重新回到血液中。亨勒袢在髓质中建立浓度梯度，使集合管能够重吸收水。最终的尿液含有水、尿素和多余的离子，经输尿管输送至膀胱储存，然后排出体外。
9. Immune System and Defence | 免疫系统与防御
The immune system defends the body against pathogens such as bacteria, viruses and fungi. It is divided into non-specific (innate) and specific (adaptive) defences. Non-specific defences include physical barriers like skin and mucous membranes, chemical barriers like stomach acid and lysozyme in tears, and phagocytosis by white blood cells. Inflammation and fever are also non-specific responses.
免疫系统保护身体免受病原体（如细菌、病毒和真菌）侵害。它分为非特异性（先天）防御和特异性（适应性）防御。非特异性防御包括物理屏障如皮肤和黏膜，化学屏障如胃酸和泪液中的溶菌酶，以及白细胞的吞噬作用。炎症和发热也是非特异性应答。
Specific immunity targets particular pathogens and provides memory. Lymphocytes play key roles: B lymphocytes produce antibodies that bind to antigens on pathogens, marking them for destruction. T lymphocytes (helper T cells) activate B cells and cytotoxic T cells, which kill infected cells. After an infection, memory cells remain in the body, enabling a faster and stronger response upon re-exposure. Vaccination mimics a natural infection to induce immunity without causing disease, by using weakened or inactivated pathogens or their antigens.
特异性免疫针对特定病原体并提供记忆。淋巴细胞起关键作用：B淋巴细胞产生抗体，抗体与病原体上的抗原结合，标记它们以供摧毁。T淋巴细胞（辅助性T细胞）激活B细胞和细胞毒性T细胞，后者杀伤受感染的细胞。感染后，记忆细胞留在体内，使再次接触时能产生更快、更强的应答。疫苗接种通过使用减毒或灭活的病原体或其抗原，模拟自然感染以诱导免疫而不引起疾病。
10. Homeostasis and Thermoregulation | 稳态与体温调节
Homeostasis is the maintenance of a constant internal environment despite external changes. It relies on negative feedback mechanisms. Temperature regulation is a classic example. The body's core temperature is kept around 37°C, regulated by the thermoregulatory centre in the hypothalamus of the brain. Receptors in the skin and hypothalamus detect temperature changes.
稳态是指尽管外部环境变化，仍能维持相对稳定的内环境。它依赖负反馈机制。体温调节是一个经典例子。人体核心温度维持在37°C左右，由大脑下丘脑的体温调节中枢调控。皮肤和下丘脑中的感受器检测温度变化。
When body temperature rises, the hypothalamus triggers responses to increase heat loss: vasodilation (widening of skin arterioles to bring warm blood close to the surface), increased sweating (evaporation cools the skin) and pilorelaxation (hairs lie flat). When body temperature falls, heat conservation mechanisms are activated: vasoconstriction (narrowing of skin arterioles to reduce heat loss), shivering (muscle contractions generate heat) and piloerection (hairs stand on end to trap an insulating layer of air). The pancreas and liver also adjust metabolism to influence heat production.
当体温升高，下丘脑启动促散热反应：血管舒张（皮肤小动脉扩张，使温血靠近体表）、出汗增加（蒸发冷却皮肤）和毛发平躺。当体温下降，保热机制激活：血管收缩（皮肤小动脉收缩以减少散热）、战栗（肌肉收缩产热）和毛发竖立（直毛鸡皮疙瘩，形成隔热空气层）。胰腺和肝脏也会调节代谢以影响产热。
11. Reproduction and Development | 生殖与发育
Human reproduction involves the fusion of male and female gametes (sperm and egg) to form a zygote, which develops into an embryo and then a foetus. The male reproductive system produces sperm in the testes and delivers them via the penis. Sperm are small, motile cells with a head containing the nucleus and acrosome (enzymes to penetrate the egg). The female reproductive system produces eggs in the ovaries; typically one egg is released each menstrual cycle during ovulation. The egg travels through the fallopian tube, where fertilisation may occur.
人类生殖涉及精子和卵子（配子）融合形成合子，合子发育成胚胎再到胎儿。男性生殖系统在睾丸生成精子，经阴茎输送。精子是小而能动的细胞，头部含有细胞核和顶体（含穿透卵子的酶）。女性生殖系统在卵巢生成卵子；通常每个月经周期排出一个卵子（排卵）。卵子沿输卵管移动，可能在壶腹部受精。
After fertilisation, the zygote undergoes mitosis and forms a morula then a blastocyst, which implants into the uterine lining (endometrium). The placenta develops, allowing exchange of nutrients, gases and wastes between maternal and foetal blood without the two bloods mixing. The foetus is protected in the amniotic sac and obtains oxygen and nutrients from the mother via the umbilical cord. Hormones such as progesterone maintain the pregnancy. During birth, the cervix dilates and uterine contractions push the baby out.
受精后，合子进行有丝分裂，形成桑椹胚再到囊胚，囊胚植入子宫内膜。胎盘发育，使母体与胎儿的血液之间能够交换营养、气体和废物，但两者不混合。胎儿受羊膜囊保护，并通过脐带从母体获得氧和营养。孕酮等激素维持妊娠。分娩时，宫颈扩张，子宫收缩将婴儿推出。
12. Key IB Practical Skills | IB 关键实验技能
In IB Science, you may be assessed on practical skills relating to the human body. Common investigations include measuring the effect of exercise on heart rate and breathing rate, testing for the presence of nutrients (starch, glucose, protein, fat) using Benedict's solution, iodine, Biuret reagent and ethanol, modelling enzyme action with variable temperature/pH, and microscopy of specialised cells (e.g. blood smear, cheek cells). You should be able to identify variables (independent, dependent, control), design a reliable method, record data systematically, and evaluate the validity and limitations of the experiment.
在IB科学中，你可能被评估与人体相关的实验技能。常见的探究包括：测量运动对心率和呼吸率的影响；使用本尼迪克特试剂、碘液、双缩脲试剂和乙醇检测营养物质（淀粉、葡萄糖、蛋白质、脂肪）；模拟不同温度/pH下酶的作用；以及显微镜观察特化细胞（如血涂片、口腔上皮细胞）。你需要能识别变量（自变量、因变量、控制变量），设计可靠的方法，系统记录数据，并评价实验的有效性和局限性。
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