IB生物HL 气体交换 血红蛋白 稳态调节
人体生理在 IB 生物里已经不是一个可以挑着学的选修模块。按 IBO 官网公布的 IB Biology guide（2025 年首次考试），这部分内容分散在 Theme B（B3.1 气体交换、B3.2 运输、B3.3 肌肉与运动，后者只有 HL 学）和 Theme C（C3.1 讲各系统之间的整合）。下面按这条线把机制串起来，每一节的顺序都是先讲结构再讲功能，因为答题时拿分的正是这个对应关系。
The human physiology content in IB Biology is no longer a single optional topic. In the guide with first assessment in 2025 it is spread across Theme B, which holds gas exchange, transport and, at HL only, muscle and motility, and Theme C, where the integration of systems is examined. What follows traces the causal chains that carry marks, from surface area to haemoglobin and from the sarcomere to thermoregulation. Each section names the structure first because the structure is what the answer has to point at.
表面积与体积之比 与 Fick 定律 限定扩散速率
单细胞生物靠整个体表就能完成气体交换，因为它离表面近。细胞变大以后体积按立方增长，表面积只按平方增长，单位体积能分到的表面越来越少。细胞中心到表面的距离也变远，而扩散在毫米尺度上慢到跟不上代谢。这就是为什么大型、活跃的生物必须长出一块折叠得很厉害的交换面。Fick 定律把这件事写成三道乘除：速率与表面积和浓度差成正比，与厚度成反比。IB 的题常给一个具体结构，让你指认它改变的是哪一个量。很多学生以为肺泡壁薄只是为了省地方，其实薄对应的是扩散距离这一项。
Diffusion is fast over micrometres and hopeless over millimetres, which sets the size limit for any organism that relies on its body surface. Volume grows as the cube of length while surface grows as the square, so a larger animal has less exchange area for each unit of respiring tissue; that is why lungs, gills and leaves are all folded. Fick's law turns the argument into three quantities, with area and concentration difference in the numerator and thickness in the denominator. Examiners like to name one alveolar feature and ask which quantity it alters, so learn each structure with its own variable attached.
I型与II型肺泡细胞 表面活性物质 与 0.5 微米的扩散距离
两个肺里大约有 3 亿到 5 亿个肺泡，摊开的总面积在 70 平方米这个量级，全部塞在胸腔这么小的地方。肺泡壁是单层扁平的 I 型肺泡细胞，外面紧贴单层内皮构成的毛细血管，气体跨过的距离大约 0.5 微米。II 型肺泡细胞体积更圆，负责分泌表面活性物质，这层磷脂把水膜的表面张力压低，呼气末肺泡才不会黏在一起塌掉。II 型细胞还能分裂，补充受损的 I 型细胞。阅卷时能看出学生有没有把结构和 Fick 定律挂上钩，只写肺泡壁很薄而不提扩散距离的那类答案通常只拿到一半分。还有一个高频混淆：表面活性物质的作用不是增加表面积，它管的是防止塌陷。肺泡里的氧气分压高于血液，二氧化碳分压低于血液，两个方向靠各自的梯度同时进行。
An adult pair of lungs holds something in the region of 300 to 500 million alveoli, which unfold to a total surface of about 70 square metres. Type I pneumocytes form that surface and are flattened to a fraction of a micrometre, which shortens the path the gases take; type II pneumocytes are the rounded cells that secrete surfactant and can divide to replace damaged type I cells. Surfactant lowers the surface tension of the fluid lining, so alveoli do not stick shut at the end of a forced expiration. Students often describe the wall as thin because thin sounds efficient, yet the mark comes from tying the thinness to diffusion distance and to nothing else.
膈肌 外肋间肌 与 胸腔容积和肺内压的变化
通气解决的是浓度梯度问题，不是扩散本身。膈肌收缩时由穹顶变平，外肋间肌把肋骨拉起并外翻，胸腔容积变大，肺内压降到低于大气压，空气顺着压力差进入。安静呼气不需要肌肉，肺和胸廓的弹性回缩就够了，用力呼气时内肋间肌和腹肌才参与。潮气量是每次平静呼吸进出的气量，成人约 0.5 升；肺活量是尽力吸气后再尽力呼出的最大气量，约 4.5 到 5 升。通气率等于潮气量乘以呼吸频率，题目给肺量计图时先读刻度和单位再乘。常见的错误是把通气、气体交换和细胞呼吸混成一件事，这三者分别发生在肺、肺泡壁和线粒体里。
Ventilation exists to keep a steep gradient, not to push oxygen into the blood by itself. The diaphragm flattens and the external intercostals lift the ribs, chest volume rises and the pressure inside the lungs falls below atmospheric, so air flows in; quiet expiration needs no muscle at all because elastic recoil does the work. Tidal volume is the air moved in one resting breath, around half a cubic decimetre, while vital capacity is the largest volume that can be forced out after a full inhalation, roughly 4.5 to 5. Ventilation rate is tidal volume multiplied by breathing frequency, and a spirometer trace should be checked for scale before any multiplication.
心脏双泵 房室瓣 半月瓣 与心动周期的时序
心脏是两台泵并在一起，右心把缺氧血送去肺，左心把含氧血送去全身，所以左心室壁最厚。心房收缩把血推进心室，心室收缩时房室瓣关闭，腱索拉住瓣膜不让它翻回去。心室内的压力超过主动脉和肺动脉里的压力以后，半月瓣被推开，血液射出；心室舒张时半月瓣关闭，防止血液倒流回心室。右心房壁上的窦房结是起搏点，心肌自己有节律性，不需要神经命令就能收缩。房室结把兴奋往下传时会稍作延迟，让心房先完成排空。冠状动脉给心肌自己供氧，它堵住就是冠心病。很多学生以为心脏收缩一次四个腔同时用力，实际上心房和心室的收缩是错开的。
The heart is two pumps in one organ, with the right side sending deoxygenated blood to the lungs and the left side driving oxygenated blood around the body, which is why the left ventricular wall is the thickest muscle in the system. Atrial systole tops up the ventricles; ventricular systole closes the atrioventricular valves, held shut by the tendinous cords, and once pressure exceeds that in the aorta and pulmonary artery the semilunar valves are pushed open. The sinoatrial node in the right atrial wall sets the rhythm and cardiac muscle is myogenic, so no nerve is needed for a beat, while the atrioventricular node delays the impulse so the atria empty first. A common slip in exams is treating systole as all four chambers contracting together.
心输出量 每搏输出量 与压力感受器到延髓的反馈
心输出量等于心率乘以每搏输出量，静息状态大约是每分钟 70 次乘以 70 毫升，一分钟接近 5 升。运动时这两个量同时上升，输出量能翻好几倍。血压的短期调节靠压力感受器，它们分布在颈动脉窦和主动脉弓的血管壁上，被拉伸时发放冲动的频率改变。化学感受器监测的是血液 pH 以及氧和二氧化碳的浓度。信号送到延髓，延髓通过交感和副交感神经改变心率和每搏输出量。呼吸频率也归这条回路管：二氧化碳升高，溶于水生成碳酸，pH 下降，化学感受器报警，通气加深加快。阅卷时很能看出学生有没有分清感受器测的是什么量，把压力感受器写成测 pH 的那句话直接丢分。
Cardiac output is heart rate multiplied by stroke volume, which at rest works out at roughly 70 beats per minute times 70 cubic centimetres, about 5 cubic decimetres per minute. Baroreceptors sit in the walls of the carotid sinus and aortic arch and respond to stretch, while chemoreceptors report pH and the concentrations of oxygen and carbon dioxide. Both feed the medulla, which adjusts heart rate and stroke volume through sympathetic and parasympathetic nerves, and the same loop raises ventilation when carbon dioxide dissolves to form carbonic acid and pH falls. Answers that have baroreceptors sensing pH lose the mark at once.
血红蛋白四个亚基 与氧解离曲线的 S 形
血红蛋白由四条多肽链组成，每条链上一个血红素，中心的亚铁离子各结合一个氧分子，所以一分子最多带四个氧。第一个氧结合以后其余亚基的构象改变，后面三个更容易结合，这就叫协同结合，曲线因此呈 S 形而不是双曲线。S 形的含义是：在肺泡的高氧分压下几乎饱和，到了组织的中低氧分压下容易卸载。胎儿血红蛋白对氧的亲和力高于成人血红蛋白，可以隔着胎盘从母血里取氧。肌红蛋白的曲线是双曲线，亲和力更高，作用是在肌肉里存氧。比较三条曲线时先看 P50 再看斜率，不要只记形状。常见的错误是把曲线左移当成释放更多氧，左移其实表示亲和力升高、更难卸载。
Haemoglobin is a tetramer of four polypeptide chains, each carrying a haem group with a ferrous ion that binds one oxygen molecule, so the protein carries a maximum of four. Loading the first oxygen shifts the conformation of the remaining subunits and makes the next three easier to bind, which is why the dissociation curve is sigmoid rather than hyperbolic. Fetal haemoglobin has a higher affinity than adult haemoglobin and can therefore take oxygen across the placenta, while myoglobin is hyperbolic and holds oxygen for muscle use. When the three curves are compared, read P50 first and gradient second.
波尔效应 与二氧化碳的碳酸氢根运输
波尔效应说的是二氧化碳和酸让血红蛋白的氧亲和力下降，曲线右移。原因在于质子与血红蛋白结合以后稳定了低亲和力的构象，所以在同样的氧分压下释放出的氧更多。正在收缩的肌肉产生大量二氧化碳，局部 pH 下降、温度升高，恰好是最需要氧的地方，这条机制正好对上。二氧化碳在血里有三种走法：少量溶解在血浆里，一部分与血红蛋白的氨基结合成氨基甲酰血红蛋白，大部分在红细胞内经碳酸酐酶催化变成碳酸氢根再运走。碳酸氢根离开红细胞时氯离子进来平衡电荷，这个交换叫氯转移。题目问运输形式时要把三种都写出来，只写碳酸氢根会漏掉分数。
The Bohr shift describes the fall in oxygen affinity caused by carbon dioxide and acid: protons stabilise the low-affinity conformation, the curve moves right and more oxygen is released at any given partial pressure. A contracting muscle produces carbon dioxide, its local pH falls and its temperature rises, exactly where extra oxygen is needed. Carbon dioxide travels in three forms, a small fraction dissolved in plasma, a portion bound to the amino groups of haemoglobin as carbaminohaemoglobin, and the majority converted inside red cells by carbonic anhydrase to hydrogen carbonate. That hydrogen carbonate leaves the cell in exchange for chloride, the chloride shift, and answers should name all three routes.
HL 肌节 滑动学说 与钙离子和横桥循环
骨骼肌的收缩单位是肌节，两端由 Z 线界定，里面是粗肌丝肌球蛋白和细肌丝肌动蛋白。收缩时细肌丝向肌节中央滑进去，I 带和 H 区变短，A 带的长度不变，这一点经常被拿来出判断题。静止状态下原肌球蛋白挡住肌动蛋白上的结合位点，钙离子从肌浆网释放出来后先与肌钙蛋白结合，位点才露出来。肌球蛋白头抓住肌动蛋白形成横桥，头部摆动把细肌丝拉动一小段。ATP 结合到头部使它和肌动蛋白分离，ATP 水解后头部复位，循环往复。没有 ATP 时横桥解不开，这正是尸僵的成因。很多学生以为肌肉收缩靠肌丝本身变短，实际是两组肌丝相互滑过。
The sarcomere is the contractile unit, bounded by two Z discs and containing thick myosin and thin actin filaments. During contraction the actin filaments slide towards the centre, so the I band and the H zone shorten while the A band keeps its length, a detail that turns up in data questions. Tropomyosin blocks the binding sites at rest, calcium released from the sarcoplasmic reticulum binds troponin, the sites are exposed, and myosin heads form cross-bridges that pull the thin filaments along. ATP binding detaches the head and hydrolysis resets it, and without ATP the cross-bridges cannot release, which is why muscle stiffens after death.
下丘脑 血管舒缩 发抖 与棕色脂肪产热
体温调节的中枢在下丘脑，它把来自皮肤和血液的温度信息放在一起比较，然后下达指令。环境变热时皮肤附近的小动脉舒张，更多血液走到体表散热，汗腺分泌增加，汗液蒸发带走潜热。环境变冷时这些小动脉收缩，血液更多走深层以减少散热，骨骼肌不自主地快速收缩产热，也就是发抖。棕色脂肪组织里的呼吸链和解偶联蛋白配合，把能量直接变成热而不做成 ATP，婴儿身上这条路占比很大。立毛肌收缩让毛竖起来，人类这层毛已经退化，实际效果有限。答题时把这些反应都归到下丘脑和自主神经这一条链上，不要只说身体自己会调节。
The hypothalamus compares temperature information from the skin and the blood, then issues the response. In the heat, arterioles near the skin surface dilate, more blood reaches the surface, sweat glands secrete and the evaporation of sweat removes latent heat. In the cold, those arterioles constrict, skeletal muscle contracts rapidly and involuntarily as shivering, and brown adipose tissue runs uncoupled respiration that releases energy as heat instead of making ATP. Hair erection belongs to the same reflex but does little in humans, whose coat has largely gone.
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