📚 Blood Circulation: WJEC A-Level Biology | 血液循环 考点精讲
The circulatory system forms the core of transport in mammals, and for WJEC A-Level Biology you must master its anatomy, physiology, and regulatory mechanisms. This revision guide covers every major topic, from the structure of the heart to the oxygen dissociation curve, with clear bilingual explanations to support your learning and exam success.
循环系统是哺乳动物体内运输的核心，在 WJEC A-Level 生物考试中，你必须掌握其解剖结构、生理功能和调节机制。这份考点精讲涵盖从心脏结构到氧合解离曲线的所有主要主题，配以清晰的双语解释，助力你的学习与考试成功。
1. Overview of the Circulatory System | 循环系统概述
The circulatory system is a mass transport system that ensures rapid, efficient distribution of essential substances. Mammals possess a closed, double circulation: blood is confined within vessels and passes through the heart twice during one complete circuit. The pulmonary circulation sends blood to the lungs for gas exchange, while the systemic circulation delivers oxygen and nutrients to all body tissues and collects metabolic wastes. This arrangement separates oxygenated and deoxygenated blood, maintaining high pressure in the systemic circuit and protecting the delicate pulmonary capillaries.
循环系统是一个物质运输系统，能快速高效地分配必需物质。哺乳动物具有闭合式双循环：血液完全在血管内流动，在一次完整的循环中两次经过心脏。肺循环将血液输送到肺部进行气体交换，体循环则将氧气和营养物质输送到全身组织并收集代谢废物。这种安排使富氧血和缺氧血分开，维持了体循环中的高压力，同时保护了脆弱的肺毛细血管。
The system includes the heart as a pump, a network of arteries, veins and capillaries, and blood as the transport medium. Exchange of materials occurs at the capillaries, where their thin walls and huge total cross‑sectional area facilitate diffusion. Understanding the relationship between structure and function is a key skill required by the WJEC specification.
该系统包括作为泵的心脏、由动脉、静脉和毛细血管组成的网络以及作为运输介质的血液。物质交换发生在毛细血管处，其薄壁和巨大的总横截面积利于扩散。理解结构与功能之间的关系是 WJEC 考纲要求的一项关键技能。
2. Structure of the Heart | 心脏结构
The heart is a muscular organ with four chambers: the right and left atria (upper chambers) and the right and left ventricles (lower chambers). The right atrium receives deoxygenated blood from the body via the superior and inferior vena cava. The left atrium receives oxygenated blood from the lungs through the pulmonary veins. Blood moves from atria to ventricles through atrioventricular valves – the tricuspid valve on the right and the bicuspid (mitral) valve on the left. The ventricles then pump blood out: the right ventricle into the pulmonary artery and the left ventricle into the aorta. Semilunar valves guard the exits of the ventricles, preventing backflow.
心脏是一个有四个腔室的肌肉器官：右心房和左心房（上腔室）以及右心室和左心室（下腔室）。右心房通过上腔静脉和下腔静脉接收来自身体的缺氧血。左心房通过肺静脉接收来自肺部的富氧血。血液通过房室瓣从心房流向心室——右侧为三尖瓣，左侧为二尖瓣。心室随后将血泵出：右心室将血泵入肺动脉，左心室将血泵入主动脉。半月瓣守卫着心室的出口，防止血液倒流。
The left ventricular wall is significantly thicker than the right because it must generate sufficient pressure to propel blood through the entire systemic circulation. The cardiac muscle (myocardium) is supplied with oxygenated blood by the coronary arteries. Knowledge of heart chambers, valves, and associated vessels is frequently tested in diagrams and labelling exercises.
左心室壁明显厚于右心室壁，因为它必须产生足够的压力推动血液流经整个体循环。心肌通过冠状动脉获得含氧血的供应。心脏腔室、瓣膜和相关血管的知识经常以图表和标注题的形式进行考查。
3. Cardiac Cycle and Heartbeat | 心动周期与心跳
The cardiac cycle is the sequence of events in one complete heartbeat, comprising systole (contraction) and diastole (relaxation). Atrial systole pushes the final volume of blood into the ventricles. Ventricular systole follows, forcing the atrioventricular valves shut (first heart sound, ‘lub’) and ejecting blood into the arteries. As the ventricles relax, the semilunar valves close (second heart sound, ‘dub’) to prevent backflow from the arteries. The cycle then repeats.
心动周期是一次完整心跳中发生的事件序列，包括收缩期和舒张期。心房收缩将最后的血量推入心室。随后心室收缩，使房室瓣关闭（第一心音，“lub”），并将血液射入动脉。当心室舒张时，半月瓣关闭（第二心音，“dub”），防止动脉血倒流。然后周期重复。
The heartbeat is initiated by the sinoatrial node (SAN), the natural pacemaker located in the wall of the right atrium. The SAN generates an electrical impulse that spreads across both atria, causing them to contract simultaneously. The impulse then reaches the atrioventricular node (AVN), where a brief delay allows the ventricles to fill completely. The signal travels down the Bundle of His and along Purkinje fibres to trigger ventricular contraction from the apex upward, ensuring efficient ejection.
心跳由窦房结（SAN）启动，它是位于右心房壁的天然起搏器。SAN 发出电脉冲，传遍两个心房，使其同时收缩。然后冲动到达房室结（AVN），在此短暂延迟，让心室完全充盈。信号沿希氏束传下，再经浦肯野纤维传导，从心尖向上引发心室收缩，确保高效射血。
4. Blood Vessels: Arteries, Veins, and Capillaries | 血管：动脉、静脉和毛细血管
Arteries carry blood away from the heart. Their walls are thick, containing elastic fibres and smooth muscle, which enable them to withstand and maintain high pressure. The elasticity allows arteries to stretch during ventricular systole and recoil during diastole, smoothing the blood flow. Arterioles can constrict or dilate to regulate blood flow into capillary beds. Veins return blood to the heart; they have thinner walls, a wider lumen, and possess valves that prevent backward flow. Skeletal muscle contraction assists venous return by compressing veins and pushing blood towards the heart.
动脉将血液带离心脏。其管壁厚，含有弹性纤维和平滑肌，使其能承受并维持高压。弹性使动脉能在心室收缩时扩张、舒张时回缩，从而平稳血流。小动脉可以收缩或舒张以调节进入毛细血管床的血流量。静脉将血液送回心脏；它们管壁较薄，管腔较宽，并具有防止倒流的瓣膜。骨骼肌的收缩通过挤压静脉、推动血液流向心脏，辅助静脉回流。
Capillaries are microscopic vessels with walls only one endothelial cell thick. This minimises the diffusion distance for gases, nutrients, and waste products between blood and tissue fluid. Capillary networks provide a huge total cross‑sectional area, slowing blood flow and allowing sufficient time for exchange. The structure of each blood vessel type thus directly relates to its specialised function.
毛细血管是仅由一层内皮细胞构成的微小血管，最大限度缩短了血液与组织液之间气体、营养物质和废物的扩散距离。毛细血管网提供了巨大的总横截面积，减缓血流，为物质交换提供充足时间。因此，每种血管的结构直接与其特殊功能相适应。
5. Composition and Functions of Blood | 血液的组成与功能
Blood is a specialised connective tissue composed of plasma (about 55%) and formed elements. Plasma is a pale yellow liquid consisting mainly of water, which acts as a solvent for transporting glucose, amino acids, mineral ions, hormones, plasma proteins, urea, and carbon dioxide. Red blood cells (erythrocytes) are biconcave discs that lack a nucleus and most organelles in mammals, maximising space for haemoglobin. Haemoglobin binds oxygen in the lungs and releases it in tissues. White blood cells (leucocytes) defend the body against infection, and platelets (thrombocytes) are cell fragments essential for blood clotting.
血液是一种特殊的结缔组织，由血浆（约55%）和有形成分组成。血浆是一种淡黄色液体，主要成分是水，作为溶剂运输葡萄糖、氨基酸、无机离子、激素、血浆蛋白、尿素和二氧化碳。红细胞呈双凹圆盘状，在哺乳动物中无细胞核和大多数细胞器，最大化血红蛋白的容纳空间。血红蛋白在肺部结合氧气并在组织释放。白细胞防御机体免受感染，血小板是参与血液凝固必不可少的细胞碎片。
Carbon dioxide is primarily carried as hydrogen carbonate ions (HCO₃⁻) in plasma after conversion inside red blood cells. Plasma proteins such as albumin help maintain osmotic balance, while globulins and fibrinogen play roles in immunity and clotting respectively. WJEC questions often ask about the adaptations of erythrocytes and the multiple roles of plasma.
二氧化碳主要在红细胞内转化后以碳酸氢根离子（HCO₃⁻）形式在血浆中运输。血浆蛋白如白蛋白有助于维持渗透平衡，而球蛋白和纤维蛋白原分别在免疫和凝血中发挥作用。WJEC 考题常涉及红细胞的适应性和血浆的多种作用。
6. Systemic and Pulmonary Circulation | 体循环与肺循环
The right side of the heart deals with deoxygenated blood returning from the body. It pumps this blood through the pulmonary artery to the lungs, where CO₂ diffuses out and O₂ diffuses in. Oxygenated blood returns to the left atrium via the pulmonary veins. This pulmonary circuit operates at relatively low pressure to prevent damage to the thin‑walled capillaries surrounding the alveoli.
心脏右侧处理从身体返回的缺氧血。它将此血液通过肺动脉泵入肺部，在此 CO₂ 扩散出去而 O₂ 扩散进来。富氧血经肺静脉返回左心房。这一肺循环在较低压力下运行，以防损伤肺泡周围的薄壁毛细血管。
The left side of the heart pumps oxygenated blood into the aorta and through a branching network of arteries to all organs and tissues. In the systemic capillaries, O₂ and nutrients are delivered, and wastes are collected. Deoxygenated blood then travels back through venules and veins, emptying into the right atrium. The high pressure generated by the left ventricle is essential to overcome the total peripheral resistance of the extensive systemic circulation.
心脏左侧将富氧血泵入主动脉，并通过分支动脉网络输送到所有器官和组织。在体毛细血管中，O₂ 和营养物质被释放，废物被收集。缺氧血随后经微静脉和静脉流回，汇入右心房。左心室产生的高压对于克服广大体循环的总外周阻力至关重要。
The double circulatory system ensures that blood is re‑oxygenated before being distributed again, maintaining a steep concentration gradient for efficient gas exchange and nutrient delivery.
双循环系统确保血液在再次分配前被重新氧合，维持了陡峭的浓度梯度，以实现高效的气体交换和营养物质输送。
7. Control of Heart Rate | 心率的调控
Heart rate is modulated by the cardiovascular centre in the medulla oblongata of the brain. Sympathetic nerves increase heart rate by releasing noradrenaline at the SAN, which raises the frequency of action potentials. Parasympathetic fibres of the vagus nerve release acetylcholine, slowing the heart rate. Under resting conditions, vagal tone predominates, keeping the heart rate below the intrinsic rate of the SAN.
心率由大脑延髓的心血管中枢调节。交感神经通过在窦房结释放去甲肾上腺素加快心率，提高动作电位的频率。迷走神经的副交感纤维释放乙酰胆碱，减慢心率。在静息状态下，迷走紧张占主导地位，使心率低于窦房结的固有频率。
Hormonal regulation is provided by adrenaline and noradrenaline from the adrenal medulla, which accelerate the heart during stress or exercise. Chemoreceptors in the carotid and aortic bodies are sensitive to increased CO₂, decreased pH, or decreased O₂ in the blood, sending signals to increase ventilation and heart rate. Baroreceptors in the carotid sinus and aortic arch detect changes in blood pressure and initiate homeostatic responses. These pathways exemplify the WJEC requirement to explain coordination and feedback mechanisms.
激素调控由肾上腺髓质分泌的肾上腺素和去甲肾上腺素提供，在应激或运动时加速心跳。颈动脉体和主动脉体的化学感受器对血液中 CO₂ 升高、pH 降低或 O₂ 降低敏感，发送信号以增加通气和心率。颈动脉窦和主动脉弓的压力感受器检测血压变化并启动稳态反应。这些通路体现了 WJEC 对阐释协调与反馈机制的要求。
8. Blood Pressure and Its Regulation | 血压及其调节
Blood pressure is the hydrostatic force exerted by blood on the walls of blood vessels. It is expressed as systolic pressure (peak during ventricular contraction) over diastolic pressure (lowest during relaxation). Mean arterial pressure (MAP) is calculated as cardiac output × total peripheral resistance. Normal values are essential knowledge, but the principles of regulation are more important.
血压是血液对血管壁产生的静水压力，用收缩压（心室收缩时的峰值）与舒张压（舒张时的最低值）表示。平均动脉压（MAP）的计算公式为心输出量 × 总外周阻力。正常值属于基础知识，但调节原理更为重要。
Short‑term regulation involves the baroreceptor reflex: an increase in blood pressure stretches baroreceptors, which send impulses to the medulla, resulting in decreased sympathetic output and increased parasympathetic output. This reduces heart rate and causes vasodilation, lowering pressure. Conversely, a drop in pressure triggers the opposite response. Long‑term control involves the renin‑angiotensin‑aldosterone system (RAAS) and antidiuretic hormone (ADH), which adjust blood volume and vasoconstriction. Understanding these systems is frequently assessed in WJEC exam questions on homeostasis.
短期调节涉及压力感受器反射：血压升高牵拉压力感受器，向延髓发送冲动，导致交感输出减少、副交感输出增加。这使心率降低并引起血管舒张，从而降低血压。反之，血压下降会触发相反的反应。长期控制涉及肾素‑血管紧张素‑醛固酮系统（RAAS）和抗利尿激素（ADH），它们调节血容量和血管收缩。理解这些系统经常在 WJEC 关于稳态的考题中被评估。
9. Oxygen Dissociation Curves | 氧合血红蛋白解离曲线
The oxygen dissociation curve plots the partial pressure of oxygen (pO₂) on the x‑axis against the percentage saturation of haemoglobin with oxygen on the y‑axis. The curve is sigmoidal (S‑shaped) because haemoglobin exhibits cooperative binding: the binding of one O₂ molecule changes the quaternary structure, making it easier for subsequent molecules to bind. This gives haemoglobin a high affinity for oxygen in the lungs (high pO₂) and a lower affinity in respiring tissues (low pO₂), promoting efficient unloading.
氧合血红蛋白解离曲线以氧分压（pO₂）为横坐标、血红蛋白氧饱和度为纵坐标。曲线呈 S 形，因为血红蛋白具有协同结合效应：第一个 O₂ 分子结合后改变了四级结构，使后续分子更容易结合。这使得血红蛋白在肺部（高 pO₂）对氧有高亲和力，在呼吸组织（低 pO₂）亲和力降低，促进高效卸载。
The curve shifts right under conditions of increased CO₂ (the Bohr effect), increased temperature, or decreased pH (increased H⁺). This rightward shift decreases haemoglobin’s oxygen affinity, increasing oxygen delivery to active tissues. Fetal haemoglobin (HbF) has a higher affinity for oxygen than adult haemoglobin, so its dissociation curve lies to the left. This allows the fetus to obtain oxygen from maternal blood across the placenta.
在 CO₂ 增加（波尔效应）、温度升高或 pH 降低（H⁺ 增加）的情况下，曲线右移。右移降低了血红蛋白对氧的亲和力，增加了对活跃组织的氧气供应。胎儿血红蛋白（HbF）对氧的亲和力高于成人血红蛋白，因此其解离曲线左移。这使得胎儿能够从母体血液中通过胎盘获取氧气。
10. Transport of Carbon Dioxide | 二氧化碳的运输
Carbon dioxide is transported from tissues to the lungs in three main ways. A small percentage dissolves directly in plasma. Some CO₂ binds to the amino groups of haemoglobin and plasma proteins to form carbamino compounds. However, the majority (approximately 70–85%) is converted into hydrogen carbonate ions (HCO₃⁻) inside red blood cells. The enzyme carbonic anhydrase catalyses the reaction: CO₂ + H₂O → H₂CO₃. Carbonic acid then dissociates: H₂CO₃ → H⁺ + HCO₃⁻.
二氧化碳以三种主要方式从组织运输到肺部。一小部分直接溶解在血浆中。部分 CO₂ 与血红蛋白和血浆蛋白的氨基结合形成氨基甲酸化合物。然而，绝大多数（约70–85%）在红细胞内转化为碳酸氢根离子（HCO₃⁻）。碳酸酐酶催化反应：CO₂ + H₂O → H₂CO₃。然后碳酸解离：H₂CO₃ → H⁺ + HCO₃⁻。
The HCO₃⁻ diffuses out of the red blood cell into the plasma in exchange for chloride ions (Cl⁻), a process known as the chloride shift. This maintains electrical neutrality. The H⁺ ions are buffered by haemoglobin, forming haemoglobinic acid, which prevents significant changes in blood pH. In the lungs, these reactions reverse, and CO₂ is regenerated and exhaled. The Bohr effect is directly linked to this process, as H⁺ binding promotes oxygen release.
HCO₃⁻ 扩散出红细胞进入血浆，同时氯离子（Cl⁻）反向交换，这一过程称为氯离子转移，以维持
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