📚 Blood Circulation: Key Points | 血液循环 考点精讲
The circulatory system is a fundamental topic in IGCSE Biology, responsible for transporting oxygen, nutrients, hormones and waste products around the body. Understanding the structure of the heart, the types of blood vessels, the double circulation and the composition of blood is essential. This revision guide covers all the key points you need, with precise explanations in both English and Chinese to support bilingual learners.
循环系统是 IGCSE 生物的重要课题，负责运输氧气、养分、激素和废物。掌握心脏结构、血管类型、双循环以及血液成分至关重要。这份考点精讲涵盖所有核心知识点，并用中英双语详细解释，帮助双语学习者巩固理解。
1. The Heart: Structure and Function | 心脏：结构与功能
The human heart is a muscular organ about the size of a fist, located in the chest cavity between the lungs. It is divided into four chambers: the right atrium, right ventricle, left atrium and left ventricle. The left and right sides are separated by a thick muscular wall called the septum, preventing oxygenated and deoxygenated blood from mixing.
人类心脏是一个拳头大小的肌肉器官，位于胸腔两肺之间。心脏分为四个腔室：右心房、右心室、左心房和左心室。左右两侧被一层厚肌肉壁（室间隔）隔开，防止含氧血和脱氧血混合。
The atria are the upper chambers that receive blood: the right atrium receives deoxygenated blood from the body via the venae cavae, while the left atrium receives oxygenated blood from the lungs via the pulmonary veins. The ventricles are the lower, thicker-walled chambers that pump blood out of the heart. The right ventricle pumps deoxygenated blood to the lungs through the pulmonary artery, and the left ventricle pumps oxygenated blood to the rest of the body through the aorta. The left ventricle has the thickest muscular wall because it must generate enough pressure to send blood around the entire systemic circuit.
心房是接收血液的上部腔室：右心房通过腔静脉接收来自全身的脱氧血，左心房通过肺静脉接收来自肺部的含氧血。心室是下部、壁更厚的腔室，负责将血液泵出心脏。右心室将脱氧血经肺动脉泵至肺部，左心室将含氧血经主动脉泵至全身。左心室壁最厚，因为它必须产生足够的压力将血液输送到整个体循环。
Valves inside the heart ensure one-way blood flow and prevent backflow. The atrioventricular valves (tricuspid on the right, bicuspid or mitral on the left) lie between atria and ventricles. Semilunar valves are found at the base of the pulmonary artery and the aorta. The ‘lub-dub’ heart sounds are caused by the closing of these valves.
心脏内的瓣膜确保血液单向流动，防止倒流。房室瓣（右侧三尖瓣，左侧二尖瓣）位于心房与心室之间。半月瓣位于肺动脉和主动脉根部。“咚哒”心音由这些瓣膜关闭产生。
2. Blood Vessels: Arteries, Veins and Capillaries | 血管：动脉、静脉和毛细血管
The three main types of blood vessels are arteries, veins and capillaries. Each is structurally adapted to its function.
三种主要的血管类型是动脉、静脉和毛细血管，每一种在结构上都与其功能相适应。
| Feature 特征 | English Description | 中文描述 |
|---|---|---|
| Arteries 动脉 | Carry blood away from the heart. Thick muscular wall with elastic fibres to withstand high pressure. Lumen is relatively small. | 将血液从心脏运走。管壁厚，有肌肉与弹性纤维，以承受高压。管腔相对较小。 |
| Veins 静脉 | Carry blood towards the heart. Thinner wall, wider lumen, contain valves to prevent backflow. Blood is under lower pressure. | 将血液送回心脏。管壁较薄，管腔较宽，含有静脉瓣防止倒流。血液压力较低。 |
| Capillaries 毛细血管 | Microscopic vessels formed by a single layer of endothelial cells. Allow exchange of gases, nutrients and waste between blood and tissues. Walls are one cell thick for efficient diffusion. | 由单层内皮细胞构成的微小血管。允许血液与组织之间进行气体、养分和废物的交换。管壁仅一个细胞厚，利于高效扩散。 |
Arteries branch into smaller arterioles and then into capillaries, which form extensive networks. Capillaries join to form venules, which eventually merge into veins. The elastic recoil of arteries helps maintain blood pressure during diastole. Skeletal muscles pressing on veins assist venous return.
动脉分支成小动脉，再形成毛细血管，构成广泛网络。毛细血管汇合成小静脉，最终合并为静脉。动脉的弹性回缩有助于在舒张期维持血压。骨骼肌挤压静脉帮助血液回流。
3. The Double Circulatory System | 双循环系统
Mammals, including humans, have a double circulatory system, meaning blood passes through the heart twice in one complete circuit. This consists of the pulmonary circulation and the systemic circulation.
包括人类在内的哺乳动物拥有双循环系统，即血液在一次完整的循环中两次经过心脏。这包括肺循环和体循环。
In pulmonary circulation, deoxygenated blood is pumped from the right ventricle to the lungs via the pulmonary artery. In the lung capillaries, carbon dioxide diffuses out and oxygen diffuses into the blood. The now oxygenated blood returns to the left atrium through the pulmonary veins.
在肺循环中，脱氧血从右心室经肺动脉泵入肺部。在肺毛细血管中，二氧化碳扩散出，氧气扩散入血液。含氧血随后通过肺静脉返回左心房。
In systemic circulation, oxygenated blood is pumped from the left ventricle into the aorta and distributed to all body tissues. After delivering oxygen and nutrients, deoxygenated blood returns to the right atrium via the superior and inferior venae cavae. The double circuit ensures that oxygenated and deoxygenated blood are kept separate, and it allows the systemic circuit to be at high pressure, delivering substances efficiently.
在体循环中，含氧血从左心室泵入主动脉，分布到全身组织。在输送氧气和养分后，脱氧血通过上腔静脉和下腔静脉返回右心房。双循环确保含氧血和脱氧血分开，并允许体循环处于高压状态，高效输送物质。
4. Blood Composition: Plasma and Cells | 血液成分：血浆和血细胞
Blood is a liquid connective tissue composed of plasma (about 55%) and formed elements – red blood cells, white blood cells and platelets. Plasma is straw-coloured and mainly water (about 90%), with dissolved substances such as glucose, amino acids, mineral ions, hormones, plasma proteins and waste products like urea. It also transports carbon dioxide in solution and as bicarbonate ions.
血液是一种液态结缔组织，由血浆（约55%）和血细胞（红细胞、白细胞和血小板）组成。血浆呈淡黄色，主要成分是水（约90%），溶解有葡萄糖、氨基酸、无机离子、激素、血浆蛋白以及尿素等废物。血浆还以溶解态和碳酸氢根离子形式运输二氧化碳。
Red blood cells are produced in red bone marrow and are the most numerous blood cells. White blood cells are part of the immune system and platelets are cell fragments involved in clotting. All blood cells ultimately originate from stem cells in the bone marrow.
红细胞在红骨髓中生成，是数量最多的血细胞。白细胞是免疫系统的一部分，血小板是参与凝血过程的细胞碎片。所有血细胞最终都来自骨髓中的造血干细胞。
5. Red Blood Cells and Oxygen Transport | 红细胞与氧气运输
Red blood cells (erythrocytes) are highly specialised for oxygen transport. They are biconcave discs, which increases surface area for gas exchange, and they have no nucleus, allowing more space for haemoglobin. Haemoglobin is an iron-containing protein that reversibly binds with oxygen to form oxyhaemoglobin.
红细胞（红血球）高度特化，用于运输氧气。它们呈双凹圆盘状，增大了气体交换表面积，且无细胞核，为血红蛋白提供更多空间。血红蛋白是一种含铁的蛋白质，能与氧气可逆结合形成氧合血红蛋白。
In the lungs, where oxygen concentration is high, haemoglobin loads oxygen. In the respiring tissues, where oxygen concentration is low, oxyhaemoglobin dissociates, releasing oxygen. Some carbon dioxide is also carried by haemoglobin, but most is transported as bicarbonate ions in plasma.
在肺部氧气浓度高时，血红蛋白结合氧气。在耗氧组织氧气浓度低时，氧合血红蛋白解离，释放氧气。部分二氧化碳也由血红蛋白携带，但大部分以碳酸氢根离子形式在血浆中运输。
6. White Blood Cells and Platelets | 白细胞与血小板
White blood cells (leucocytes) defend the body against pathogens. There are two main types: phagocytes and lymphocytes. Phagocytes, such as neutrophils and macrophages, carry out phagocytosis – they engulf and digest microorganisms. Lymphocytes produce antibodies and contribute to immunological memory. White blood cells have a nucleus and can change shape to pass through capillary walls.
白细胞（白血球）保护机体抵御病原体。主要有两种类型：吞噬细胞和淋巴细胞。吞噬细胞，如中性粒细胞和巨噬细胞，执行吞噬作用——吞噬并消化微生物。淋巴细胞产生抗体，参与免疫记忆。白细胞有细胞核，并能变形穿过毛细血管壁。
Platelets are small, irregularly shaped cell fragments that cluster at the site of a wound. They release chemicals that convert the soluble plasma protein fibrinogen into insoluble fibrin threads, forming a mesh that traps red blood cells and forms a clot. This prevents excessive blood loss and the entry of microorganisms.
血小板是小型、形状不规则的细胞碎片，在伤口处聚集。它们释放化学物质，将可溶性血浆蛋白纤维蛋白原转化为不溶性的纤维蛋白丝，形成网状结构，网住红细胞而形成血凝块。这可以防止过量失血和微生物侵入。
7. The Cardiac Cycle and Heartbeat Control | 心动周期与心跳控制
The cardiac cycle consists of the sequence of events in one heartbeat. It includes diastole (relaxation) and systole (contraction). During diastole, the atria and ventricles relax, and blood flows into the atria and passively into the ventricles. During atrial systole, the atria contract to push remaining blood into the ventricles. Ventricular systole follows: ventricles contract, the atrioventricular valves close, and blood is ejected into the arteries. The cycle repeats about 70 times per minute at rest.
心动周期指一次心跳的一系列事件，包括舒张期和收缩期。舒张时，心房和心室松弛，血液流入心房并被动进入心室。心房收缩时，心房收缩将余血推入心室。随后心室收缩：心室收缩，房室瓣关闭，血液被射入动脉。安静状态下，该周期约每分钟重复70次。
The heartbeat is initiated by a natural pacemaker called the sinoatrial node (SAN), located in the wall of the right atrium. The SAN sends out electrical impulses that spread across the atria, causing them to contract. The impulses then reach the atrioventricular node (AVN), which delays the signal slightly to allow ventricles to fill before contracting. The signal then travels down the bundle of His and through Purkinje fibres, stimulating ventricular contraction. This coordination ensures efficient pumping.
心跳由位于右心房壁的天然起搏器——窦房结（SAN）发起。窦房结发出电冲动，传遍心房，导致心房收缩。冲动随后到达房室结（AVN），该处略微延迟信号，使心室在收缩前得以充盈。信号再沿希氏束下传并经浦肯野纤维，刺激心室收缩。这一协调过程确保了高效泵血。
8. Coronary Heart Disease | 冠心病
The heart muscle itself is supplied with blood through the coronary arteries, which branch off the aorta. Coronary heart disease (CHD) occurs when these arteries are narrowed or blocked, usually by the build-up of fatty plaques (atherosclerosis). This reduces blood flow and therefore oxygen supply to the heart muscle, which can lead to angina (chest pain) and, if a vessel is completely blocked, a heart attack (myocardial infarction).
心肌自身由从主动脉分支出来的冠状动脉供血。冠心病发生于这些动脉变窄或堵塞时，通常由脂质斑块堆积（动脉粥样硬化）引起。这减少了血流量，从而降低心肌供氧，可能导致心绞痛（胸痛），若血管完全堵塞，则引发心脏病发作（心肌梗死）。
Risk factors for CHD include a diet high in saturated fats and cholesterol, smoking, lack of exercise, high blood pressure, diabetes and genetic predisposition. Preventive measures involve regular physical activity, a balanced diet low in saturated fat, maintaining a healthy body weight and not smoking.
冠心病的危险因素包括高饱和脂肪和高胆固醇饮食、吸烟、缺乏运动、高血压、糖尿病和遗传易感性。预防措施包括规律锻炼、低饱和脂肪的均衡饮食、保持健康体重和不吸烟。
9. Blood Pressure, Tissue Fluid and the Lymphatic System | 血压、组织液与淋巴系统
Blood pressure is the force exerted by blood against the walls of arteries. It is measured using a sphygmomanometer and expressed as systolic pressure over diastolic pressure (e.g. 120/80 mmHg). Systolic pressure occurs during ventricular contraction, and diastolic pressure during ventricular relaxation. High blood pressure (hypertension) can damage blood vessels and increase the risk of stroke and CHD.
血压是血液对动脉管壁施加的压力。使用血压计测量，表示为收缩压/舒张压（如 120/80 mmHg）。收缩压为心室收缩时的压力，舒张压为心室舒张时的压力。高血压会损伤血管，增加中风和冠心病的风险。
At the arterial end of a capillary, the higher hydrostatic pressure forces fluid out of the blood, forming tissue fluid that bathes the cells. At the venous end, most of this fluid returns to the capillary due to osmotic pull from plasma proteins. The excess fluid is drained into lymphatic capillaries, becoming lymph. Lymph is transported through lymph vessels, passing through lymph nodes where pathogens and debris are filtered, and eventually returns to the blood circulation via the subclavian veins.
在毛细血管动脉端，较高的静水压使液体渗出血管，形成组织液，浸润细胞。在静脉端，大部分组织液因血浆蛋白的渗透拉力而返回毛细血管。多余的液体流入毛细淋巴管，成为淋巴。淋巴经淋巴管运输，通过淋巴结时过滤病原体和碎屑，最终经锁骨下静脉返回血循环。
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