📚 IGCSE WJEC Biology: Blood Circulation – Key Points | IGCSE WJEC 生物：血液循环 考点精讲
Blood circulation is a fundamental topic in IGCSE WJEC Biology, covering the structure and function of the heart, blood vessels, and blood components, as well as the double circulatory system. A thorough understanding of how blood moves around the body is essential for grasping how oxygen, nutrients, and waste products are transported, and how the body maintains a stable internal environment.
血液循环是 IGCSE WJEC 生物的基础核心课题，涵盖心脏的结构与功能、血管类型、血液成分以及双循环系统。透彻理解血液如何在体内流动，对于掌握氧气、营养物质和废物的运输方式，以及身体如何维持稳定的内环境至关重要。
1. Overview of the Circulatory System | 循环系统概述
The human circulatory system is a closed, double circulation consisting of the heart, a network of blood vessels, and blood. It ensures that oxygenated blood and deoxygenated blood do not mix, making the delivery of oxygen to tissues highly efficient. The system is also responsible for transporting hormones, nutrients, carbon dioxide, and urea, as well as distributing heat throughout the body.
人体的循环系统是一个封闭的双循环系统，由心脏、血管网络和血液组成。它确保富氧血与缺氧血不会混合，从而高效地为组织供氧。该系统还负责运输激素、营养物质、二氧化碳和尿素，并在全身分配热量。
2. External and Internal Structure of the Heart | 心脏的外部与内部结构
The heart is a muscular organ located in the chest cavity, protected by the ribcage. It is made of cardiac muscle and is divided into four chambers: the right atrium, right ventricle, left atrium, and left ventricle. The left ventricle has a much thicker muscular wall than the right because it needs to pump blood all around the body, whereas the right ventricle only pumps blood to the lungs.
心脏是位于胸腔内的肌肉器官，受肋骨保护。它由心肌构成，分为四个腔室：右心房、右心室、左心房和左心室。左心室的肌壁比右心室厚得多，因为它需要将血液泵送到全身，而右心室仅将血液泵送到肺部。
The heart is surrounded by a double membrane called the pericardium. The septum separates the left and right sides to prevent oxygenated blood from mixing with deoxygenated blood. The atria are smaller and located above the ventricles; their walls are thinner because they only push blood into the ventricles.
心脏被一层叫作心包膜的双层膜包裹。室间隔将左右两侧分开，防止富氧血与缺氧血混合。心房较小，位于心室上方，其壁较薄，因为它们仅将血液推入心室。
3. Chambers, Valves and Blood Flow | 心腔、瓣膜与血流方向
Blood enters the heart through the atria and leaves through the ventricles. Valves inside the heart ensure one-way flow and prevent backflow. The tricuspid valve sits between the right atrium and right ventricle, while the bicuspid (mitral) valve lies between the left atrium and left ventricle. Semilunar valves are located at the exits of the pulmonary artery and the aorta.
血液通过心房进入心脏，并通过心室离开。心脏内部的瓣膜确保血液单向流动，防止回流。三尖瓣位于右心房与右心室之间，二尖瓣（僧帽瓣）位于左心房与左心室之间。半月瓣位于肺动脉和主动脉的出口处。
When the ventricles contract, the atrioventricular valves close to stop blood from going back into the atria; at the same time, the semilunar valves open, allowing blood to be ejected into the arteries. When the ventricles relax, the semilunar valves close, and the atrioventricular valves open to let the next volume of blood in from the atria.
当心室收缩时，房室瓣关闭以防止血液回流心房；同时半月瓣打开，让血液射入动脉。当心室舒张时，半月瓣关闭，房室瓣打开，让下一批血液从心房进入心室。
4. Blood Vessels: Arteries, Veins and Capillaries | 血管：动脉、静脉和毛细血管
Three main types of blood vessels form the transport network. Each type is adapted to its function:
三种主要血管构成运输网络，各自适应其功能：
| Vessel 血管 | Structure 结构 | Function 功能 |
|---|---|---|
| Artery 动脉 | Thick, muscular, elastic walls; narrow lumen | Carry blood away from the heart under high pressure; stretch and recoil |
| Vein 静脉 | Thin walls with less muscle; wide lumen; valves present | Carry blood back to the heart under low pressure; valves prevent backflow |
| Capillary 毛细血管 | One-cell-thick walls; very narrow lumen | Allow exchange of gases, nutrients and waste between blood and tissues |
Arteries branch into smaller arterioles, which further lead to capillary beds. Capillaries join to form venules, which then merge into veins. This arrangement ensures a seamless flow from high-pressure distribution to low-pressure collection.
动脉分支为更小的小动脉，再通向毛细血管网。毛细血管汇合形成小静脉，再合并为静脉。这种布局确保了从高压分配到低压收集的顺畅血流。
5. Composition and Functions of Blood | 血液的组成与功能
Blood is a tissue composed of plasma, red blood cells, white blood cells, and platelets. Plasma is the liquid portion, making up about 55% of blood volume. It transports dissolved substances such as glucose, amino acids, carbon dioxide, urea, hormones, and heat.
血液是由血浆、红细胞、白细胞和血小板组成的组织。血浆是液体部分，约占血液体积的55%。它运输溶解的物质，如葡萄糖、氨基酸、二氧化碳、尿素、激素和热量。
Red blood cells (erythrocytes) contain haemoglobin, which binds oxygen to form oxyhaemoglobin. They are biconcave discs, have no nucleus, and are highly flexible, enabling them to squeeze through narrow capillaries. White blood cells (leukocytes) are part of the immune system and defend against pathogens. Platelets are cell fragments that play a key role in blood clotting.
红细胞含有血红蛋白，能结合氧气形成氧合血红蛋白。它们呈双凹圆盘形，无细胞核，高度灵活，能挤过狭窄的毛细血管。白细胞是免疫系统的一部分，负责抵御病原体。血小板是细胞碎片，在血液凝固中起关键作用。
6. The Double Circulatory System | 双循环系统
Mammals have a double circulation: pulmonary circulation between the heart and lungs, and systemic circulation between the heart and the rest of the body. This separation allows oxygenated blood to be delivered at high pressure to the body, while deoxygenated blood is sent to the lungs at a lower pressure to avoid damage to the delicate lung capillaries.
哺乳动物具有双循环：肺循环连接心脏与肺部，体循环连接心脏与身体其余部分。这种分离使富氧血能以高压输送到身体各处，而缺氧血以较低的压力送至肺部，以避免损伤脆弱的肺毛细血管。
The right side of the heart handles deoxygenated blood into the pulmonary circulation, while the left side handles oxygenated blood into the systemic circulation. This dual system maximises the efficiency of oxygen delivery and meets the high metabolic demands of warm-blooded organisms.
心脏右侧将缺氧血送入肺循环，左侧将富氧血送入体循环。这种双重系统最大限度地提高了氧气输送效率，满足了恒温生物的高代谢需求。
7. Pulmonary Circulation in Detail | 肺循环详解
Pulmonary circulation starts when deoxygenated blood from the body enters the right atrium through the superior and inferior vena cavae. It passes to the right ventricle, which contracts and pumps blood through the pulmonary artery to the lungs. In the lungs, carbon dioxide diffuses out of the blood and oxygen diffuses in across the alveolar–capillary membrane.
肺循环始于来自身体的缺氧血通过上腔静脉和下腔静脉进入右心房。血液进入右心室，右心室收缩将血液通过肺动脉泵入肺部。在肺部，二氧化碳从血液中扩散出去，氧气通过肺泡-毛细血管膜扩散进来。
The now oxygenated blood returns to the heart via the pulmonary veins into the left atrium. Note that the pulmonary artery is the only artery carrying deoxygenated blood, and the pulmonary veins are the only veins carrying oxygenated blood.
此时富氧血通过肺静脉返回心脏，进入左心房。请注意，肺动脉是唯一输送缺氧血的动脉，肺静脉是唯一输送富氧血的静脉。
8. Systemic Circulation in Detail | 体循环详解
Systemic circulation begins when oxygenated blood moves from the left atrium into the left ventricle. The left ventricle, with its thick muscular wall, contracts powerfully to push blood into the aorta. The aorta branches into a network of arteries that deliver oxygen and nutrients to all organs and tissues, including the brain, liver, kidneys, and muscles.
体循环始于富氧血从左心房进入左心室。左心室具有厚实的肌壁，强力收缩将血液推入主动脉。主动脉分支形成动脉网络，将氧气和营养物质输送到所有器官和组织，包括大脑、肝脏、肾脏和肌肉。
At the tissues, oxygen and nutrients are exchanged for carbon dioxide and waste products through the thin capillary walls. Deoxygenated blood then returns to the heart via venules and veins, eventually entering the right atrium through the vena cavae, completing the circuit.
在组织处，氧气和营养物质通过薄薄的毛细血管壁与二氧化碳和废物进行交换。然后缺氧血通过小静脉和静脉返回心脏，最终通过腔静脉进入右心房，完成循环。
9. Coronary Circulation and Heart Disease | 冠脉循环与心脏病
The heart muscle itself needs a constant supply of oxygen and nutrients, which is provided by the coronary arteries. These arteries branch off the aorta and spread over the surface of the heart. Blockage or narrowing of coronary arteries by fatty plaques (atherosclerosis) can lead to coronary heart disease (CHD).
心肌本身需要持续的氧气和营养供应，这由冠状动脉提供。这些动脉从主动脉分支，分布在心脏表面。冠状动脉被脂肪斑块（动脉粥样硬化）堵塞或狭窄可导致冠心病。
If blood flow to part of the heart muscle is completely blocked, a myocardial infarction (heart attack) occurs, causing damage or death of cardiac tissue. Risk factors include smoking, high-fat diet, lack of exercise, high blood pressure, and genetic predisposition. Lifestyle changes and, in some cases, surgery such as stent insertion or bypass grafts can manage CHD.
如果流向部分心肌的血流完全阻断，就会发生心肌梗死（心脏病发作），导致心脏组织受损或坏死。危险因素包括吸烟、高脂饮食、缺乏运动、高血压和遗传倾向。改变生活方式以及在某些情况下的支架置入或搭桥手术可以控制冠心病。
10. Controlling Heart Rate | 心率控制
The heart rate is controlled by the pacemaker region in the right atrium, known as the sinoatrial node (SAN). The SAN generates electrical impulses that spread across the atria, causing them to contract. The impulse then passes to the atrioventricular node (AVN), which delays it slightly before sending it down the Bundle of His and Purkinje fibres to the ventricles, ensuring they contract from the bottom up.
心率由右心房中的起搏区域——窦房结控制。窦房结产生电冲动，传遍心房，使其收缩。冲动随后传导至房室结，后者略微延迟后再通过希氏束和浦肯野纤维传至心室，确保心室从底部向上收缩。
The autonomic nervous system and hormones like adrenaline can modify the heart rate. During exercise, the heart rate increases to deliver more oxygen to muscles. At rest, the parasympathetic nervous system slows the heart down.
自主神经系统和肾上腺素等激素可以调节心率。运动时心率加快，以向肌肉输送更多氧气。休息时，副交感神经系统使心率减慢。
11. Blood Pressure and Health | 血压与健康
Blood pressure is the force exerted by blood against the artery walls. It is measured in millimetres of mercury (mmHg) and recorded as systolic pressure over diastolic pressure. The systolic pressure represents the pressure when ventricles contract; diastolic pressure is when ventricles relax.
血压是血液对动脉壁施加的压力。它以毫米汞柱为单位测量，记录为收缩压/舒张压。收缩压代表心室收缩时的压力，舒张压是心室舒张时的压力。
Persistent high blood pressure (hypertension) can damage blood vessel linings, promote atherosclerosis, and increase the risk of heart attack, stroke, and kidney failure. Low blood pressure (hypotension) may cause dizziness or fainting. A healthy diet, regular exercise, and limiting salt and alcohol intake help maintain normal blood pressure.
持续的高血压会损伤血管内壁，促进动脉粥样硬化，并增加心脏病发作、中风和肾衰竭的风险。低血压可能导致头晕或昏厥。健康饮食、经常锻炼、限制盐和酒精摄入有助于维持正常血压。
12. Key Exam Tips for Blood Circulation | 血液循环考点总结
When answering questions on circulation, always refer to the direction of blood flow and name the relevant chambers, vessels, and valves. Be very clear about which side of the heart contains oxygenated or deoxygenated blood. Use labelled diagrams when required, and practice drawing and labelling the heart.
在回答血液循环考题时，务必指出血流方向，并说出相关腔室、血管和瓣膜的名称。要非常清楚心脏的哪一侧含有富氧血或缺氧血。需要时使用标注清晰的图表，并练习绘制和标记心脏结构。
Common exam topics include: adaptations of red blood cells, differences between arteries and veins, the role of valves, the double circulatory system, and lifestyle factors that affect heart health. Make sure you can explain the sequence of events in the cardiac cycle and connect the structure of blood vessels to their function.
常见考点包括：红细胞的适应特征、动脉与静脉的区别、瓣膜的作用、双循环系统以及影响心脏健康的生活方式因素。确保你能解释心动周期的事件顺序，并将血管结构与功能联系起来。
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