📚 The Human Heart and Circulation | 人体心脏与循环系统
The human circulatory system is a complex network responsible for transporting oxygen, nutrients, hormones and waste products throughout the body. At its core lies the heart, a muscular pump that drives blood through a closed system of vessels. Understanding the structure and function of this system is fundamental to IGCSE Edexcel Science, as it illustrates how organ systems work together to maintain life.
人体循环系统是一个复杂的网络，负责将氧气、营养物质、激素和废物输送到全身。这个系统的核心是心脏——一个肌肉泵，推动血液在封闭的血管系统中流动。理解这个系统的结构和功能是 IGCSE Edexcel 科学的基础，因为它展示了器官系统如何协同工作以维持生命。
1. Structure of the Heart | 心脏的结构
The human heart is a four-chambered organ located in the thoracic cavity, slightly to the left of the midline. The upper chambers are called atria (singular: atrium) and the lower chambers are ventricles. The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs. The right ventricle pumps deoxygenated blood to the lungs, and the left ventricle pumps oxygenated blood to the rest of the body.
人类心脏是一个四腔室器官，位于胸腔内，中线偏左。上腔室称为心房，下腔室称为心室。右心房接收来自身体的缺氧血，左心房接收来自肺部的富氧血。右心室将缺氧血泵送到肺部，而左心室将富氧血泵送至全身。
The walls of the ventricles are thicker than those of the atria because they must generate higher pressure to push blood further. Notably, the left ventricle has the thickest muscular wall, as it needs to pump blood forcefully through the aorta to the entire body.
心室壁比心房壁厚，因为它们必须产生更高的压力将血液推送得更远。值得注意的是，左心室具有最厚的肌肉壁，因为它需要通过主动脉将血液有力地泵送到全身。
Valves between the atria and ventricles (atrioventricular valves) and between the ventricles and major arteries (semilunar valves) prevent backflow of blood. The tricuspid valve lies on the right side, and the bicuspid (mitral) valve on the left.
心房与心室之间的瓣膜（房室瓣）以及心室与主要动脉之间的瓣膜（半月瓣）防止血液回流。三尖瓣位于右侧，二尖瓣（僧帽瓣）位于左侧。
2. The Double Circulatory System | 双循环系统
Mammals, including humans, have a double circulatory system, meaning that blood passes through the heart twice in one complete circuit. The first pathway is the pulmonary circulation, where deoxygenated blood is pumped from the right ventricle to the lungs via the pulmonary arteries. In the lungs, gas exchange occurs, and oxygenated blood returns to the left atrium through the pulmonary veins.
哺乳动物，包括人类，拥有双循环系统，这意味着血液在一次完整的循环中两次经过心脏。第一条路径是肺循环，缺氧血从右心室经肺动脉泵送到肺部。在肺部进行气体交换后，富氧血通过肺静脉返回左心房。
The second pathway is the systemic circulation, where oxygenated blood is pumped from the left ventricle through the aorta to all body tissues, supplying oxygen and nutrients. Deoxygenated blood then returns to the right atrium via the superior and inferior venae cavae.
第二条路径是体循环，富氧血从左心室通过主动脉泵送到全身所有组织，供应氧气和养分。然后缺氧血经由上腔静脉和下腔静脉返回右心房。
This dual system ensures that oxygenated and deoxygenated blood remain separate, and the blood is repressurised after passing through the lungs, allowing efficient delivery of oxygen to active tissues.
这种双系统确保富氧血和缺氧血保持分离，并且血液在通过肺部后重新增压，从而高效地将氧气输送到活跃的组织。
3. Blood Vessels: Arteries, Veins and Capillaries | 血管：动脉、静脉和毛细血管
Arteries carry blood away from the heart under high pressure. Their walls are thick, muscular and elastic, enabling them to withstand and maintain pressure. The largest artery, the aorta, branches into smaller arteries and arterioles.
动脉在高压下将血液带离心脏。它们的管壁厚实、富有肌肉和弹性，使其能够承受并维持压力。最大的动脉——主动脉，分支成较小的动脉和小动脉。
Veins carry blood towards the heart at lower pressure. Their walls are thinner and less muscular than arteries, and they contain valves to prevent the backflow of blood. Skeletal muscle contraction helps push venous blood back towards the heart.
静脉在较低的压力下将血液带回心脏。它们的管壁比动脉薄、肌肉较少，并且含有瓣膜防止血液回流。骨骼肌的收缩有助于将静脉血推回心脏。
Capillaries are microscopic vessels with walls only one cell thick. They form extensive networks that allow efficient exchange of gases, nutrients and waste between blood and tissue fluid. The thin walls minimise diffusion distance for substances.
毛细血管是管壁仅有一个细胞厚的微小血管。它们形成广泛的网络，使得血液与组织液之间能够高效地交换气体、营养物质和废物。薄壁最大限度地缩短了物质的扩散距离。
4. Composition of Blood | 血液的组成
Blood is a fluid connective tissue consisting of plasma and formed elements. Plasma, a straw-coloured liquid, makes up about 55% of blood volume and transports dissolved substances such as glucose, amino acids, urea, hormones, carbon dioxide and heat.
血液是一种液态结缔组织，由血浆和有形成分组成。血浆是一种淡黄色液体，约占血液体积的55%，运输溶解的物质，如葡萄糖、氨基酸、尿素、激素、二氧化碳和热量。
Red blood cells (erythrocytes) are biconcave discs without a nucleus (in mature cells); they contain the protein haemoglobin, which binds reversibly to oxygen to form oxyhaemoglobin. This allows them to carry oxygen from the lungs to tissues.
红细胞（红血球）是双凹圆盘状，成熟后无细胞核；它们含有血红蛋白，能可逆地与氧气结合形成氧合血红蛋白。这使得它们能将氧气从肺部携带到组织。
White blood cells (leucocytes) are part of the immune system. Lymphocytes produce antibodies, while phagocytes engulf and digest pathogens. Both types help defend the body against infection.
白细胞（白血球）是免疫系统的一部分。淋巴细胞产生抗体，而吞噬细胞吞噬并消化病原体。这两种类型都有助于身体抵御感染。
Platelets are small cell fragments that play a key role in blood clotting at the site of a wound. They release chemicals that convert soluble fibrinogen into insoluble fibrin, forming a mesh that traps blood cells and forms a scab.
血小板是小的细胞碎片，在伤口部位的血凝过程中起关键作用。它们释放化学物质，将可溶性纤维蛋白原转化为不溶性纤维蛋白，形成网状结构，捕获血细胞并形成痂。
5. The Heartbeat and Cardiac Cycle | 心跳与心动周期
The cardiac cycle describes the sequence of events in one heartbeat. It consists of two phases: diastole (relaxation) and systole (contraction). During diastole both atria and ventricles relax, and blood flows passively from the veins into the atria and then into the ventricles.
心动周期描述了一次心跳中事件的顺序。它由两个阶段组成：舒张期（放松）和收缩期（收缩）。在舒张期，心房和心室均舒张，血液被动地由静脉流入心房，然后进入心室。
Atrial systole follows, where the atria contract to push remaining blood into the ventricles. Ventricular systole then occurs: the ventricles contract, the atrioventricular valves snap shut (producing the first heart sound ‘lub’), and blood is forced into the pulmonary artery and aorta. When the ventricles relax, the semilunar valves close (producing the second sound ‘dub’) and prevent backflow.
接着是心房收缩期，心房收缩将剩余的血液推入心室。随后心室收缩：心室收缩，房室瓣猛然关闭（产生第一心音‘lub’），血液被压入肺动脉和主动脉。当心室舒张时，半月瓣关闭（产生第二心音‘dub’）并防止回流。
6. Controlling the Heart Rate | 心率控制
The heart rate is controlled by the pacemaker, a group of specialised cells in the right atrium (the sinoatrial node). These cells generate electrical impulses that spread through the atria and to the ventricles, coordinating contraction. The autonomic nervous system and hormones modify the pacemaker’s intrinsic rate.
心率由起搏点——右心房内的一组特殊细胞（窦房结）控制。这些细胞产生电冲动，传播到心房和心室，协调收缩。自主神经系统和激素可以调节起搏点的固有频率。
During exercise or stress, the hormone adrenaline is released from the adrenal glands. Adrenaline binds to receptors on the pacemaker, increasing the rate of impulse generation and thus raising heart rate. This prepares the body for ‘fight or flight’ by increasing oxygen delivery to muscles.
在运动或压力期间，肾上腺素从肾上腺释放出来。肾上腺素与起搏点上的受体结合，增加冲动的产生速率，从而提升心率。这通过增加对肌肉的氧气供应使身体做好‘战斗或逃跑’的准备。
7. Coronary Heart Disease | 冠心病
Coronary heart disease occurs when the coronary arteries, which supply oxygenated blood to the heart muscle itself, become narrowed or blocked by a build-up of fatty deposits called atheroma. This reduces blood flow to cardiac muscle, causing chest pain (angina) and possibly a heart attack (myocardial infarction).
冠心病是指为心肌本身供应富氧血的冠状动脉由于脂肪沉积物（粥样斑块）积聚而变窄或堵塞。这减少了到达心肌的血流，引起胸痛（心绞痛）甚至可能造成心脏病发作（心肌梗死）。
Risk factors include a diet high in saturated fat and cholesterol, smoking, lack of exercise, high blood pressure, diabetes and genetic predisposition. Lifestyle changes and medication can reduce the risk.
风险因素包括高饱和脂肪和胆固醇饮食、吸烟、缺乏运动、高血压、糖尿病和遗传易感性。改变生活方式和药物治疗可以降低风险。
8. Transport of Substances | 物质运输
The circulatory system transports a wide range of substances. Oxygen is carried mainly bound to haemoglobin inside red blood cells. Carbon dioxide is transported in the blood plasma as dissolved CO₂, as hydrogen carbonate ions (HCO₃⁻), and bound to haemoglobin. Glucose, amino acids, salts and hormones are dissolved in the plasma.
循环系统运输各种各样的物质。氧气主要与红细胞内的血红蛋白结合运输。二氧化碳以溶解的 CO₂、碳酸氢根离子 (HCO₃⁻) 以及与血红蛋白结合的形式在血浆中运输。葡萄糖、氨基酸、盐和激素溶解在血浆中。
Urea, a waste product from the breakdown of proteins in the liver, is carried in plasma to the kidneys for excretion. Excess heat generated by active muscles is distributed by blood flow and lost through the skin, helping to regulate body temperature.
尿素——肝脏分解蛋白质产生的废物——在血浆中被携带至肾脏排出。活跃肌肉产生的多余热量通过血流分布，经皮肤散失，从而帮助调节体温。
9. Comparing Circulatory Systems | 循环系统的比较
Fish have a single circulatory system: blood passes through the heart once per circuit. Deoxygenated blood is pumped from the heart to the gills, where it becomes oxygenated, and then continued directly to the rest of the body before returning to the heart. This is less efficient for a high metabolic rate because blood pressure drops significantly after the gills.
鱼类拥有单循环系统：血液在每次循环中仅一次经过心脏。缺氧血从心脏泵送到鳃，在那里变为富氧血，然后直接继续流向全身，最后返回心脏。对于高代谢率来说，这一方式效率较低，因为血压在鳃后会显著下降。
Mammals and birds have a double circulatory system, as described, which maintains high pressure and oxygen delivery. This adaptation supports endothermy and active lifestyles. Amphibians have a three-chambered heart with some mixing of oxygenated and deoxygenated blood, which is adequate for their lower metabolic demands.
如前所述，哺乳动物和鸟类拥有双循环系统，维持较高的压力和氧气供应。这种适应支持了恒温性和活跃的生活方式。两栖动物具有三腔室心脏，富氧血和缺氧血存在部分混合，这足以满足它们较低的代谢需求。
10. Key Exam Tips | 考试要点
When labelling the heart, always trace the path of blood flow logically. Remember that the right side handles deoxygenated blood, and the left side handles oxygenated blood. Use arrows to indicate direction and label valves accurately. A common error is confusing the left and right from the examiner's perspective; the heart is drawn as if you are looking at someone in front of you, so the left side appears on the right of the diagram.
在标示心脏时，要始终按照逻辑追踪血液流动路径。记住右侧处理缺氧血，左侧处理富氧血。使用箭头标示方向并准确标示瓣膜。一个常见错误是从审阅者的角度混淆左右；心脏的绘制如同你面向一个人，因此左侧会出现在图画的右边。
In blood vessel comparisons, emphasise structural adaptations: thick wall and narrow lumen in arteries to withstand pressure; wide lumen and valves in veins to aid low‑pressure flow; one‑cell‑thick capillary wall for efficient exchange. Relate the double circulation to separation of oxygenated/deoxygenated blood and maintenance of pressure.
在比较血管时，要强调结构适应：动脉壁厚、管腔窄以承受压力；静脉腔宽并有瓣膜以帮助低压流动；毛细血管壁仅一个细胞厚以实现高效交换。将双循环与富氧血和缺氧血的分离以及压力的维持联系起来。
When explaining heart rate control, use key terms: sinoatrial node (pacemaker), adrenaline, autonomic nervous system. For disease, link lifestyle factors to the biological process of atheroma formation and reduced blood flow.
在解释心率控制时，使用关键术语：窦房结（起搏点）、肾上腺素、自主神经系统。对于疾病，将生活方式因素与粥样斑块形成和血流减少的生物学过程联系起来。
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