📚 The Heart and Circulation | 心脏与循环
The circulatory system is at the centre of IGCSE Biology (Edexcel). It delivers oxygen, glucose and other substances to every cell while removing carbon dioxide and waste. At the core of this system sits the heart – a muscular pump that never rests. This article will walk you through the structure of the human heart, the double circulation, the role of blood vessels and the composition of blood. You will also learn how to calculate cardiac output and why lifestyle choices affect heart health.
循环系统是IGCSE生物（Edexcel）的核心内容。它将氧气、葡萄糖等物质输送到每一个细胞，同时清除二氧化碳和废物。这个系统的核心是心脏——一个永不休息的肌肉泵。本文将带你了解人类心脏的结构、双循环、血管的作用以及血液的组成。你还会学到如何计算心输出量，以及生活方式的选择为何会影响心脏健康。
1. Location and External Anatomy | 位置与外部解剖
The human heart lies in the thoracic cavity behind the sternum, slightly tilted to the left. It is roughly the size of a clenched fist and is enclosed in a tough protective sac called the pericardium. The outer wall of the heart itself is composed of three layers: the epicardium, the myocardium (the thick muscular layer) and the endocardium. The myocardium is especially thick in the left ventricle because it must pump blood around the entire body.
人类心脏位于胸腔内胸骨后方，略微偏向左侧。它大约有一个紧握的拳头大小，并被一层坚韧的保护性囊——心包所包裹。心壁本身由三层组成：心外膜、心肌层（厚厚的肌肉层）和心内膜。左心室的心肌层尤其厚实，因为它必须将血液泵送到全身各处。
2. Internal Chambers and Septum | 内部腔室与隔膜
Inside, the heart is divided into four chambers: two atria at the top and two ventricles at the bottom. The right atrium receives deoxygenated blood from the body through the vena cava, while the left atrium receives oxygenated blood from the lungs via the pulmonary veins. A thick muscular wall called the septum separates the left and right sides, preventing oxygenated and deoxygenated blood from mixing. This separation is essential for maintaining a high concentration gradient for oxygen in the tissues.
心脏内部被分为四个腔室：上方是两个心房，下方是两个心室。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氧合血。一层被称为隔膜的厚肌肉壁分隔左右两侧，防止氧合血和脱氧血混合。这种分隔对于维持组织内高氧气浓度梯度至关重要。
3. Heart Valves and One-Way Flow | 心瓣膜与单向流动
Blood flows in one direction only thanks to four sets of valves. The tricuspid valve sits between the right atrium and right ventricle, and the bicuspid (mitral) valve lies between the left atrium and left ventricle. When the ventricles contract, these atrioventricular valves snap shut, producing the 'lub' sound. The semilunar valves – the pulmonary valve and the aortic valve – guard the exits to the pulmonary artery and the aorta. They close when the ventricles relax, creating the 'dub' sound. Tendinous cords and papillary muscles anchor the valve flaps, stopping them from turning inside out.
血液只能单向流动，这要归功于四组瓣膜。三尖瓣位于右心房和右心室之间，二尖瓣位于左心房和左心室之间。当心室收缩时，这些房室瓣猛然关闭，产生“lub”音。半月瓣——肺动脉瓣和主动脉瓣——守卫着肺动脉和主动脉的出口。它们在心室舒张时关闭，产生“dub”音。腱索和乳头肌拉住瓣膜小叶，防止它们翻向心房。
4. Pathway of Blood Through the Heart | 血液流经心脏的路径
m → bicuspid valve → left ventricle → aortic semilunar valve → aorta → body. This sequence ensures that blood always moves from an area of higher pressure to lower pressure, never backwards.
脱氧血从身体返回 → 上腔静脉和下腔静脉 → 右心房 → 三尖瓣 → 右心室 → 肺动脉半月瓣 → 肺动脉 → 肺。在肺部，二氧化碳被换为氧气。然后氧合血 → 肺静脉 → 左心房 → 二尖瓣 → 左心室 → 主动脉半月瓣 → 主动脉 → 全身。这一序列确保血液总是从高压区流向低压区，绝不会逆流。
5. The Cardiac Cycle | 心动周期
One complete heartbeat consists of systole (contraction) and diastole (relaxation). During atrial systole, both atria contract, pushing blood into the relaxed ventricles. Ventricular systole follows: the ventricles contract, forcing the atrioventricular valves shut and opening the semilunar valves. Blood is ejected into the pulmonary artery and aorta. Finally, diastole occurs – the whole heart relaxes, the semilunar valves close and the atrioventricular valves open, allowing the atria to fill passively. A typical resting heart rate is about 70 beats per minute.
一次完整心跳包括收缩期和舒张期。在心房收缩期，两个心房收缩，将血液推入舒张的心室。随后是心室收缩期：心室收缩，迫使房室瓣关闭并推开半月瓣，血液射入肺动脉和主动脉。最后是舒张期——整个心脏舒张，半月瓣关闭，房室瓣重新打开，心房被动地充满血液。典型的静息心率约为每分钟70次。
6. Double Circulation: Pulmonary and Systemic | 双循环：肺循环与体循环
Mammals have a double circulatory system. The right side of the heart drives the pulmonary circulation: blood is pumped at relatively low pressure to the lungs to pick up oxygen and release carbon dioxide. The left side powers the systemic circulation: oxygenated blood is sent at high pressure to the head, muscles, digestive organs and all other tissues. The advantage of double circulation is that the blood can travel quickly to the gas-exchange surface and then return to the heart to be re-pressurised before it rushes to the rest of the body. This keeps metabolic rates high.
哺乳动物拥有双循环系统。右心驱动肺循环：血液以相对较低的压力被泵送到肺部，以获取氧气并排出二氧化碳。左心驱动体循环：氧合血以高压被输送到头部、肌肉、消化器官及所有其他组织。双循环的优势在于，血液可以快速抵达气体交换表面，然后返回心脏重新加压，再冲向身体其他部位。这使代谢率得以保持高水平。
7. Blood Vessels: Arteries, Veins and Capillaries | 血管：动脉、静脉和毛细血管
Arteries carry blood away from the heart. They have thick walls rich in smooth muscle and elastic fibres, which allow them to withstand and smooth out the high-pressure pulses. Veins return blood to the heart; their walls are thinner and they contain valves to prevent backflow. Capillaries are tiny, one-cell-thick vessels where substances are exchanged with the tissues. Their thin walls and narrow diameter maximise diffusion of oxygen, glucose and waste products.
动脉将血液带离心脏。它们拥有厚厚的管壁，富含平滑肌和弹性纤维，这使它们能够承受并缓和高压脉搏。静脉将血液送回心脏；它们的管壁较薄，内部有瓣膜以防止倒流。毛细血管是仅一个细胞厚度的细小血管，物质在此与组织交换。它们极薄的管壁和狭窄的直径，使氧气、葡萄糖和废物的扩散效率达到最大。
8. Composition and Functions of Blood | 血液的组成与功能
Blood is a fluid tissue made of plasma (55%) and cellular components (45%). Plasma is a straw-coloured liquid that transports dissolved glucose, amino acids, hormones, mineral ions and waste products such as urea. Red blood cells contain haemoglobin, which binds oxygen to form oxyhaemoglobin. White blood cells – lymphocytes and phagocytes – defend the body against pathogens. Platelets are cell fragments that trigger blood clotting when a vessel is damaged. All these components are manufactured in the bone marrow.
血液是一种流体组织，由血浆（55%）和细胞成分（45%）组成。血浆是一种淡黄色液体，运输溶解的葡萄糖、氨基酸、激素、矿物离子以及尿素等废物。红细胞含有血红蛋白，它能结合氧气形成氧合血红蛋白。白细胞——淋巴细胞和吞噬细胞——防御病原体入侵。血小板是细胞碎片，当血管受损时触发血液凝固。所有这些成分都在骨髓中制造。
9. Coronary Circulation and Heart Health | 冠状循环与心脏健康
The heart muscle itself needs a constant supply of oxygenated blood. This is provided by the coronary arteries that branch off the aorta and wrap around the heart. If these arteries become narrowed by fatty plaques (cholesterol, calcium and other substances), the condition is called coronary heart disease. A reduced blood supply can cause angina (chest pain) or a heart attack if a coronary artery becomes completely blocked. Risk factors include a diet high in saturated fat, smoking, high blood pressure, obesity and lack of exercise.
心肌自身需要持续的氧合血供应。这由冠状动脉提供，它们从主动脉分出并环绕心脏。如果这些动脉因脂肪斑块（胆固醇、钙和其他物质）而变窄，就称为冠心病。血液供应减少可导致心绞痛，若冠状动脉完全堵塞则引起心脏病发作。危险因素包括高饱和脂肪饮食、吸烟、高血压、肥胖和缺乏运动。
10. Measuring Cardiac Output | 测量心输出量
Cardiac output (CO) is the volume of blood pumped by each ventricle per minute. It depends on two factors: heart rate (HR), the number of beats per minute, and stroke volume (SV), the volume pumped per beat.
心输出量（CO）是每个心室每分钟泵出的血液量。它取决于两个因素：心率（HR），即每分钟心跳次数，以及每搏输出量（SV），即每跳动一次泵出的血量。
Cardiac Output = Heart Rate × Stroke Volume
oximately 5 L/min. During exercise, both heart rate and stroke volume increase, so cardiac output can rise dramatically to meet the higher oxygen demand of muscles.
例如，心率70次/分，每搏输出量70 mL，心输出量为 70 × 70 = 4900 mL/min，约5 L/min。运动时，心率和每搏输出量都增加，心输出量会大幅上升，以满足肌肉增加的氧需求。
11. Common Heart Diseases and Risk Factors | 常见心脏病与风险因素
Apart from coronary heart disease, hypertension (persistently high blood pressure) can damage artery walls and make the heart work harder, possibly leading to heart failure. A faulty heart valve may cause inefficient flow and a murmur. Lifestyle changes such as eating more unsaturated fats, consuming less salt, exercising regularly and not smoking significantly lower the risk. Statins are drugs that reduce blood cholesterol, while aspirin can prevent clot formation. In severe cases, a stent may be inserted to hold an artery open, or a bypass graft can route blood around a blocked section.
除冠心病外，高血压（持续性血压增高）会损伤动脉壁，增加心脏负荷，可能导致心力衰竭。心瓣膜失灵可造成血流不畅和杂音。改变生活方式，如多摄入不饱和脂肪、减少盐摄入、规律锻炼和不吸烟，能显著降低风险。他汀类药物可降低血胆固醇，阿司匹林可防止血栓形成。情况严重时，可植入支架撑开动脉，或进行旁路移植术让血液绕过堵塞段。
12. Summary and Key Exam Points | 总结与关键考点
four major valves accurately. Compare arteries, veins and capillaries in terms of wall thickness, lumen size and presence of valves. Explain why the left ventricle wall is thicker than the right (systemic circuit requires higher pressure). Be ready to calculate cardiac output and interpret data on heart rate before and after exercise. Finally, link lifestyle factors to coronary heart disease using clear biological reasoning, such as plaque narrowing the lumen and reducing blood flow to cardiac muscle.
记住单向流动路径：腔静脉 → 右心房 → 右心室 → 肺动脉 → 肺 → 肺静脉 → 左心房 → 左心室 → 主动脉 → 全身。准确标注四个腔室和四个主要瓣膜。从管壁厚度、管腔大小和有无瓣膜三个角度比较动脉、静脉和毛细血管。解释为何左心室壁比右心室壁更厚（体循环需要更高压力）。准备好计算心输出量，并分析运动前后心率的数据。最后，用清晰的生物学推理将生活方式因素与冠心病联系起来，例如斑块使管腔变窄，减少流向心肌的血流。
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