📚 The Mammalian Cardiovascular System | 哺乳动物心血管系统
The mammalian cardiovascular system is a closed, double circulation that efficiently transports oxygen, nutrients, hormones, and waste products around the body. It consists of a muscular four-chambered heart, a network of blood vessels, and blood. This highly coordinated system ensures that deoxygenated blood travels to the lungs for gas exchange while oxygenated blood is delivered under high pressure to all tissues. Understanding its structure and function is fundamental to A-Level Biology and crucial for exploring topics such as health, disease, and homeostasis.
哺乳动物的心血管系统是一个封闭的双循环系统，能高效地将氧气、营养物质、激素和代谢废物运送至全身各处。它由肌肉发达的四个腔室的心脏、血管网络和血液组成。这一高度协调的系统确保缺氧血前往肺部进行气体交换，而富氧血则在高压下被输送到所有组织。理解其结构和功能是A-Level生物学的基础，也是探索健康、疾病与稳态等主题的关键。
1. Introduction to the Mammalian Cardiovascular System | 哺乳动物心血管系统简介
The cardiovascular system is the body's main transport network. In mammals, it is a closed system, meaning blood never leaves the vessels. The heart acts as a dual pump: the right side pumps blood to the lungs (pulmonary circulation), and the left side pumps blood to the rest of the body (systemic circulation). This separation of oxygenated and deoxygenated blood allows for a high metabolic rate, as tissues receive a constant supply of oxygen-rich blood.
心血管系统是身体的主要运输网络。在哺乳动物中，它是一个封闭系统，这意味着血液永远不会离开血管。心脏充当双重泵：右侧将血液泵送到肺部（肺循环），左侧将血液泵送到身体其余部分（体循环）。这种富氧血和缺氧血的分离允许较高的代谢率，因为组织能持续获得富氧血液供应。
The system comprises the heart, blood vessels (arteries, veins, capillaries), and blood. Homeostatic regulation of blood pressure, blood volume, and flow distribution is achieved through neural and hormonal mechanisms. Damage or failure of any component can lead to severe consequences, underscoring the importance of maintaining cardiovascular health.
该系统由心脏、血管（动脉、静脉、毛细血管）和血液组成。通过神经和激素机制实现对血压、血容量和血流分配的稳态调节。任何组成部分的损伤或衰竭都可能导致严重后果，这突显了维护心血管健康的重要性。
2. The Double Circulation | 双循环
Mammals have a double circulation, which means blood passes through the heart twice during one complete circuit of the body. The pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs via the pulmonary artery. Gas exchange occurs in the pulmonary capillaries, where blood releases CO₂ and takes up O₂. The now oxygenated blood returns to the left atrium through the pulmonary veins.
哺乳动物具有双循环，即血液在一次完整的体内循环中两次经过心脏。肺循环将缺氧血从右心室通过肺动脉运送到肺部。气体交换发生在肺毛细血管中，血液释放二氧化碳并摄取氧气。随后含氧血通过肺静脉返回左心房。
The systemic circulation delivers oxygenated blood to all body tissues. The left ventricle pumps blood through the aorta, which branches into smaller arteries and arterioles, eventually reaching capillaries. After delivering oxygen and nutrients and picking up wastes, deoxygenated blood returns to the right atrium via venules and veins, primarily the superior and inferior venae cavae.
体循环将富氧血输送到所有身体组织。左心室将血液泵出经过主动脉，主动脉分支成更小的动脉和小动脉，最终到达毛细血管。在输送氧气和营养物质并收集废物后，缺氧血通过小静脉和静脉返回右心房，主要经由上腔静脉和下腔静脉。
This separation ensures that systemic blood pressure is high enough to reach all tissues efficiently, while the pulmonary circuit operates at a lower pressure to protect the delicate lung capillaries. The double circulation is key to maintaining the high metabolic demands of endothermic mammals.
这种分离确保了体循环血压足够高以有效到达所有组织，而肺循环在较低压力下运行以保护脆弱的肺毛细血管。双循环是维持吸热哺乳动物高代谢需求的关键。
3. Structure of the Heart | 心脏结构
The mammalian heart is a hollow, cone-shaped organ located in the thoracic cavity. It is enclosed by a protective double-walled sac called the pericardium. The heart wall is composed of cardiac muscle (myocardium), which is myogenic and capable of self-initiated contraction. Internally, the heart is divided into four chambers: two atria (upper chambers) and two ventricles (lower chambers).
哺乳动物的心脏是一个位于胸腔内的空心锥形器官。它被一个称为心包的保护性双层囊包围。心壁由心肌组成，心肌是肌源性的，能够自发收缩。内部心脏分为四个腔室：两个心房（上腔）和两个心室（下腔）。
- Right atrium receives deoxygenated blood from the body via the venae cavae. 右心房通过腔静脉接收来自身体的缺氧血。
- Right ventricle pumps blood to the lungs via the pulmonary artery. 右心室通过肺动脉将血液泵入肺部。
- Left atrium receives oxygenated blood from the lungs via the pulmonary veins. 左心房通过肺静脉接收来自肺部的富氧血。
- Left ventricle pumps oxygenated blood to the body via the aorta. Its wall is much thicker than that of the right ventricle, as it must generate higher pressure. 左心室通过主动脉将富氧血泵送到身体各处。其壁比右心室厚得多，因为它必须产生更高的压力。
Valves prevent backflow of blood. The atrioventricular (AV) valves include the tricuspid valve (right side) and the bicuspid (mitral) valve (left side). These are attached to papillary muscles by chordae tendineae, which prevent the valves from inverting. The semilunar valves (pulmonary and aortic) are located at the exits of the ventricles and prevent blood from flowing back into the heart after contraction.
瓣膜防止血液倒流。房室瓣包括三尖瓣（右侧）和二尖瓣（左侧）。它们通过腱索与乳头肌相连，防止瓣膜翻转。半月瓣（肺动脉瓣和主动脉瓣）位于心室出口处，防止收缩后血液回流到心脏。
The septum separates the left and right sides, ensuring complete separation of oxygenated and deoxygenated blood. The coronary arteries supply the heart muscle itself with oxygenated blood, branching off from the aorta just above the aortic semilunar valve.
隔膜将左右两侧分开，确保富氧血和缺氧血完全分离。冠状动脉为心肌本身提供富氧血，从主动脉半月瓣上方的主动脉分支出来。
4. The Cardiac Cycle | 心动周期
The cardiac cycle is the sequence of events that occurs during one heartbeat. It consists of systole (contraction) and diastole (relaxation) of both atria and ventricles. At rest, the cycle lasts about 0.8 seconds in humans. The atrial systole first forces remaining blood into the ventricles; then ventricular systole closes the AV valves ('lub' sound), builds pressure, and opens the semilunar valves to eject blood. Ventricular diastole follows, during which the semilunar valves close ('dub' sound) and the ventricles refill passively.
心动周期是指一次心跳期间发生的一系列事件，包括心房和心室的收缩期和舒张期。在静息状态下，人类的周期大约持续0.8秒。首先心房收缩将剩余的血液挤入心室；随后心室收缩关闭房室瓣（产生“lub”音），压力升高，并打开半月瓣将血液射出。接着心室舒张，此时半月瓣关闭（产生“dub”音），心室被动充盈。
aches similar peaks. The volume of blood remaining in the ventricle after contraction is the end-systolic volume, while the end-diastolic volume is the fully relaxed filling volume. Stroke volume = end-diastolic volume − end-systolic volume.
重要的压力变化发生。左心室内压力在收缩期间从接近0 mmHg上升到约120 mmHg，同时主动脉压达到相似峰值。心室收缩后剩余的血液体积称为收缩末期容积，而完全舒张时的充盈体积是舒张末期容积。每搏输出量 = 舒张末期容积 − 收缩末期容积。
5. Control of Heart Rate | 心率的控制
The heart is myogenic, with its intrinsic rhythm set by the sinoatrial node (SAN) located in the right atrium. The SAN generates electrical impulses that spread across the atria, causing atrial contraction. The impulse then reaches the atrioventricular node (AVN), which delays it slightly to allow ventricular filling before passing it down the bundle of His and Purkinje fibres, initiating coordinated ventricular contraction from the apex upwards.
心脏是肌源性的，其内在节律由位于右心房的窦房结（SAN）设定。SAN产生电脉冲，传遍心房，引起心房收缩。然后脉冲到达房室结（AVN），稍作延迟以允许心室充盈，再通过希氏束和浦肯野纤维向下传导，从心尖向上发起协调的心室收缩。
Heart rate is modified by the autonomic nervous system and hormones. The cardiovascular centre in the medulla oblongata receives input from baroreceptors (detecting blood pressure) and chemoreceptors (detecting O₂, CO₂, and pH). Sympathetic nerves release noradrenaline, increasing heart rate and stroke volume; parasympathetic (vagus) nerves release acetylcholine, slowing the heart. Adrenaline from the adrenal medulla also increases heart rate during stress or exercise.
心率由自主神经系统和激素调节。延髓中的心血管中枢接收来自压力感受器（检测血压）和化学感受器（检测O₂、CO₂和pH值）的输入。交感神经释放去甲肾上腺素，提高心率和每搏输出量；副交感神经（迷走神经）释放乙酰胆碱，减慢心跳。肾上腺髓质释放的肾上腺素在应激或运动时也会增加心率。
6. Blood Vessels: Arteries, Veins and Capillaries | 血管：动脉、静脉和毛细血管
The three main types of blood vessels have structures adapted to their functions. Arteries carry blood away from the heart; they have thick, elastic walls to withstand high pressure. The tunica media contains smooth muscle and elastin fibres, allowing vasoconstriction and vasodilation. Veins return blood to the heart under low pressure. They have thinner walls, a wider lumen, and contain valves to prevent backflow. Capillaries are the site of exchange; their walls are a single layer of endothelial cells, providing a short diffusion distance.
三种主要类型的血管结构与其功能相适应。动脉将血液带离心脏；它们具有厚实的弹性管壁以承受高压。中膜含有平滑肌和弹性蛋白纤维，允许血管收缩和舒张。静脉在低压下将血液送回心脏。它们管壁较薄，管腔较宽，并含有瓣膜以防止倒流。毛细血管是交换场所；其管壁为单层内皮细胞，提供了短的扩散距离。
| Feature / 特征 | Artery / 动脉 | Vein / 静脉 | Capillary / 毛细血管 |
|---|---|---|---|
| Wall thickness / 管壁厚度 | Thick / 厚 | Thin / 薄 | Very thin (one cell thick) / 极薄（单细胞厚） |
| Lumen / 管腔 | Narrow / 窄 | Wide / 宽 | Very narrow / 极窄 |
| Valves / 瓣膜 | Absent (except aortic/pulmonary) / 无（主动脉/肺动脉除外） | Present / 有 | Absent / 无 |
| Blood flow / 血流方向 | Away from heart / 离开心脏 | Towards heart / 流向心脏 | From arterioles to venules / 从小动脉到小静脉 |
Arterioles are small branches of arteries that regulate blood flow into capillary beds via contraction of smooth muscle. Venules receive blood from capillaries and merge into veins. The elasticity of arterial walls helps maintain blood pressure during diastole by recoiling.
小动脉是动脉的小分支，通过平滑肌收缩调节进入毛细血管床的血流量。小静脉接收来自毛细血管的血液，并汇合成静脉。动脉壁的弹性通过在舒张期回缩帮助维持血压。
7. Blood Composition and Functions | 血液成分与功能
Blood is a specialised connective tissue composed of plasma (about 55%) and formed elements (about 45%). Plasma is 90% water, carrying dissolved substances such as glucose, amino acids, hormones, antibodies, urea, and plasma proteins (albumin, fibrinogen, globulins). It also transports carbon dioxide as bicarbonate ions and dissolved CO₂.
血液是一种特化的结缔组织，由血浆（约55%）和有形成分（约45%）组成。血浆90%是水，携带溶解的物质，如葡萄糖、氨基酸、激素、抗体、尿素和血浆蛋白（白蛋白、纤维蛋白原、球蛋白）。它还以碳酸氢根离子和溶解态CO₂的形式运输二氧化碳。
Formed elements include:
- Red blood cells (erythrocytes): biconcave discs lacking a nucleus and organelles; packed with haemoglobin to transport oxygen. 红细胞（红血球）：双凹圆盘状，无细胞核和细胞器；充满血红蛋白以运输氧气。
- White blood cells (leukocytes): involved in immune defence, including phagocytes (neutrophils, macrophages) and lymphocytes (B and T cells). 白细胞（白血球）：参与免疫防御，包括吞噬细胞（中性粒细胞、巨噬细胞）和淋巴细胞（B细胞和T细胞）。
- Platelets (thrombocytes): small cell fragments that play a key role in blood clotting. 血小板：在血液凝固中起关键作用的小细胞碎片。
Red blood cells are produced in the bone marrow through erythropoiesis, regulated by the hormone erythropoietin (EPO) from the kidneys in response to low oxygen levels. They live about 120 days and are destroyed in the liver and spleen.
红细胞在骨髓中通过红细胞生成过程产生，受肾脏因低氧水平而释放的促红细胞生成素（EPO）调节。它们寿命约120天，在肝脏和脾脏中被破坏。
8. Oxygen Transport and the Oxyhaemoglobin Dissociation Curve | 氧气运输与氧合血红蛋白解离曲线
Oxygen is transported mainly by binding reversibly to the iron-containing haem group in haemoglobin (Hb), forming oxyhaemoglobin (HbO₂). Each Hb molecule can bind up to four O₂ molecules. The binding is cooperative, meaning the binding of one oxygen molecule increases the affinity for the next. This gives the oxyhaemoglobin dissociation curve its characteristic sigmoid (S) shape.
氧气主要通过可逆地与血红蛋白中的含铁血红素基团结合来运输，形成氧合血红蛋白（HbO₂）。每个血红蛋白分子最多可结合四个O₂分子。结合是协同的，即一个氧分子的结合增加了对下一个氧分子的亲和力。这赋予了氧合血红蛋白解离曲线特有的S形形状。
Hb + 4O₂ ⇌ Hb(O₂)₄
The curve plots percentage saturation of haemoglobin with oxygen against partial pressure of oxygen (pO₂). In the lungs, where pO₂ is high (~12–13 kPa), Hb is almost fully saturated (~97%). In respiring tissues, low pO₂ (~2–5 kPa) leads to oxygen unloading. A shift to the right (Bohr shift) occurs with increased CO₂, increased temperature, or decreased pH (higher H⁺ concentration), reducing Hb's affinity for oxygen and enhancing unloading to active tissues.
该曲线描绘了血红蛋白氧饱和度与氧分压（pO₂）的关系。在肺部，pO₂高（约12-13 kPa），Hb几乎完全饱和（~97%）。在呼吸组织中，低pO₂（约2-5 kPa）导致氧气释放。当CO₂增加、温度升高或pH下降（H⁺浓度增加）时，曲线右移（波尔效应），降低了Hb对氧的亲和力，并增强了向活跃组织的氧气卸放。
Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, with its dissociation curve shifted to the left, allowing efficient oxygen transfer across the placenta.
胎儿血红蛋白比成人血红蛋白对氧的亲和力更高，其解离曲线左移，允许通过胎盘高效地转移氧气。
9. Carbon Dioxide Transport | 二氧化碳运输
Carbon dioxide is produced by respiring cells and transported in the blood in three main ways: about 5% dissolved in plasma, 10% bound to haemoglobin as carbaminohaemoglobin, and 85% as hydrogencarbonate ions (HCO₃⁻) in plasma. The conversion occurs inside red blood cells, catalysed by the enzyme carbonic anhydrase.
二氧化碳由呼吸细胞产生，以三种主要形式在血液中运输：约5%溶解在血浆中，10%与血红蛋白结合形成氨基甲酰血红蛋白，以及85%以碳酸氢根离子（HCO₃⁻）形式存在于血浆中。这种转化发生在红细胞内，由碳酸酐酶催化。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
The HCO₃⁻ ions diffuse out of the red blood cell in exchange for Cl⁻ ions (the chloride shift), maintaining electrochemical neutrality. In the lungs, the low pCO₂ drives the reverse reaction, releasing CO₂ to be exhaled. The Bohr effect links CO₂ transport and O₂ unloading: increased H⁺ from carbonic acid dissociation lowers pH, promoting oxygen release.
HCO₃⁻离子通过氯离子转移（氯离子移换）与Cl⁻交换而扩散出红细胞，维持电化学中性。在肺部，低pCO₂推动逆反应，释放CO₂呼出。波尔效应将CO₂运输与O₂卸放联系起来：碳酸解离产生的H⁺增加，降低pH，促进氧的释放。
10. Cardiovascular Disease: Atherosclerosis and Coronary Heart Disease | 心血管疾病：动脉粥样硬化与冠心病
Atherosclerosis is the progressive narrowing and hardening of arteries due to the build-up of plaques. Plaques consist of cholesterol, lipids, calcium, and fibrous tissue. The process begins with damage to the endothelial lining, often caused by high blood pressure or toxins from smoking. Monocytes are attracted, develop into macrophages, and ingest oxidised LDL cholesterol, forming foam cells. These accumulate, forming a fatty streak that becomes a fibrous plaque. This narrows the lumen, restricts blood flow, and makes the artery less elastic.
动脉粥样硬化是由于斑块积聚导致动脉逐渐狭窄和硬化的过程。斑块由胆固醇、脂质、钙和纤维组织组成。这个过程始于内皮衬里的损伤，通常由高血压或吸烟产生的毒素引起。单核细胞被吸引，发育成巨噬细胞，并吞噬氧化的低密度脂蛋白胆固醇，形成泡沫细胞。它们积聚形成脂肪条纹，进而成为纤维斑块。这使管腔变窄，限制血流，并使动脉弹性降低。
Coronary heart disease (CHD) occurs when the coronary arteries are affected by atherosclerosis. Reduced blood flow to the heart muscle can cause angina (chest pain) upon exertion. If a plaque ruptures, a blood clot (thrombus) may form, completely blocking the artery. This leads to myocardial infarction (heart attack), where cardiac muscle cells are starved of oxygen and die. If a thrombus travels and blocks a vessel elsewhere (e.g. in the brain), it can cause a stroke.
冠心病（CHD）发生在冠状动脉受动脉粥样硬化影响时。流向心肌的血流减少时，用力可引起心绞痛（胸痛）。如果斑块破裂，可能形成血栓，完全阻塞动脉。这会导致心肌梗死（心脏病发作），心肌细胞因缺氧而死亡。如果血栓移动并阻塞其他部位的血管（例如大脑），则可能导致中风。
11. Risk Factors and Prevention | 风险因素与预防
Several factors increase the risk of developing cardiovascular disease. These include:
- High blood cholesterol, especially elevated LDL ('bad' cholesterol) and low HDL ('good' cholesterol). 高血胆固醇，尤其是高水平的低密度脂蛋白（“坏”胆固醇）和低水平的高密度脂蛋白（“好”胆固醇）。
- Hypertension (high blood pressure) damages arterial walls, accelerating plaque formation. 高血压损伤动脉壁，加速斑块形成。
- Smoking introduces carbon monoxide and nicotine; CO reduces oxygen carriage, nicotine constricts vessels, and chemicals damage endothelium. 吸烟引入一氧化碳和尼古丁；一氧化碳降低携氧能力，尼古丁收缩血管，化学物质损伤内皮。
- Diet high in saturated fats, trans fats, and salt, and low in fibre and antioxidants. 饮食富含饱和脂肪、反式脂肪和盐，而缺乏纤维和抗氧化剂。
- Lack of exercise, obesity, and type 2 diabetes also contribute. 缺乏运动、肥胖和2型糖尿病也起促进作用。
Prevention and management involve lifestyle changes such as regular physical activity, a balanced diet rich in unsaturated fats and fresh vegetables, smoking cessation, and limiting alcohol. Medication (statins to lower cholesterol, antihypertensives, and anticoagulants) may be used when risk is high. Public health campaigns target awareness of these risk factors.
预防和管理涉及生活方式的改变，如定期体育活动、富含不饱和脂肪和新鲜蔬菜的均衡饮食、戒烟以及限制饮酒。当风险较高时，可能会使用药物（他汀类药物降低胆固醇、降压药和抗凝剂）。公共卫生宣传运动旨在提高对这些风险因素的认识。
12. Summary | 总结
The mammalian cardiovascular system exemplifies a highly evolved transport network tailored for homeothermy and high metabolic demand. Its double circulation, four-chambered heart, and specialised vessels enable the precise delivery of oxygen and substrates while efficiently removing waste. The intricate regulation of heart rate, blood flow, and blood composition demonstrates the integration of neural, hormonal, and local controls. Understanding cardiovascular physiology is essential for diagnosing and managing common diseases such as atherosclerosis and hypertension, and it provides a foundation for further studies in biology and medicine.
哺乳动物的心血管系统是一个高度进化的运输网络，适应于恒温和高代谢需求。其双循环、四腔心脏和特化的血管实现了氧气和底物的精确输送，同时高效地清除废物。心率、血流和血液成分的复杂调控展示了神经、激素和局部控制的整合。理解心血管生理学对于诊断和管理如动脉粥样硬化和高血压等常见疾病至关重要，并为进一步学习生物学和医学奠定基础。
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