📚 The Circulatory System: Heart and Blood Vessels | 循环系统：心脏与血管
The circulatory system, also known as the cardiovascular system, is responsible for transporting oxygen, nutrients, hormones, and waste products throughout the body. It consists of the heart, blood vessels, and blood, working together to maintain homeostasis and support cellular respiration.
循环系统，又称心血管系统，负责在全身运输氧气、营养物质、激素和废物。它由心脏、血管和血液组成，共同维持体内稳态并为细胞呼吸提供支持。
1. Functions of the Circulatory System | 循环系统的功能
The circulatory system has several essential functions that keep the body alive and healthy. These include transporting oxygen from the lungs to all body cells and carrying carbon dioxide from cells back to the lungs for exhalation.
循环系统有几项维持身体生命和健康的基本功能，包括将氧气从肺部输送到全身细胞，并将二氧化碳从细胞运回肺部呼出。
It also transports nutrients absorbed from the digestive system to tissues, distributes hormones from endocrine glands to target organs, and helps regulate body temperature by adjusting blood flow near the skin surface.
它还将消化系统吸收的营养物质输送到组织，把激素从内分泌腺运送到靶器官，并通过调节皮肤附近的血流来帮助调节体温。
- Transport – carries oxygen, nutrients, and waste products.
- 运输 – 携带氧气、营养物质和废物。
- Protection – white blood cells defend against pathogens.
- 保护 – 白细胞防御病原体。
- Regulation – distributes heat and hormones.
- 调节 – 分配热量和激素。
2. Structure of the Heart | 心脏的结构
The heart is a muscular organ located in the chest, slightly to the left. It is divided into four chambers: the right atrium, right ventricle, left atrium, and left ventricle. The septum separates the right and left sides to prevent oxygenated and deoxygenated blood from mixing.
心脏是位于胸部略偏左的肌肉器官，分为四个腔室：右心房、右心室、左心房和左心室。中隔将左右两侧分开，防止含氧血和缺氧血混合。
The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs. The left side receives oxygenated blood from the lungs and pumps it to the rest of the body. The left ventricle has a thicker muscular wall than the right ventricle because it must pump blood to the entire body.
心脏右侧接收来自全身的缺氧血并将其泵送到肺部；左侧接收来自肺部的含氧血并将其泵送到全身。左心室比右心室的肌壁更厚，因为它需要将血液泵送到全身。
| Chamber | 腔室 | Blood received | 接收血液 | Pumps to | 泵送至 |
| Right atrium 右心房 | Deoxygenated from body 全身缺氧血 | Right ventricle 右心室 |
| Right ventricle 右心室 | Deoxygenated from right atrium 右心房缺氧血 | Lungs (via pulmonary artery) 肺（经肺动脉） |
| Left atrium 左心房 | Oxygenated from lungs 肺部含氧血 | Left ventricle 左心室 |
| Left ventricle 左心室 | Oxygenated from left atrium 左心房含氧血 | Body (via aorta) 全身（经主动脉） |
3. Heart Valves and the Cardiac Cycle | 心脏瓣膜与心动周期
Valves in the heart ensure that blood flows in one direction only. The atrioventricular valves (tricuspid on the right, bicuspid/mitral on the left) are located between the atria and ventricles. The semilunar valves are located at the exits of the ventricles, at the pulmonary artery and aorta.
心脏内的瓣膜确保血液只能沿一个方向流动。房室瓣（右侧为三尖瓣，左侧为二尖瓣）位于心房和心室之间；半月瓣位于心室的出口处，即肺动脉和主动脉的根部。
The cardiac cycle consists of three stages: diastole (relaxation), atrial systole (atria contract), and ventricular systole (ventricles contract). When the atria contract, blood is pushed into the ventricles. When the ventricles contract, high pressure closes the atrioventricular valves and opens the semilunar valves, forcing blood out of the heart.
心动周期包括三个阶段：舒张期（放松）、心房收缩期（心房收缩）和心室收缩期（心室收缩）。心房收缩时，血液被推入心室；心室收缩时，高压关闭房室瓣并打开半月瓣，将血液泵出心脏。
Cardiac output = Heart rate × Stroke volume
心输出量 = 心率 × 每搏输出量
Exercise increases both heart rate and stroke volume, leading to a higher cardiac output to supply more oxygen to active muscles.
运动时心率和每搏输出量都会增加，使心输出量升高，从而为活跃的肌肉提供更多氧气。
4. Types of Blood Vessels | 血管的类型
Blood travels through three main types of blood vessels: arteries, veins, and capillaries. Each type has a structure adapted to its function.
血液通过三种主要的血管流动：动脉、静脉和毛细血管。每种结构都适应其功能。
- Arteries – carry blood away from the heart, usually oxygenated (except pulmonary artery).
- 动脉 – 将血液运离心脏，通常含氧（肺动脉除外）。
- Veins – carry blood toward the heart, usually deoxygenated (except pulmonary vein).
- 静脉 – 将血液运回心脏，通常缺氧（肺静脉除外）。
- Capillaries – microscopic vessels where exchange of substances occurs.
- 毛细血管 – 发生物质交换的微小血管。
5. Structure of Arteries and Veins | 动脉与静脉的结构
Arteries have thick, elastic muscular walls to withstand the high pressure produced by the heart’s pumping. They have a small lumen and no valves, except at the base of the aorta and pulmonary artery.
动脉具有厚而富有弹性的肌肉壁，以承受心脏泵血产生的高压。它们的管腔较小，除了主动脉和肺动脉基部外没有瓣膜。
Veins have thinner, less elastic walls because the blood pressure is much lower. They have a larger lumen and contain valves to prevent backflow of blood, especially in the limbs.
静脉壁较薄，弹性较小，因为血压低得多。它们管腔较大，并含有瓣膜，防止血液逆流，尤其是在四肢。
Capillaries have walls that are only one cell thick, allowing rapid diffusion of oxygen, carbon dioxide, nutrients, and waste products between blood and tissues.
毛细血管壁仅由一层细胞构成，允许氧气、二氧化碳、营养物质和废物在血液与组织之间快速扩散。
6. The Double Circulatory System | 双循环
Mammals, including humans, have a double circulatory system: the pulmonary circulation and the systemic circulation. Blood passes through the heart twice in one complete circuit of the body.
哺乳动物（包括人类）具有双循环系统：肺循环和体循环。血液在一次完整的全身循环中两次经过心脏。
In pulmonary circulation, deoxygenated blood travels from the right ventricle to the lungs via the pulmonary artery, becomes oxygenated, and returns to the left atrium via the pulmonary vein.
在肺循环中，缺氧血从右心室经肺动脉到达肺部，变为含氧血，然后经肺静脉返回左心房。
In systemic circulation, oxygenated blood travels from the left ventricle to the body via the aorta, delivers oxygen and nutrients, collects waste products, and returns as deoxygenated blood to the right atrium via the vena cava.
在体循环中，含氧血从左心室经主动脉到达全身，输送氧气和营养物质，收集废物，然后作为缺氧血经腔静脉返回右心房。
Right heart → Lungs → Left heart → Body → Right heart
右心 → 肺 → 左心 → 全身 → 右心
The advantage of a double circulation is that oxygenated and deoxygenated blood are kept separate, allowing high pressure and efficient oxygen delivery to tissues.
双循环的优点是含氧血和缺氧血被分开，从而能够保持较高的压力，并高效地向组织输送氧气。
7. Composition of Blood | 血液的组成
Blood is a tissue composed of plasma and cellular components: red blood cells, white blood cells, and platelets.
血液是由血浆和细胞成分（红细胞、白细胞和血小板）组成的组织。
Plasma is the liquid part of blood, carrying dissolved substances such as glucose, amino acids, hormones, urea, and carbon dioxide (partly as hydrogencarbonate ions).
血浆是血液中的液体部分，携带溶解物质，如葡萄糖、氨基酸、激素、尿素和二氧化碳（部分以碳酸氢根离子形式）。
- Red blood cells – contain haemoglobin, which binds to oxygen to form oxyhaemoglobin. They are biconcave discs with no nucleus, increasing their surface area for oxygen absorption.
- 红细胞 – 含有血红蛋白，可与氧气结合形成氧合血红蛋白。它们呈双凹圆盘状，无细胞核，增大了吸收氧气的表面积。
- White blood cells – defend against infection. Phagocytes engulf pathogens; lymphocytes produce antibodies.
- 白细胞 – 防御感染。吞噬细胞吞噬病原体；淋巴细胞产生抗体。
- Platelets – are cell fragments involved in blood clotting.
- 血小板 – 是参与血液凝固的细胞碎片。
8. The Blood Clotting Process | 血液凝固过程
When a blood vessel is damaged, platelets aggregate at the site and release chemicals that convert a soluble protein (fibrinogen) into insoluble fibrin. Fibrin forms a mesh that traps red blood cells, creating a clot.
当血管受损时，血小板在受损部位聚集并释放化学物质，将可溶性蛋白质（纤维蛋白原）转化为不溶性纤维蛋白。纤维蛋白形成网状结构，网罗红细胞，形成血凝块。
Blood clotting prevents excessive bleeding and prevents pathogens from entering the body. After a clot forms, the skin can heal underneath.
血液凝固可防止过度出血，并防止病原体进入人体。血凝块形成后，皮肤可在其下方愈合。
| Step | 步骤 | Event | 事件 |
| 1 | Platelets stick to the wound. 血小板粘附在伤口处。 |
| 2 | Platelets release clotting factors. 血小板释放凝血因子。 |
| 3 | Fibrinogen → fibrin. 纤维蛋白原 → 纤维蛋白。 |
| 4 | Fibrin mesh traps cells to form a clot. 纤维蛋白网网罗细胞形成凝块。 |
9. The Lymphatic System and Tissue Fluid | 淋巴系统与组织液
As blood flows through capillaries, plasma fluid leaks out through the capillary walls into the spaces between cells. This fluid, called tissue fluid, contains oxygen, glucose, and other nutrients but lacks red blood cells and large plasma proteins.
当血液流经毛细血管时，血浆液通过毛细血管壁渗入细胞之间的间隙。这种液体称为组织液，含有氧气、葡萄糖和其他营养物质，但没有红细胞和大分子血浆蛋白。
Not all tissue fluid returns directly into capillaries. Some enters the lymphatic vessels and becomes lymph. The lymphatic system eventually returns this fluid to the bloodstream near the heart via the thoracic duct.
并非所有组织液都直接回到毛细血管。部分组织液进入淋巴管成为淋巴。淋巴系统最终通过胸导管将这种液体送回心脏附近的血流中。
10. Cardiovascular Disease and Healthy Lifestyle | 心血管疾病与健康生活方式
Cardiovascular disease, such as coronary heart disease, occurs when the coronary arteries become narrowed or blocked by fatty deposits (atheroma). This reduces blood flow to the heart muscle, causing angina or a heart attack.
心血管疾病，如冠心病，是当冠状动脉被脂肪沉积物（动脉粥样斑）变窄或堵塞时发生的。这会减少流向心肌的血流，导致心绞痛或心脏病发作。
Risk factors include a diet high in saturated fat and salt, smoking, lack of exercise, obesity, high blood pressure, and genetic predisposition.
危险因素包括高饱和脂肪和高盐饮食、吸烟、缺乏运动、肥胖、高血压和遗传易感性。
To maintain a healthy cardiovascular system, one should exercise regularly, eat a balanced diet rich in fruits and vegetables, avoid smoking, and manage stress.
为了维持心血管系统健康，应定期运动，摄入富含水果和蔬菜的均衡饮食，避免吸烟，并管理压力。
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