📚 Blood Circulation in IB Biology | IB 生物：血液循环考点精讲
Understanding blood circulation is fundamental to grasping how mammals transport nutrients, gases, and wastes. This revision guide covers the structure of the heart, blood vessels, the cardiac cycle, and regulatory mechanisms, all aligned with the IB Biology syllabus and relevant to CCEA examination requirements.
掌握血液循环是理解哺乳动物如何运输营养物质、气体和废物的基础。本复习指南涵盖心脏结构、血管、心动周期及调节机制，贴合 IB 生物学大纲并兼顾 CCEA 考试要求。
1. The Need for a Transport System | 为什么需要运输系统
Larger organisms cannot rely on diffusion alone because the surface area to volume ratio decreases with size. A circulatory system overcomes this limitation by moving substances rapidly over long distances. Open circulatory systems (e.g. insects) have blood bathing organs directly, but closed systems (e.g. vertebrates) keep blood within vessels, allowing higher pressure and more efficient delivery.
较大的生物不能仅靠扩散，因为表面积与体积比随体型增大而减小。循环系统通过远距离快速运输物质来克服这一限制。开放循环系统（如昆虫）的血液直接浸润器官，而封闭系统（如脊椎动物）将血液限制在血管内，从而产生更高的压力和更高效的运输。
2. Structure of the Heart | 心脏的结构
The human heart is a four-chambered, muscular pump. The right atrium and right ventricle handle deoxygenated blood; the left atrium and left ventricle manage oxygenated blood. Key features include: atrioventricular valves (tricuspid on the right, bicuspid/mitral on the left) preventing backflow from ventricles to atria; semilunar valves (pulmonary and aortic) preventing backflow from arteries into ventricles; and the septum separating oxygenated and deoxygenated blood to maintain concentration gradients.
人类心脏是一个四腔的肌肉泵。右心房和右心室处理缺氧血，左心房和左心室处理含氧血。关键结构包括：房室瓣（右侧三尖瓣、左侧二尖瓣）防止血液从心室回流到心房；半月瓣（肺动脉瓣和主动脉瓣）防止血液从动脉回流入心室；室间隔将含氧血和缺氧血分开，维持浓度梯度。
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 have thick layers of smooth muscle and elastic fibres to withstand and maintain pressure. Veins carry blood toward the heart at lower pressure; they contain valves to prevent backflow and rely on skeletal muscle contraction to assist return. Capillaries are the site of exchange; their walls are a single layer of endothelial cells, allowing rapid diffusion of gases, nutrients, and wastes.
动脉将血液在高压下带离心脏。它们的管壁有厚层的平滑肌和弹性纤维，以承受并维持压力。静脉在较低压下将血液送回心脏；它们含有瓣膜防止回流，并依靠骨骼肌收缩辅助回流。毛细血管是物质交换的场所；管壁仅一层内皮细胞，允许气体、营养物质和废物快速扩散。
4. Blood Composition | 血液的组成
Blood consists of plasma (about 55%) and formed elements. Plasma transports dissolved substances such as glucose, amino acids, hormones, carbon dioxide, urea, and plasma proteins. Red blood cells (erythrocytes) contain haemoglobin for oxygen transport. White blood cells (leucocytes) are involved in immune responses. Platelets (thrombocytes) are cell fragments essential for blood clotting.
血液由血浆（约55%）和有形成分组成。血浆运输溶解的物质，如葡萄糖、氨基酸、激素、二氧化碳、尿素和血浆蛋白。红细胞含有血红蛋白，负责氧气的运输。白细胞参与免疫反应。血小板是细胞碎片，对凝血至关重要。
5. The Cardiac Cycle | 心动周期
The cardiac cycle describes the sequence of contraction and relaxation in one heartbeat. Atrial systole: atria contract, forcing blood into the ventricles through open AV valves. Ventricular systole: ventricles contract, closing AV valves (first heart sound “lub”), opening semilunar valves, and ejecting blood into the pulmonary artery and aorta. Diastole: both atria and ventricles relax; semilunar valves close (second heart sound “dub”), and the heart fills passively.
心动周期描述了一次心跳中收缩与舒张的顺序。心房收缩期：心房收缩，将血液通过打开的房室瓣挤入心室。心室收缩期：心室收缩，关闭房室瓣（第一心音“lub”），打开半月瓣，将血液射入肺动脉和主动脉。舒张期：心房和心室均松弛；半月瓣关闭（第二心音“dub”），心脏被动充盈。
6. Control of the Heartbeat | 心跳的调控
The heartbeat is myogenic – initiated by the sinoatrial node (SAN) in the right atrium. The SAN generates electrical impulses that spread across the atria, causing atrial contraction. The impulses then reach the atrioventricular node (AVN), which delays the signal to allow ventricular filling. The signal travels down the bundle of His and Purkinje fibres, triggering ventricular contraction from the apex upward. The autonomic nervous system can alter heart rate via the medulla oblongata: sympathetic nerves increase rate, parasympathetic (vagus) nerves decrease it.
心跳是肌源性的——由右心房的窦房结（SAN）启动。SAN产生的电冲动传播到心房，引起心房收缩。冲动随后到达房室结（AVN），此处延迟信号以确保心室充盈。信号沿希氏束和浦肯野纤维下传，从心尖向上触发心室收缩。自主神经系统可通过延髓改变心率：交感神经使其加快，副交感神经（迷走神经）使其减慢。
7. Cardiac Output and Blood Pressure | 心输出量与血压
Cardiac output is the volume of blood pumped by each ventricle per minute, calculated as:
Cardiac Output = Heart Rate × Stroke Volume
Stroke volume is the volume of blood ejected per beat. Blood pressure is the force exerted by blood against vessel walls, highest in arteries during systole (systolic pressure) and lowest during diastole (diastolic pressure). Typical values are 120/80 mmHg. Hypertension can lead to cardiovascular disease.
心输出量是每分钟每个心室泵出的血量，计算方法为：
心输出量 = 心率 × 每搏输出量
每搏输出量是每次心跳射出的血量。血压是血液对血管壁施加的力，在动脉收缩期最高（收缩压），舒张期最低（舒张压）。正常值约为 120/80 mmHg。高血压可导致心血管疾病。
8. Double Circulation: Pulmonary and Systemic | 双循环：肺循环和体循环
Mammals have a double circulation: the pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium. The systemic circuit carries oxygenated blood from the left ventricle to all body tissues and returns deoxygenated blood to the right atrium. This separation maintains high pressure in the systemic circuit while protecting the delicate lung capillaries in the pulmonary circuit.
哺乳动物具有双循环：肺循环将缺氧血从右心室输送到肺部，并将含氧血回流至左心房。体循环将含氧血从左心室输送到全身组织，并将缺氧血回流至右心房。这种分离在体循环中维持高压，同时保护了肺循环中脆弱的肺部毛细血管。
9. Exchange at Capillaries: Transport of Oxygen and Carbon Dioxide | 毛细血管处的交换：氧气和二氧化碳的运输
Oxygen diffuses from alveoli into pulmonary capillaries, binding to haemoglobin to form oxyhaemoglobin. In tissues, the dissociation curve shows that as partial pressure of oxygen falls, haemoglobin releases oxygen. Carbon dioxide is transported in three ways: dissolved in plasma (about 7%), bound to haemoglobin as carbaminohaemoglobin (about 23%), and mostly as hydrogen carbonate ions (about 70%) after being converted by carbonic anhydrase inside red blood cells. The Bohr effect describes how increased CO₂ or acidity reduces haemoglobin’s affinity for oxygen, enhancing unloading in respiring tissues.
氧气从肺泡扩散入肺毛细血管，与血红蛋白结合形成氧合血红蛋白。在组织中，根据氧解离曲线，当氧分压下降时，血红蛋白释放氧气。二氧化碳以三种方式运输：溶解于血浆（约7%）、与血红蛋白结合为氨基甲酸血红蛋白（约23%）、以及大部分（约70%）在红细胞内经碳酸酐酶转化形成的碳酸氢根离子。玻尔效应指出，二氧化碳或酸性增加会降低血红蛋白对氧的亲和力，促进在呼吸组织中的卸载。
10. Formation of Tissue Fluid and Lymph | 组织液和淋巴的形成
At the arterial end of a capillary, hydrostatic pressure forces water and small solutes out through gaps between endothelial cells, forming tissue fluid. Large proteins and cells remain in the capillary. At the venous end, the lower hydrostatic pressure and the osmotic effect of plasma proteins draw most fluid back. Excess fluid is drained by the lymphatic system and returned to the blood via the subclavian veins. Lymph nodes filter pathogens and house lymphocytes.
在毛细血管动脉端，静水压将水和小溶质挤出内皮细胞间隙，形成组织液。大分子蛋白和细胞留在毛细血管内。在静脉端，较低的静水压和血浆蛋白的渗透作用将大部分液体吸回。多余的液体由淋巴系统引流，经锁骨下静脉返回血液。淋巴结滤除病原体并容纳淋巴细胞。
11. Atherosclerosis and Coronary Heart Disease | 动脉粥样硬化与冠心病
Atherosclerosis is the narrowing and hardening of arteries due to the build-up of plaques (cholesterol, fatty deposits, fibrous tissue) under the endothelium. This reduces blood flow and increases the risk of blood clot formation. If a coronary artery is blocked, myocardial infarction (heart attack) can occur. Risk factors include high LDL cholesterol, smoking, obesity, lack of exercise, and genetic predisposition. Lifestyle changes and medical interventions can reduce risk.
动脉粥样硬化是由于斑块（胆固醇、脂肪沉积、纤维组织）在内皮下积聚导致动脉变窄变硬。这会减少血流量，增加血栓形成的风险。若冠状动脉堵塞，可能发生心肌梗死（心脏病发作）。风险因素包括高LDL胆固醇、吸烟、肥胖、缺乏运动及遗传易感性。改变生活方式和医疗干预可降低风险。
12. Key Data and Comparisons | 关键数据与对比
| 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 (1 cell) / 极薄（单细胞） |
| Valves / 瓣膜 | Absent / 无 | Present / 有 | Absent / 无 |
| Lumen / 管腔 | Small / 小 | Large / 大 | Extremely small / 极小 |
| Pressure / 压力 | High / 高 | Low / 低 | Decreasing / 逐渐降低 |
Use this comparison to answer questions about structure–function relationships. Arteries must withstand high pressure; veins have valves to counteract low pressure; capillaries maximise surface area for diffusion.
利用此表回答结构与功能关系的问题。动脉必须承受高压；静脉有瓣膜以补偿低压；毛细血管最大化扩散表面积。
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