📚 Blood Circulation: Essential Concepts for IB and AQA Biology | 血液循环：IB/AQA生物核心考点精讲
The circulatory system is a vital transport network in animals, ensuring that every cell receives oxygen and nutrients while metabolic wastes are removed. For IB and AQA Biology students, understanding the structure and function of the heart, blood vessels, and blood components is essential. This article covers key concepts with bilingual explanations to aid revision.
循环系统是动物体内至关重要的运输网络，确保每个细胞获得氧气和营养物质，同时清除代谢废物。对于 IB 和 AQA 生物课程的学生，理解心脏、血管和血液成分的结构与功能是必不可少的。本文通过双语讲解覆盖核心考点，助力复习。
1. Introduction to the Circulatory System | 循环系统简介
The mammalian circulatory system is a closed system, meaning blood is always contained within vessels. It is also a double circulatory system, where blood passes through the heart twice per complete circuit: once to the lungs (pulmonary circulation) and once to the rest of the body (systemic circulation).
哺乳动物的循环系统是一个封闭系统，即血液始终在血管内流动。它也是一个双循环系统，血液在一次完整的循环中两次经过心脏：一次去往肺部（肺循环），另一次去往身体其他部分（体循环）。
The heart acts as a dual pump: the right side pumps deoxygenated blood to the lungs, while the left side pumps oxygenated blood to the body. This separation ensures efficient oxygen delivery and high metabolic rates in endothermic animals.
心脏充当双重泵：右侧将缺氧血泵入肺部，左侧将富氧血泵送到全身。这种分隔确保了高效的氧气输送，满足了恒温动物较高的新陈代谢率。
2. Structure of the Heart | 心脏的结构
The human heart has four chambers: two upper atria (left and right) and two lower ventricles (left and right). The right atrium receives deoxygenated blood from the vena cava, while the left atrium receives oxygenated blood from the pulmonary veins.
人类心脏有四个腔室：两个上方的房室（左心房和右心房）和两个下方的心室（左心室和右心室）。右心房从上腔静脉接收缺氧血，左心房从肺静脉接收富氧血。
The ventricles have thicker muscular walls than the atria because they pump blood out of the heart. The left ventricle is particularly thick, as it must generate enough pressure to propel blood throughout the entire systemic circuit.
心室的肌壁比心房更厚，因为它们要将血液泵出心脏。左心室的肌壁尤其厚实，因为它需要产生足够的压力将血液推送至整个体循环。
The septum is the wall separating the left and right sides of the heart, preventing oxygenated and deoxygenated blood from mixing. Atrioventricular valves (tricuspid on the right, bicuspid/mitral on the left) and semilunar valves (pulmonary and aortic) ensure one-way blood flow and prevent backflow.
室间隔是分隔心脏左右两侧的壁，防止富氧血和缺氧血混合。房室瓣（右侧三尖瓣，左侧二尖瓣/僧帽瓣）和半月瓣（肺动脉瓣和主动脉瓣）确保血液单向流动并防止倒流。
3. Cardiac Cycle and Valves | 心动周期与瓣膜
The cardiac cycle consists of three main stages: atrial systole, ventricular systole, and diastole. During atrial systole, the atria contract, pushing blood into the ventricles. At this time, the atrioventricular valves are open and the semilunar valves are closed.
心动周期包括三个主要阶段：心房收缩期、心室收缩期和舒张期。心房收缩时，心房收缩将血液推入心室。此时房室瓣开放，半月瓣关闭。
Ventricular systole follows, where the ventricles contract, raising pressure to close the AV valves (producing the 'lub' sound) and open the semilunar valves, ejecting blood into the aorta and pulmonary artery.
接下来是心室收缩期，心室收缩使压力升高，关闭房室瓣（产生“lub”心音）并打开半月瓣，将血液射入主动脉和肺动脉。
During diastole, both atria and ventricles are relaxed, allowing the heart to fill passively. The semilunar valves snap shut (producing the 'dub' sound) to prevent backflow of blood from arteries.
在舒张期，心房和心室均松弛，心脏被动充盈。半月瓣关闭（产生“dub”心音）以防止动脉血液倒流。
4. 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 under high pressure; they have thick, elastic muscular walls to withstand and maintain pressure. The pulse we feel is the expansion and recoil of arterial walls.
三种主要的血管类型在结构上与其功能相适应。动脉将血液从心脏输送到身体各处，承受较高的压力；它们拥有厚而富有弹性的肌肉壁，以承受并维持血压。我们感知到的脉搏就是动脉壁的扩张和回弹。
Veins carry blood back to the heart at lower pressure. They have thinner walls, wider lumens, and contain one-way valves to prevent backflow. The contraction of skeletal muscles assists venous return.
静脉将血液送回心脏，压力较低。它们的壁较薄、管腔较宽，并具有单向瓣膜以防止倒流。骨骼肌的收缩有助于静脉回流。
Capillaries are microscopic vessels where exchange of gases, nutrients, and wastes occurs. Their walls are a single cell thick (endothelium), minimizing diffusion distance. They form extensive networks to increase surface area.
毛细血管是进行气体、营养物质和废物交换的微细血管。它们的壁仅由单层细胞（内皮）构成，最大限度地缩短了扩散距离。它们形成广泛的网络以增加表面积。
| Vessel | Structure | Function |
|---|---|---|
| Artery | Thick muscle and elastic fibres | Carry blood at high pressure away from heart |
| Vein | Thin muscle, valves, wide lumen | Return blood to heart at low pressure |
| Capillary | Single cell layer, narrow | Exchange of materials |
The table above summarises the main features of each vessel type.
上表总结了每种血管类型的主要特征。
5. Composition of Blood | 血液的组成
Blood is composed of plasma (about 55%) and cellular components (about 45%). Plasma is a pale yellow liquid consisting mainly of water, with dissolved substances such as glucose, amino acids, hormones, urea, and plasma proteins (e.g., fibrinogen for clotting).
血液由血浆（约55%）和细胞成分（约45%）组成。血浆是一种淡黄色液体，主要成分为水，并溶解有葡萄糖、氨基酸、激素、尿素以及血浆蛋白（如用于凝血的血纤维蛋白原）等。
Red blood cells (erythrocytes) are biconcave discs that lack a nucleus, maximising space for haemoglobin. Their primary role is oxygen transport. White blood cells (leukocytes) are part of the immune system, defending against pathogens. Platelets are fragments of cells that initiate blood clotting.
红细胞（红血球）呈双凹圆盘状，没有细胞核，最大化地容纳血红蛋白。其主要功能是运输氧气。白细胞是免疫系统的一部分，抵御病原体。血小板是细胞碎片，可启动血液凝固。
Adaptations of red blood cells include a large surface area-to-volume ratio, flexibility to squeeze through narrow capillaries, and haemoglobin molecules that bind oxygen reversibly.
红细胞的适应性特征包括较大的表面积体积比、可变形性以穿过狭窄的毛细血管，以及能与氧气可逆结合的血红蛋白分子。
6. Double Circulation: Pulmonary and Systemic | 双循环：肺循环与体循环
In the double circulation, the right side of the heart drives the pulmonary circuit:
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