📚 Blood Circulation: AQA A-Level Biology | A-Level AQA 生物：血液循环 考点精讲
The circulatory system in mammals is a closed, double circulation that delivers oxygen and nutrients while removing waste products. In AQA A-Level Biology, you must understand cardiac anatomy, the cardiac cycle, blood vessels, gas transport, tissue fluid formation and fetal adaptations. This article covers all key learning points with paired English and Chinese explanations to aid revision.
哺乳动物的循环系统是一个封闭的双循环系统，负责输送氧气和营养并移除废物。在 AQA A-Level 生物学中，你必须掌握心脏解剖、心动周期、血管、气体运输、组织液形成和胎儿适应等考点。本文以中英对照的形式梳理全部核心知识，帮助高效复习。
1. Overview of the Double Circulatory System | 双循环系统概述
In a double circulatory system, blood passes through the heart twice in one complete circuit. The right side pumps deoxygenated blood to the lungs (pulmonary circulation) where gas exchange occurs, while the left side pumps oxygenated blood to the rest of the body (systemic circulation). This separation maintains high pressure in the systemic circuit, which is essential for efficient delivery of oxygen and nutrients to respiring tissues.
在双循环系统中，血液每完成一次全身循环会两次经过心脏。右心将缺氧血泵入肺部进行气体交换（肺循环），左心将富氧血泵到身体其余部分（体循环）。这种分开的回路保证了体循环中的高压，这对于向呼吸组织高效输送氧气和营养物质至关重要。
Double circulation also prevents mixing of oxygenated and deoxygenated blood, allowing the systemic system to deliver blood with a high oxygen concentration. The heart’s structure, with interventricular and interatrial septa, ensures complete separation of the two sides.
双循环还能防止含氧血与缺氧血混合，使体循环能够输送高浓度氧气的血液。心脏的室间隔和房间隔保证了左右两侧的完全分隔。
2. External and Internal Structure of the Heart | 心脏的外部与内部结构
The heart is a hollow, muscular organ located in the thoracic cavity, covered by the pericardium. Externally, the coronary arteries supply the cardiac muscle with oxygenated blood. Internally, it has four chambers: the right and left atria (upper) and the right and left ventricles (lower). The atria are thin-walled receiving chambers, while the ventricles have thick muscular walls to generate high pressure.
心脏是位于胸腔内的中空肌性器官，外包心包膜。外部的冠状动脉为心肌供氧。内部有四个腔室：左心房、右心房（上方）和左心室、右心室（下方）。心房是接受血液的薄壁腔室，而心室肌壁厚实，能产生高压。
Key valves prevent backflow: the atrioventricular (AV) valves (tricuspid on the right, bicuspid/mitral on the left) are held by chordae tendineae to prevent inversion; the semilunar valves (pulmonary and aortic) prevent blood from returning to the ventricles from the arteries. The left ventricle has a much thicker myocardium than the right because it must pump blood around the entire body.
关键瓣膜防止血液倒流：房室瓣（右侧三尖瓣，左侧二尖瓣）由腱索牵拉防止翻转；半月瓣（肺动脉瓣和主动脉瓣）防止血液从动脉回流进心室。左心室的心肌远厚于右心室，因为它需要将血液泵送到全身。
3. The Cardiac Cycle | 心动周期
The cardiac cycle describes the sequence of events in one heartbeat. It consists of diastole (relaxation) and systole (contraction). During atrial systole, the atria contract, forcing blood into the ventricles through the open AV valves. Ventricular systole then follows: the ventricles contract, closing the AV valves (‘lub’ sound) and opening the semilunar valves as pressure rises, ejecting blood into the pulmonary artery and aorta.
心动周期描述一次心跳中的连续事件，包括舒张期和收缩期。心房收缩时，心房通过打开的房室瓣将血液挤入心室。随后进入心室收缩期：心室收缩，压力升高使房室瓣关闭（产生’Lub’的心音），打开半月瓣，将血液射入肺动脉和主动脉。
During ventricular diastole, the ventricles relax, semilunar valves shut (‘dub’ sound) to prevent backflow, and the atria passively fill with blood from the veins. The cycle then repeats. Pressure changes are recorded on a graph showing aortic, ventricular and atrial pressure traces. Understanding the relationship between pressure and valve opening/closing is a typical AQA exam focus.
在心室舒张期，心室松弛，半月瓣关闭（’Dub’心音）防止倒流，心房从静脉被动充盈血液。然后周期重复。压力变化可用主动脉压、心室压和心房压曲线图记录。理解压力与瓣膜开关的关系是 AQA 常见的考点。
4. Coordination of the Cardiac Cycle | 心动周期的协调
The heartbeat is myogenic; it originates within the cardiac muscle itself. The sinoatrial node (SAN) in the right atrium acts as the pacemaker, sending out electrical impulses that spread across the atria, causing atrial systole. The impulse is then delayed at the atrioventricular node (AVN) to allow complete ventricular filling.
心跳是肌源性的，即始于心肌自身。位于右心房的窦房结（SAN）充当起搏器，发出电信号传遍心房引起心房收缩。随后电信号在房室结（AVN）处延迟，以确保心室完全充盈。
From the AVN, the impulse travels down the Bundle of His through the septum, then into the Purkyne fibres which carry the signal upwards through the ventricle walls. This ensures ventricular contraction starts at the apex and moves upwards, efficiently ejecting blood. An electrocardiogram (ECG) records these electrical events in P, QRS and T waves.
电信号从房室结沿房室束（Bundle of His）向下穿过间隔，然后进入浦肯野纤维，后者将信号在心室壁内向上传导。这保证了心室收缩从心尖开始向上推进，高效射血。心电图（ECG）可记录到这些电活动，形成 P 波、QRS 波群和 T 波。
5. Blood Vessels and Their Adaptations | 血管及其适应
Arteries carry blood away from the heart. They have thick walls with abundant elastic fibres and smooth muscle. The elastic fibres allow stretch and recoil, maintaining blood pressure, while the narrow lumen helps sustain high pressure. Arterioles are smaller branches that regulate blood flow into capillary beds via vasoconstriction and vasodilation.
动脉将血液带离心脏。动脉壁厚，富含弹性纤维和平滑肌。弹性纤维能伸展和回弹，维持血压；狭窄的管腔有助于保持高压。小动脉通过收缩和舒张调节进入毛细血管床的血流。
Capillaries are the site of exchange. Their walls consist of a single layer of endothelial cells, providing a short diffusion distance and a large total cross-sectional area. They are often fenestrated (have gaps) to facilitate the formation of tissue fluid. Veins return blood to the heart at low pressure. They have a wide lumen, thinner walls, and possess valves to prevent backflow, aided by the contraction of surrounding skeletal muscles.
毛细血管是物质交换的场所，管壁仅由单层内皮细胞构成，扩散距离极短，且总横截面积很大。常有窗孔结构，促进组织液形成。静脉在低压下将血送回心脏，管腔宽大，管壁较薄，并有静脉瓣防止倒流，周围骨骼肌的收缩帮助血液回流。
6. Composition of Blood | 血液的组成
Blood is a specialized connective tissue composed of plasma (about 55%) and formed elements – erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets). Plasma is a straw-coloured liquid containing water, dissolved proteins (e.g. albumin, fibrinogen, antibodies), nutrients, hormones and waste products.
血液是特化的结缔组织，由约 55% 的血浆和血细胞组成——红细胞、白细胞和血小板。血浆是淡黄色液体，含水、溶解的蛋白质（如白蛋白、纤维蛋白原、抗体）、营养、激素和代谢废物。
Erythrocytes are biconcave discs lacking a nucleus in mammals; this shape increases surface area for oxygen uptake and allows flexibility. They contain haemoglobin for oxygen transport. Leukocytes are part of the immune system, and platelets play a key role in blood clotting. You should be able to recognise blood cell diagrams as required by AQA practical skills.
哺乳动物的红细胞呈双凹圆盘状，无细胞核；该形态增大了氧气吸收的表面积，并赋予其柔韧性。红细胞含血红蛋白用于运输氧气。白细胞参与免疫反应，血小板在凝血中起关键作用。根据 AQA 实验技能要求，应能识别血细胞图谱。
7. Tissue Fluid and Lymph Formation | 组织液和淋巴的形成
Tissue fluid bathes the cells, enabling exchange of substances. It is formed at the arteriolar end of a capillary where hydrostatic pressure inside the capillary exceeds the osmotic pressure exerted by plasma proteins. This pressure difference forces water and small solutes out through the capillary wall, leaving larger proteins and cells behind.
组织液浸泡细胞，实现物质交换。它在毛细血管的小动脉端形成，此处毛细血管内的静水压大于血浆蛋白产生的渗透压。压力差迫使水和小分子溶质透出毛细血管壁，留下大分子蛋白和血细胞。
At the venular end, the hydrostatic pressure has decreased, while the osmotic pressure (due to plasma proteins) remains largely unchanged. This draws most of the water back into the capillary, along with any dissolved waste products. The excess fluid (~10%) drains into the lymphatic system, forming lymph, which eventually returns to the blood via the subclavian veins. This return mechanism maintains the blood volume and prevents oedema.
在毛细血管的小静脉端，静水压降低，而血浆蛋白引起的渗透压基本不变，这会将大部分水连同废物一起拉回毛细血管内。大约 10% 的多余液体进入淋巴系统形成淋巴，最终经由锁骨下静脉返回血液循环。该回流机制维持了血容量，防止水肿。
8. Haemoglobin and the Oxygen Dissociation Curve | 血红蛋白与氧解离曲线
Haemoglobin (Hb) is a quaternary globular protein with four polypeptide subunits, each containing a haem group with an Fe²⁺ ion that can reversibly bind one O₂ molecule. The binding of oxygen is cooperative: binding of the first O₂ changes the shape of haemoglobin, making it easier for subsequent O₂ molecules to bind.
血红蛋白（Hb）是一种四级结构的球状蛋白，由四条多肽亚基组成，每条亚基含一个血红素辅基，其中的 Fe²⁺ 可逆地结合一个 O₂ 分子。氧的结合具有协同性：第一个 O₂ 结合后改变了血红蛋白构象，使后续氧分子更容易结合。
The oxygen dissociation curve is S-shaped (sigmoidal). At low partial pressures of oxygen (pO₂), such as in respiring tissues, haemoglobin has a low affinity and unloads oxygen readily. At high pO₂, as in the lungs, haemoglobin becomes nearly saturated. The curve can shift due to changes in CO₂ concentration, pH, and temperature – this is the Bohr effect. An AQA exam might ask you to interpret dissociation curves for fetal haemoglobin or different species.
氧解离曲线呈 S 形（S 型）。在氧分压（pO₂）较低的组织，血红蛋白亲和力低，容易释放氧气；在肺泡的高氧分压条件下，血红蛋白几乎被完全饱和。曲线的位置会因 CO₂ 浓度、pH 和体温的变化而移动——这就是波尔效应。AQA 考试可能要求解读胎儿血红蛋白或不同动物的解离曲线。
9. The Bohr Effect and Oxygen Unloading | 波尔效应与氧的释放
The Bohr effect describes the decreased affinity of haemoglobin for oxygen when the concentration of carbon dioxide is high (and pH is consequently lower). Actively respiring tissues produce CO₂, which enters red blood cells and combines with water to form carbonic acid (H₂CO₃) via the enzyme carbonic anhydrase. This dissociates into H⁺ and HCO₃⁻, lowering the pH.
波尔效应指的是当二氧化碳浓度升高（pH 因此下降）时，血红蛋白对氧的亲和力降低。活跃呼吸的组织产生 CO₂，进入红细胞后在碳酸酐酶催化下与水结合生成碳酸（H₂CO₃），并解离成 H⁺ 和 HCO₃⁻，使 pH 下降。
The increase in H⁺ ions promotes the unloading of oxygen from oxyhaemoglobin, shifting the dissociation curve to the right. This means that for the same pO₂, more oxygen is released to the respiring cells exactly where it is most needed. The Bohr effect is a crucial link between cellular respiration and oxygen delivery.
H⁺ 浓度的升高促进了氧合血红蛋白释放氧气，使氧解离曲线右移。这意味着在同样的氧分压下，更多的氧气被释放给迫切需要的呼吸细胞。波尔效应是将细胞呼吸与氧气供应精准耦合的关键机制。
10. Carbon Dioxide Transport in the Blood | 血液中二氧化碳的运输
Carbon dioxide is transported from tissues to the lungs in three main forms: about 5% dissolved directly in plasma, about 10% bound to haemoglobin as carbaminohaemoglobin, and the majority (~85%) as hydrogen carbonate ions (HCO₃⁻) in the plasma, following conversion inside red blood cells.
二氧化碳从组织运输到肺部有三种主要形式：约 5% 直接溶解在血浆中，约 10% 与血红蛋白结合成氨基甲酸血红蛋白，绝大部分（约 85%）在红细胞内转化为碳酸氢根离子（HCO₃⁻）后存在于血浆中。
Inside red blood cells, CO₂ and H₂O are converted to H₂CO₃ by carbonic anhydrase; the carbonic acid dissociates into H⁺ and HCO₃⁻. The H⁺ binds to haemoglobin (acting as a buffer) and the HCO₃⁻ diffuses out of the cell into the plasma. To maintain electrical neutrality, chloride ions (Cl⁻) move into the red blood cell — this is the chloride shift. In the lungs, these reactions reverse, and CO₂ is exhaled.
在红细胞内，CO₂ 与 H₂O 在碳酸酐酶作用下生成 H₂CO₃，碳酸解离成 H⁺ 和 HCO₃⁻。H⁺ 与血红蛋白结合（起缓冲作用），HCO₃⁻ 则扩散出细胞进入血浆。为保持电中性，氯离子（Cl⁻）移入红细胞——即氯离子转移。在肺部，这些反应反向进行，CO₂ 被呼出。
11. Fetal Circulation and Maternal Exchange | 胎儿循环与母体交换
The mammalian fetus obtains oxygen and nutrients from the mother’s blood via the placenta; its lungs are non-functional before birth. The fetal circulatory system has several structural adaptations: the foramen ovale (a hole between the right and left atria) and the ductus arteriosus (a vessel connecting the pulmonary artery to the aorta) allow most of the blood to bypass the lungs.
哺乳动物胎儿通过胎盘从母体血液获取氧气和养分，其肺在出生前无功能。胎儿循环系统具有若干结构适应：卵圆孔（左右心房间的孔洞）和动脉导管（连接肺动脉和主动脉的血管）使得大部分血液绕过肺部。
Fetal haemoglobin (HbF) has a higher affinity for oxygen than adult haemoglobin, meaning its dissociation curve is shifted to the left. This is essential because the pO₂ in the placenta is relatively low; the higher affinity enables HbF to pick up oxygen more efficiently from the maternal blood. At birth, when the baby takes its first breath, the foramen ovale closes, the ductus arteriosus contracts and eventually becomes a ligament, and the adult pattern of circulation is established.
胎儿血红蛋白（HbF）对氧的亲和力高于成人血红蛋白，因此其解离曲线左移。这至关重要，因为胎盘中氧分压较低；更高的亲和力使 HbF 更有效地从母体血液中摄取氧气。出生后第一次呼吸时，卵圆孔关闭，动脉导管收缩并最终变为韧带，建立起成体型循环模式。
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