📚 Blood Circulation (CIE A-Level Biology) | 血液循环：CIE A-Level 生物考点精讲
The circulatory system is a fundamental topic in CIE A-Level Biology, encompassing the structure and function of the heart, blood vessels, and blood itself. This article provides a concise yet comprehensive revision of the key points, ensuring you are well prepared for your examinations. You will explore the double circulation in mammals, the cardiac cycle, oxygen and carbon dioxide transport, and the homeostatic control of blood pressure.
循环系统是 CIE A-Level 生物的基础课题，涵盖心脏、血管和血液本身的结构与功能。本文对考点进行简明而全面的梳理，帮助你做好考试准备。你将学习哺乳动物的双循环、心动周期、氧气与二氧化碳的运输，以及血压的稳态调控。
1. Overview of the Circulatory System | 循环系统概述
Mammals possess a closed, double circulatory system. This means blood is confined within vessels and passes through the heart twice during one complete circuit. The right side of the heart pumps deoxygenated blood to the lungs via the pulmonary circulation, where gas exchange occurs. The left side then pumps oxygenated blood to the rest of the body through the systemic circulation, delivering oxygen and nutrients and removing waste products.
哺乳动物具有封闭的双循环系统。这意味着血液始终在血管内流动，在一次完整的循环中两次经过心脏。右心将缺氧血经肺循环泵入肺部进行气体交换，左心再将富氧血通过体循环泵至全身，输送氧气与营养物质并带走代谢废物。
The advantages of a double circulatory system include maintaining high blood pressure for efficient delivery to body tissues, while the lower-pressure pulmonary circuit protects the delicate capillaries in the lungs. This arrangement supports the high metabolic demands of endothermic animals.
双循环系统的优点在于：能为身体组织提供较高的血压以实现高效输送，而低压的肺循环则保护了肺部脆弱的毛细血管。这种配置满足了恒温动物较高的代谢需求。
2. Structure of the Heart | 心脏的结构
The heart is a muscular organ located in the thoracic cavity. It consists of four chambers: two upper atria and two lower ventricles. The atria receive blood, while the ventricles pump blood out of the heart. A thick muscular wall, the septum, separates the left and right sides, preventing the mixing of oxygenated and deoxygenated blood.
心脏是位于胸腔内的肌肉器官，由四个腔室组成：两个上部的心房和两个下部的心室。心房负责接收血液，心室则将血液泵出心脏。较厚的肌性隔壁——房间隔与室间隔——将左右两侧分开，防止富氧血与缺氧血混合。
The walls of the ventricles are thicker than those of the atria because they must contract forcefully to propel blood over long distances. The left ventricle has a particularly thick muscular wall, as it pumps blood throughout the entire systemic circulation, whereas the right ventricle only pumps blood to the nearby lungs.
心室壁比心房壁更厚，因为它们需要强有力地收缩以推动血液流经较长距离。左心室壁特别厚实，因为它要将血液泵至全身体循环，而右心室仅泵至较近的肺部。
Four valves ensure unidirectional blood flow. The atrioventricular (AV) valves (tricuspid on the right, bicuspid/mitral on the left) prevent backflow from ventricles to atria during ventricular contraction. The semilunar valves (pulmonary and aortic) prevent backflow from the arteries into the ventricles during relaxation.
四个瓣膜确保血液单向流动。房室瓣（右房室瓣为三尖瓣，左房室瓣为二尖瓣）在心室收缩时防止血液从心室倒流回心房。半月瓣（肺动脉瓣和主动脉瓣）在心室舒张时防止动脉血液反流回心室。
The cardiac muscle, or myocardium, is myogenic, meaning it can contract rhythmically without external nervous stimulation. The heartbeat is coordinated by the sinoatrial node (SAN), the atrioventricular node (AVN), and the Purkyne fibres.
心肌具有自主性，即无需外部神经刺激即可有节律地收缩。心跳由窦房结(SAN)、房室结(AVN)和浦肯野纤维协调控制。
3. Cardiac Cycle and Heartbeat | 心动周期与心跳
The cardiac cycle describes the sequence of events during one complete heartbeat. It consists of atrial systole, ventricular systole, and diastole. The cycle is initiated by the SAN, a patch of specialised cardiac muscle in the wall of the right atrium, which generates an electrical impulse. This wave of excitation spreads across the atria, causing them to contract (atrial systole) and push blood into the ventricles.
心动周期描述了一次完整心跳中发生的事件序列，包括心房收缩期、心室收缩期和舒张期。周期由右心房壁内特化的心肌——窦房结启动，它发出一阵电冲动。兴奋波传遍心房，引起心房收缩（心房收缩期），将血液挤入心室。
The impulse then reaches the AVN, located near the septum. Here, there is a slight delay, allowing the atria to finish contracting and the ventricles to fill completely before the ventricles contract. The impulse travels down the Bundle of His and spreads through the Purkyne fibres, causing ventricular systole from the apex upwards. This forces blood out through the semilunar valves into the pulmonary artery and aorta.
冲动随后到达位于隔膜附近的房室结。在此处存在短暂的延迟，使心房完成收缩、心室充分充盈，然后心室才收缩。冲动沿希氏束下传，经浦肯野纤维扩散，引起从心尖开始的向上心室收缩。这迫使血液经半月瓣射入肺动脉和主动脉。
Finally, both atria and ventricles relax during diastole. The semilunar valves close to prevent backflow, and blood enters the atria passively from the great veins. The closing of the AV valves produces the first heart sound ‘lub’, and the closing of the semilunar valves creates the second heart sound ‘dub’.
最后，心房和心室在舒张期均舒张。半月瓣关闭以防止反流，血液从大静脉被动流入心房。房室瓣关闭产生第一心音“lub”，半月瓣关闭产生第二心音“dub”。
Cardiac output (cm³ min⁻¹) = Stroke volume (cm³) × Heart rate (beats min⁻¹)
心输出量 (cm³ min⁻¹) = 每搏输出量 (cm³) × 心率 (次 min⁻¹)
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 each have a structure adapted to their specific function. Arteries carry blood away from the heart under high pressure. They have thick walls rich in smooth muscle and elastic fibres, allowing them to stretch and recoil, maintaining pressure and smoothing the pulsatile flow. The narrow lumen helps maintain high pressure.
三种主要的血管各具适配其功能的结构。动脉在高压下将血液带离心脏，管壁厚实且富含平滑肌和弹性纤维，能伸展并回弹，维持压力并使脉动血流变得平缓。较窄的管腔有助于维持高压。
Arterioles are small arteries that regulate blood flow into capillary beds by constricting or dilating. Veins carry blood back to the heart at lower pressure. They possess thinner walls with less muscle and elastic tissue, and a wider lumen. Many veins contain semilunar valves that prevent the backflow of blood, especially in the limbs where gravity acts against the flow.
微动脉是小型动脉，通过收缩或舒张调节流入毛细血管床的血流量。静脉在较低压下将血液导回心脏，管壁较薄，平滑肌与弹性组织较少，管腔较宽。许多静脉含有半月形瓣膜，可防止血液倒流，尤其在重力阻碍血流的四肢部位。
Capillaries are the site of exchange of substances between blood and tissue cells. Their walls consist of a single layer of endothelial cells, minimising diffusion distance. The network is highly branched, providing a large surface area. Capillary walls are permeable to water, small solutes, and dissolved gases, but not to large plasma proteins.
毛细血管是血液与组织细胞进行物质交换的场所。其管壁仅由单层内皮细胞构成，最大程度缩短了扩散距离。毛细血管高度分支，提供了巨大的表面积。管壁可渗透水、小分子溶质和溶解气体，但大分子血浆蛋白不能通过。
5. Blood Pressure and Its Regulation | 血压及其调节
Blood pressure is the hydrostatic force exerted by blood against the walls of blood vessels. It is highest in the aorta and arteries (systolic pressure typically around 120 mmHg in a healthy adult at rest) and falls progressively through the arterioles, capillaries, and veins, reaching nearly zero in the right atrium. This pressure gradient drives blood flow through the systemic circuit.
血压是血液对血管壁施加的静水压力。主动脉和动脉中的血压最高（健康成人静息时的收缩压通常约为 120 mmHg），并沿微动脉、毛细血管和静脉逐渐下降，至右心房时接近零。此压力梯度驱动血液流经体循环。
Blood pressure is monitored by baroreceptors located in the walls of the carotid sinus and aortic arch. When blood pressure rises, these receptors increase the frequency of action potentials sent to the medulla oblongata. The cardiovascular centre responds by increasing parasympathetic (vagus) output to the SAN, decreasing heart rate, and reducing sympathetic output to arterioles, causing vasodilation. This negative feedback loop lowers blood pressure back to the set point.
颈动脉窦和主动脉弓管壁的动脉压力感受器监测血压。当血压升高时，感受器增加传向延髓的动作电位频率。心血管中枢会增强副交感（迷走）神经对窦房结的输出，降低心率，并减弱对微动脉的交感输出，导致血管舒张。这一负反馈回路将血压降回设定值。
Conversely, a fall in blood pressure reduces baroreceptor firing, prompting an increase in sympathetic stimulation. This leads to an increase in heart rate and contractility, and vasoconstriction of arterioles, raising blood pressure.
反之，血压下降时，压力感受器发放频率降低，引发交感兴奋增强。这导致心率和收缩力提高，微动脉收缩，从而使血压升高。
6. Components of Blood | 血液的组成
Blood is a specialised connective tissue consisting of plasma and formed elements. Plasma makes up about 55% of blood volume and is a pale yellow liquid composed mostly of water (about 90%), along with dissolved proteins (albumins, globulins, fibrinogen), glucose, amino acids, ions, hormones, and waste products. It acts as a transport medium and a solvent.
血液是一种特化的结缔组织，由血浆和有形成分组成。血浆约占血液体积的55%，是淡黄色液体，主要成分为水（约90%），还含有溶解的蛋白质（白蛋白、球蛋白、纤维蛋白原）、葡萄糖、氨基酸、离子、激素和废物。作为运输介质和溶剂。
The formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). Erythrocytes are biconcave discs that lack a nucleus and most organelles; they are packed with haemoglobin, which binds and transports oxygen. This shape provides a large surface area-to-volume ratio for efficient gas diffusion.
有形成分包括红细胞、白细胞和血小板。红细胞呈双凹圆盘状，无细胞核和大多数细胞器；它们充满血红蛋白，用以结合并运输氧气。这种形状提供了较大的表面积与体积之比，有利于高效气体扩散。
Leukocytes are part of the immune system and can be divided into phagocytes (e.g., neutrophils, macrophages) that engulf pathogens by phagocytosis, and lymphocytes (B cells and T cells) that produce antibodies and coordinate the immune response. Platelets are small cell fragments that play a crucial role in blood clotting.
白细胞属于免疫系统，可分为通过吞噬作用消灭病原体的吞噬细胞（如中性粒细胞、巨噬细胞）和产生抗体并协调免疫应答的淋巴细胞（B细胞和T细胞）。血小板是小的细胞碎片，在血液凝固中起关键作用。
7. Transport of Oxygen | 氧气的运输
Oxygen is transported in the blood in two ways: about 97% is carried bound to haemoglobin inside red blood cells, forming oxyhaemoglobin; the remaining 3% is dissolved directly in the plasma. Haemoglobin is a quaternary protein consisting of four polypeptide chains, each with a haem group containing an iron ion (Fe²⁺) that can reversibly bind one O₂ molecule.
氧气以两种方式在血液中运输：约97%与红细胞内的血红蛋白结合，形成氧合血红蛋白；其余3%直接溶解在血浆中。血红蛋白是一种四级结构蛋白质，由四条多肽链组成，每条链含有一个血红素辅基，其中的亚铁离子(Fe²⁺)可逆地结合一个O₂分子。
Hb + 4O₂ ⇌ Hb(O₂)₄
Hb + 4O₂ ⇌ Hb(O₂)₄
The oxygen dissociation curve describes the relationship between the partial pressure of oxygen (pO₂) and the percentage saturation of haemoglobin with oxygen. The curve is sigmoidal (S-shaped) because binding of the first oxygen molecule induces a conformational change that facilitates the binding of subsequent oxygen molecules; this is known as positive cooperativity.
氧解离曲线描述了氧分压(pO₂)与血红蛋白氧饱和度之间的关系。该曲线呈S形，因为第一个氧分子的结合引起构象变化，有利于后续氧分子的结合；这称为正协同效应。
In the lungs, where pO₂ is high (about 13 kPa), haemoglobin becomes almost fully saturated. In respiring tissues, pO₂ is low (about 2–4 kPa), and oxygen is readily released. The steep part of the curve occurs at low pO₂ values, enabling a significant amount of oxygen to be unloaded with only a small drop in pO₂, which is particularly useful during exercise.
在肺部，pO₂较高（约13 kPa），血红蛋白几乎完全饱和。在呼吸组织，pO₂较低（约2–4 kPa），氧气容易被释放。曲线陡峭段位于低pO₂区域，这使得pO₂仅轻度下降即可释出大量氧气，在运动时尤其有益。
8. Carbon Dioxide Transport and the Bohr Effect | 二氧化碳运输与波尔效应
Carbon dioxide is transported from tissues to the lungs by three mechanisms: about 5% is dissolved directly in plasma; around 10% binds to the amino groups of haemoglobin to form carbaminohaemoglobin; and the majority, roughly 85%, is carried as hydrogencarbonate ions (HCO₃⁻) in plasma. Inside red blood cells, CO₂ reacts with water under the catalysis of carbonic anhydrase to form carbonic acid, which dissociates into hydrogencarbonate and hydrogen ions.
二氧化碳通过三种机制从组织运往肺部：约5%直接溶解于血浆；约10%与血红蛋白的氨基结合，形成氨基甲酸血红蛋白；大部分，约85%，以碳酸氢根离子(HCO₃⁻)形式存在于血浆中。在红细胞内，CO₂在水和碳酸酐酶的催化下生成碳酸，随后解离为碳酸氢根和氢离子。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
The hydrogencarbonate ions diffuse out of the red blood cells down a concentration gradient, and chloride ions move in to maintain electrical neutrality; this is the chloride shift. In the lungs, the low pCO₂ reverses the process: HCO₃⁻ re-enters the red blood cells, reacts with H⁺ to form CO₂, which then diffuses into the alveoli to be exhaled.
碳酸氢根离子顺浓度梯度扩散出红细胞，氯离子同时移入以维持电中性，即氯转移。在肺部，低pCO₂使过程逆转：HCO₃⁻重新进入红细胞，与H⁺反应生成CO₂，再扩散至肺泡呼出体外。
The Bohr effect describes the decrease in the affinity of haemoglobin for oxygen when the pH is lowered (i.e., increased CO₂ concentration or H⁺ concentration). In actively respiring tissues, high levels of CO₂ lower the pH, causing haemoglobin to release oxygen more readily. This shifts the oxygen dissociation curve to the right, enhancing oxygen unloading where it is most needed.
波尔效应描述了pH降低（即CO₂或H⁺浓度升高）时血红蛋白对氧亲和力的下降。在活跃呼吸的组织中，高浓度CO₂使pH下降，促进血红蛋白释放氧气。这将氧解离曲线右移，在最需要的部位增强氧气卸出。
9. Tissue Fluid and Lymph | 组织液与淋巴
Tissue fluid (interstitial fluid) is the medium through which cells exchange nutrients, oxygen, and waste products with the blood. It is formed by filtration of plasma from the arterial end of capillaries. The hydrostatic pressure of the blood, which is higher than the osmotic pressure of plasma proteins, forces water and small solutes out through the capillary walls. This net outward pressure is called filtration pressure.
组织液（细胞间液）是细胞与血液交换营养物质、氧气和废物的介质。它由毛细血管动脉端的血浆滤过形成。血液的静水压高于血浆蛋白的渗透压，迫使水和小分子溶质透出毛细血管壁。这种向外的净压力称为滤过压。
At the venous end of the capillary, the hydrostatic pressure has dropped significantly, while the osmotic pressure of plasma proteins (oncotic pressure) remains relatively constant. Water and solutes are therefore drawn back into the capillary by osmosis. About 90% of the filtered fluid is reabsorbed in this way.
在毛细血管静脉端，静水压显著下降，而血浆蛋白渗透压（胶体渗透压）则相对恒定。因此，水及溶质通过渗透作用被重新吸收入毛细血管。约90%的滤出液以此方式重吸收。
The remaining 10% of tissue fluid, along with any leaked proteins, is drained by the lymphatic system. Lymph vessels collect this excess fluid (now called lymph) and return it to the blood circulation via the subclavian veins. Lymph nodes along the vessels filter the lymph, removing pathogens and debris, and contain lymphocytes that mount immune responses.
剩余的10%组织液及漏出的蛋白质由淋巴系统引流。淋巴管收集这些多余液体（此时称为淋巴），经由锁骨下静脉将其送回血液循环。沿淋巴管的淋巴结滤过淋巴，清除病原体和碎片，并含有可启动免疫应答的淋巴细胞。
10. Fetal Circulation and Changes at Birth | 胎儿循环与出生时的变化
The fetal circulatory system is adapted to the intrauterine environment, where the lungs are non-functional and gas exchange occurs at the placenta. Oxygenated blood from the placenta reaches the fetus via the umbilical vein. Because the fetal lungs are collapsed, there are three key shunts: the ductus venosus, the foramen ovale, and the ductus arteriosus.
胎儿循环系统适应宫内环境，此时肺部无功能，气体交换在胎盘进行。来自胎盘的富氧血经脐静脉进入胎儿。由于胎儿肺部处于萎陷状态，存在三个关键的分流通道：静脉导管、卵圆孔和动脉导管。
The ductus venosus allows oxygenated blood from the umbilical vein to bypass the liver and enter the inferior vena cava directly. Most of this blood then passes from the right atrium to the left atrium through the foramen ovale, a valve-like opening in the septum, thus bypassing the pulmonary circulation. The blood that does enter the right ventricle is pumped into the pulmonary trunk, but most of it is shunted directly into the aorta via the ductus arteriosus, bypassing the lungs.
静脉导管使来自脐静脉的富氧血绕过肝脏，直接进入下腔静脉。大部分血液随后通过卵圆孔（隔膜上一个瓣膜样开口）从右心房进入左心房，从而绕过肺循环。少量进入右心室的血液被泵入肺动脉干，但大部分经动脉导管直接流入主动脉，绕开了肺部。
At birth, when the baby takes its first breath, the lungs expand and the pulmonary circulation resistance drops dramatically. Blood flow to the lungs increases, and the pressure in the left atrium rises above that in the right atrium, forcing the foramen ovale to close (it eventually fuses to form the fossa ovalis). The ductus arteriosus constricts due to increased oxygen levels and the fall in local prostaglandins, eventually obliterating to become the ligamentum arteriosum. The umbilical vessels also close, establishing the independent double circulatory system.
出生时，婴儿首次呼吸，肺部扩张，肺循环阻力急剧下降。流向肺部的血量增加，左心房压力升高并超过右心房，迫使卵圆孔关闭（最终融合形成卵圆窝）。动脉导管因血氧升高和局部前列腺素下降而收缩，最终闭锁成为动脉韧带。脐血管也闭锁，从而建立起独立的双循环系统。
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