📚 Composition and Functions of Blood | 血液的组成与功能
Blood is a specialised connective tissue that circulates through the cardiovascular system, delivering essential substances to cells and removing metabolic waste products. In the CIE A-Level Biology specification, a detailed understanding of blood composition, cell types, and their physiological roles is essential for topics ranging from transport mechanisms to immunity and homeostasis.
血液是一种特化的结缔组织，在心血管系统中循环，负责向细胞输送必需物质并清除代谢废物。在 CIE A-Level 生物学的考纲中，深入理解血液的组成、各类细胞及其生理功能，是掌握物质运输、免疫和稳态调节等主题的基础。
1. Overview of Blood Composition | 血液组成的总览
Blood is composed of a liquid matrix called plasma, in which formed elements — red blood cells (erythrocytes), white blood cells (leucocytes), and platelets (thrombocytes) — are suspended. On average, plasma constitutes about 55% of total blood volume, while formed elements make up approximately 45%, a proportion known as the haematocrit.
血液由称为血浆的液体基质以及悬浮于其中的有形成分——红细胞、白细胞和血小板——组成。平均而言，血浆约占血液总体积的55%，有形成分约占45%，这一比例称为红细胞压积（血细胞比容）。
- Plasma: 55% of blood volume | 血浆：占血液体积的55%
- Buffy coat (leucocytes and platelets): <1% | 白膜层（白细胞和血小板）：不足1%
- Erythrocytes: approximately 45% | 红细胞：约45%
2. Plasma: Composition and Function | 血浆的组成与功能
Plasma is a pale-yellow fluid consisting of approximately 91-92% water, with the remaining 8-9% being dissolved solutes. These solutes include plasma proteins, inorganic ions, gases, nutrients, nitrogenous waste products, and hormones.
血浆是一种淡黄色液体，约91-92%为水，其余8-9%为溶解的溶质。这些溶质包括血浆蛋白、无机离子、气体、营养物质、含氮废物以及激素。
- Plasma proteins: albumin, globulins, fibrinogen | 血浆蛋白：白蛋白、球蛋白、纤维蛋白原
- Inorganic ions: Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻ | 无机离子：Na⁺、K⁺、Ca²⁺、Cl⁻、HCO₃⁻
- Gases: O₂, CO₂, N₂ | 气体：O₂、CO₂、N₂
- Nutrients: glucose, amino acids, lipids, vitamins | 营养物质：葡萄糖、氨基酸、脂质、维生素
- Nitrogenous waste: urea, uric acid, creatinine | 含氮废物：尿素、尿酸、肌酐
Plasma functions as a transport medium for digested nutrients, hormones, and waste products, and it also plays a key role in maintaining osmotic balance and distributing heat around the body. The plasma proteins contribute to colloidal osmotic pressure, which is critical for fluid exchange at capillaries.
血浆是消化产物、激素和废物运输的介质，同时在维持渗透压平衡和全身热量分布方面发挥关键作用。血浆蛋白有助于形成胶体渗透压，这对毛细血管处的液体交换至关重要。
3. Erythrocytes: Structure and Adaptations | 红细胞的结构与适应特征
Erythrocytes are the most abundant cells in blood, with around 5 million per mm³ in adult males and 4.5 million per mm³ in adult females. Their biconcave disc shape provides a large surface-area-to-volume ratio, facilitating rapid gas diffusion.
红细胞是血液中数量最多的细胞，成年男性约每立方毫米500万，成年女性约每立方毫米450万。其双凹圆盘状赋予细胞较大的表面积与体积比，有利于气体的快速扩散。
- Biconcave shape: maximises surface area for O₂/CO₂ exchange and allows deformation through narrow capillaries | 双凹形状：最大化O₂/CO₂交换的表面积，并允许细胞在狭窄毛细血管中变形通过
- No nucleus or mitochondria: leaves more space for haemoglobin and prevents the cell from consuming the oxygen it carries | 无细胞核和线粒体：为血红蛋白腾出更多空间，同时避免细胞消耗自身携带的氧气
- Cytoplasm packed with haemoglobin (~15 g per 100 mL of blood) | 细胞质中充满血红蛋白（每100 mL血液约含15 g）
- Flexible membrane: allows passage through capillaries as narrow as 3-4 μm | 膜具有柔韧性：可穿过仅3-4 μm宽的毛细血管
In the CIE syllabus, students are expected to explain how the structure of erythrocytes is related to their function of oxygen transport. The absence of organelles ensures that all internal volume is available for haemoglobin, directly increasing oxygen-carrying capacity.
在 CIE 考纲中，学生需要解释红细胞的结构如何与其运输氧气的功能相适应。细胞器缺失确保所有内部空间都用于装载血红蛋白，直接提高了携氧能力。
4. Haemoglobin and Oxygen Transport | 血红蛋白与氧气的运输
Haemoglobin (Hb) is a conjugated protein comprising four polypeptide chains (two α and two β chains in adults), each associated with a haem group containing an Fe²⁺ ion. Each haemoglobin molecule can carry four oxygen molecules, forming oxyhaemoglobin.
血红蛋白是一种结合蛋白，由四条多肽链（成人为两条α链和两条β链）组成，每条链连有一个含Fe²⁺的血红素基团。每个血红蛋白分子可携带四个氧分子，形成氧合血红蛋白。
Hb + 4O₂ ⇌ HbO₈
This equation is a simplification; the reaction is more accurately described as a stepwise loading of O₂ onto Fe²⁺ sites. The binding of the first O₂ molecule increases the affinity of haemoglobin for subsequent O₂ molecules, a phenomenon known as cooperative binding, which gives the oxygen dissociation curve its sigmoid shape.
该方程为简化形式；更准确的描述是O₂逐步结合到Fe²⁺位点。第一个O₂分子的结合提高了血红蛋白对后续O₂分子的亲和力，这一现象称为协同结合，使氧解离曲线呈S形。
- In the lungs: high pO₂ favours loading of O₂ | 在肺部：高pO₂促进O₂的加载
- In respiring tissues: low pO₂, high pCO₂, and lower pH promote unloading | 在呼吸组织：低pO₂、高pCO₂和较低pH促进O₂的释放
- The Bohr effect: a decrease in pH shifts the oxygen dissociation curve to the right, enhancing O₂ delivery to active tissues | 波尔效应：pH降低使氧解离曲线右移，增强向活动组织的O₂释放
5. Leucocytes: Classification and Roles | 白细胞的分类与功能
Leucocytes, or white blood cells, are nucleated cells involved in immune defence. They are fewer in number than erythrocytes (about 4,000-11,000 per mm³) and are divided into granulocytes and agranulocytes based on the presence of cytoplasmic granules.
白细胞是有核细胞，参与免疫防御。其数量少于红细胞（约每立方毫米4,000-11,000个），根据细胞质中是否存在颗粒，可分为粒细胞和无粒细胞。
| Cell Type | 细胞类型 | Proportion | 比例 | Main Function | 主要功能 |
| Neutrophil | 中性粒细胞 | 60-70% | Phagocytosis of bacteria | 吞噬细菌 |
| Eosinophil | 嗜酸性粒细胞 | 2-4% | Defence against parasites; allergic responses | 抗寄生虫；参与过敏反应 |
| Basophil | 嗜碱性粒细胞 | 0.5-1% | Release histamine and heparin; inflammation | 释放组胺和肝素；参与炎症反应 |
| Monocyte | 单核细胞 | 2-8% | Differentiates into macrophages; phagocytosis | 分化为巨噬细胞；吞噬作用 |
| Lymphocyte | 淋巴细胞 | 20-30% | B cells produce antibodies; T cells kill infected cells | B细胞产生抗体；T细胞杀死感染细胞 |
The differential white blood cell count is a clinically important diagnostic tool. An elevated neutrophil count suggests a bacterial infection, while an increased eosinophil count may indicate a parasitic infection or allergy.
白细胞分类计数是临床上重要的诊断工具。中性粒细胞计数升高提示细菌感染，而嗜酸性粒细胞增多可能提示寄生虫感染或过敏。
6. Platelets and Haemostasis | 血小板与止血
Platelets (thrombocytes) are small, anucleate cell fragments derived from megakaryocytes in the bone marrow. They are essential for haemostasis, the process that stops bleeding after vascular injury.
血小板是无核的小细胞碎片，由骨髓中的巨核细胞产生。它们在止血过程中至关重要——止血是指在血管受损后阻止出血的过程。
The haemostatic response occurs in three main stages:
止血反应主要分为三个阶段：
- Vascular spasm: smooth muscle in the vessel wall contracts, reducing blood flow | 血管痉挛：血管壁平滑肌收缩，减少血流
- Platelet plug formation: platelets adhere to exposed collagen, become activated, and aggregate | 血小板栓形成：血小板粘附于暴露的胶原，被激活并聚集
- Coagulation (blood clotting): a cascade of reactions converts fibrinogen into insoluble fibrin, trapping blood cells | 凝血：一系列级联反应将纤维蛋白原转化为不溶性的纤维蛋白，网罗血细胞
Coagulation requires calcium ions (Ca²⁺) and vitamin K, which is needed for the synthesis of several clotting factors in the liver. Fibrinolysis, the breakdown of clots, is achieved by plasmin, ensuring that clots do not persist once the vessel is repaired.
凝血过程需要钙离子（Ca²⁺）和维生素K，后者是肝脏合成多种凝血因子所必需的。纤溶过程由纤溶酶完成，确保血管修复后血凝块不会持续存在。
7. Transport of Carbon Dioxide | 二氧化碳的运输
Blood plays a central role in transporting CO₂ from respiring tissues to the lungs. Approximately 70% of CO₂ is transported as hydrogencarbonate ions (HCO₃⁻), about 20% is bound to haemoglobin as carbaminohaemoglobin, and roughly 7-10% is dissolved directly in plasma.
血液在将CO₂从呼吸组织运输到肺部的过程中发挥核心作用。约70%的CO₂以碳酸氢根离子（HCO₃⁻）形式运输，约20%与血红蛋白结合形成氨基甲酰血红蛋白，约7-10%直接溶解于血浆中。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
In erythrocytes, the enzyme carbonic anhydrase accelerates the conversion of CO₂ and water to carbonic acid, which rapidly dissociates into H⁺ and HCO₃⁻. The H⁺ ions are buffered by haemoglobin (the chloride shift maintains electrical neutrality), and the HCO₃⁻ is transported in the plasma.
在红细胞中，碳酸酐酶加速CO₂与水转化为碳酸，碳酸随即解离为H⁺和HCO₃⁻。H⁺被血红蛋白缓冲（氯离子转移维持电中性），HCO₃⁻则在血浆中运输。
8. Blood as a Buffer System | 血液的缓冲系统
Blood resists changes in pH through several buffer systems, the most important being the haemoglobin buffer and the hydrogencarbonate buffer. The normal pH range of arterial blood is 7.35-7.45; deviations beyond this range indicate acidosis or alkalosis.
血液通过多种缓冲系统抵抗pH变化，最重要的是血红蛋白缓冲系统和碳酸氢盐缓冲系统。动脉血的正常pH范围为7.35-7.45；超出此范围则提示酸中毒或碱中毒。
- Haemoglobin buffer: Hb⁻/HHb buffers H⁺ produced during CO₂ transport | 血红蛋白缓冲：Hb⁻/HHb缓冲CO₂运输过程中产生的H⁺
- Hydrogencarbonate buffer: H₂CO₃/NaHCO₃ neutralises strong acids and bases | 碳酸氢盐缓冲：H₂CO₃/NaHCO₃中和强酸和强碱
- Plasma proteins also contribute minor buffering capacity | 血浆蛋白也提供少量缓冲能力
This buffering capacity is vital for enzyme function, as enzymes are highly sensitive to pH changes. In the exam, be prepared to describe the role of buffers in maintaining blood pH within a narrow range.
这种缓冲能力对酶功能至关重要，因为酶对pH变化高度敏感。在考试中，应当能够描述缓冲系统在维持血液pH于狭窄范围内所起的作用。
9. Common Clinical Applications and Exam Context | 临床常见应用与考点背景
Blood disorders are frequently used in exam questions to test understanding of blood function. Anaemia, for example, refers to a reduction in haemoglobin or erythrocyte count, leading to decreased oxygen delivery. Sickle cell anaemia results from a single amino acid substitution in the β-globin chain, causing abnormal haemoglobin polymerisation under low oxygen conditions.
血液疾病常用于试题中考查对血液功能的理解。例如，贫血指血红蛋白或红细胞数量减少，导致供氧能力下降。镰状细胞贫血源于β珠蛋白链中单个氨基酸的替换，导致异常血红蛋白在低氧条件下聚合。
HbA vs HbS: Glu (hydrophilic) → Val (hydrophobic) at position 6 of β chain
HbA 与 HbS 的差异：β链第6位谷氨酸（亲水性）→ 缬氨酸（疏水性）
Other relevant conditions include haemophilia (a genetic deficiency of clotting factors), leukaemia (malignant proliferation of leucocytes), and polycythaemia (excess erythrocytes). Understanding the underlying physiology of these conditions strengthens exam responses that require applied knowledge.
其他相关疾病包括血友病（遗传性凝血因子缺乏）、白血病（白细胞恶性增殖）和红细胞增多症（红细胞过多）。理解这些疾病背后的生理机制，有助于在考试中更好地回答应用型问题。
10. Summary of Key Exam Points | 核心考点总结
The following points are frequently assessed in CIE A-Level Biology papers and should be committed to memory.
以下要点在 CIE A-Level 生物考试中经常出现，应当熟记。
- Plasma is the liquid matrix; it transports nutrients, hormones, waste, and heat | 血浆是液体基质；运输营养物质、激素、废物和热量
- Erythrocytes are biconcave, anucleate, and packed with haemoglobin for efficient O₂ transport | 红细胞呈双凹形、无核，充满血红蛋白以高效运输O₂
- Haemoglobin binds O₂ cooperatively; the dissociation curve is sigmoid; Bohr effect shifts it right at low pH | 血红蛋白协同结合O₂；解离曲线呈S形；低pH时波尔效应使其右移
- Leucocytes are classified as granulocytes and agranulocytes; each type has a distinct immune function | 白细胞分为粒细胞和无粒细胞；每种类型具有不同的免疫功能
- Platelets initiate haemostasis via vascular spasm, platelet plug, and coagulation cascade | 血小板通过血管痉挛、血小板栓和凝血级联启动止血
- CO₂ is transported mainly as HCO₃⁻, with the chloride shift maintaining charge balance | CO₂主要以HCO₃⁻形式运输，氯离子转移维持电荷平衡
- Blood buffers maintain pH at 7.35-7.45; Hb and HCO₃⁻ systems are primary | 血液缓冲系统维持pH在7.35-7.45；Hb和HCO₃⁻系统是主要的
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