📚 Blood Composition: Red Blood Cells, White Blood Cells, Platelets and Plasma | 血液组成：红细胞、白细胞、血小板和血浆
Blood is a specialised tissue that performs vital transport, defence and regulatory functions. It consists of cells and cell fragments suspended in a liquid called plasma. Understanding the composition of blood is essential in IGCSE Biology, as it links directly to respiration, immunity and homeostasis.
血液是一种承担运输、防御和调节功能的特化组织。它由悬浮在液体（血浆）中的细胞和细胞碎片组成。理解血液的组成是 IGCSE 生物学的关键，因为它直接与呼吸、免疫和稳态等主题相关联。
1. Overview of Blood Composition | 血液组成概述
Blood is composed of several components: red blood cells (erythrocytes), white blood cells (leukocytes), platelets (thrombocytes) and plasma. These elements work together to carry oxygen, fight infection, clot wounds and transport nutrients, waste and hormones.
血液由多种成分组成：红细胞、白细胞、血小板和血浆。这些成分协同工作，运送氧气、抵抗感染、凝合伤口以及运输营养、废物和激素。
A sample of centrifuged blood separates into layers. The densest layer at the bottom consists of red blood cells, a thin buffy coat in the middle contains white blood cells and platelets, and the top layer is straw‑coloured plasma.
离心的血液样本会分成不同层次。最底层密度最大的是红细胞，中间薄薄的白色层含有白细胞和血小板，最上层是淡黄色的血浆。
2. Red Blood Cells (Erythrocytes) | 红细胞
Red blood cells are the most numerous cells in the blood. They are small, biconcave discs that lack a nucleus and other organelles. Their cytoplasm is packed with the iron‑containing protein haemoglobin, which binds and transports oxygen.
红细胞是血液中数量最多的细胞。它们是没有细胞核和其他细胞器的小型双凹圆盘状细胞。细胞质中充满含铁的血红蛋白，用以结合和运输氧气。
The red blood cell's shape gives it a large surface area to volume ratio, allowing rapid diffusion of oxygen. The absence of a nucleus creates more space for haemoglobin, so each cell can carry about 250 million molecules of oxygen when fully saturated.
红细胞的形状使其具有很大的表面积与体积比，有利于氧气快速扩散。没有细胞核为血红蛋白腾出更多空间，因此每个红细胞在完全饱和时可携带约 2.5 亿个氧分子。
Red blood cells are produced in the bone marrow and have a lifespan of about 120 days. Old cells are broken down in the spleen and liver, and the iron from haemoglobin is recycled.
红细胞在骨髓中产生，寿命约为 120 天。衰老的细胞在脾脏和肝脏中被分解，血红蛋白中的铁被回收利用。
3. White Blood Cells (Leukocytes) | 白细胞
White blood cells are part of the immune system. They are larger than red blood cells, have a nucleus, and are far less numerous. They defend the body against pathogens such as bacteria, viruses and foreign substances.
白细胞是免疫系统的一部分。它们比红细胞大，有细胞核，数量远少于红细胞。它们抵御细菌、病毒等病原体及外来物质的侵害。
There are two main groups: phagocytes and lymphocytes. Phagocytes engulf and digest pathogens through phagocytosis. Lymphocytes produce antibodies that specifically target antigens on pathogens, and some become memory cells for long‑term immunity.
白细胞主要分为吞噬细胞和淋巴细胞。吞噬细胞通过吞噬作用吞入并消化病原体。淋巴细胞产生抗体，特异性针对病原体上的抗原，部分会成为记忆细胞以提供长期免疫。
4. Platelets (Thrombocytes) | 血小板
Platelets are not complete cells; they are small fragments of larger cells called megakaryocytes. They have no nucleus and are much smaller than red blood cells. Their main role is to initiate blood clotting at sites of injury.
血小板并非完整的细胞，而是名为巨核细胞的大细胞碎裂而成的小碎片。它们没有细胞核，比红细胞小得多，主要功能是在受伤部位启动血液凝固。
When a blood vessel is damaged, platelets stick to the exposed collagen fibres, become activated and release chemical signals that attract more platelets. This forms a temporary platelet plug that reduces blood loss.
当血管受损时，血小板会粘附在暴露的胶原纤维上，被激活并释放化学信号，吸引更多血小板前来聚集，形成临时血小板栓塞，减少血液流失。
5. Blood Plasma | 血浆
Plasma is the pale yellow liquid part of blood, making up about 55% of its total volume. It is about 90% water and acts as a solvent for a wide range of substances. The remaining 10% consists of dissolved solutes, including plasma proteins, nutrients, waste products, ions and hormones.
血浆是血液中的淡黄色液体部分，约占总血容量的 55%。它大约 90% 是水，可作为多种物质的溶剂。剩下的 10% 由溶解物质组成，包括血浆蛋白、营养物质、代谢废物、离子和激素。
Plasma proteins such as albumin, globulins and fibrinogen have important functions. Albumin helps maintain osmotic balance, globulins include antibodies, and fibrinogen is essential for blood clotting.
血浆蛋白如白蛋白、球蛋白和纤维蛋白原具有重要功能。白蛋白帮助维持渗透压平衡，球蛋白中包含抗体，纤维蛋白原对血液凝固至关重要。
6. Adaptations of Red Blood Cells | 红细胞的适应特征
Red blood cells are highly adapted for oxygen transport. Their biconcave shape maximises surface area for gas exchange; the absence of a nucleus provides more room for haemoglobin; and they are flexible, allowing them to squeeze through narrow capillaries.
红细胞高度适应氧气运输。双凹圆盘形使气体交换表面积最大化；无细胞核为血红蛋白提供更多空间；它们还有弹性，能够挤过狭窄的毛细血管。
Haemoglobin inside red blood cells binds oxygen reversibly, forming oxyhaemoglobin in high oxygen concentrations (lungs) and releasing oxygen in tissues where oxygen concentration is low. This cooperative binding makes oxygen delivery efficient.
红细胞内的血红蛋白可逆结合氧气，在氧气浓度高处（肺部）形成氧合血红蛋白，在氧气浓度低的组织处释放氧气。这种协同结合使氧气输送非常高效。
7. Phagocytes and Lymphocytes: Two Key White Blood Cells | 吞噬细胞和淋巴细胞：两种关键白细胞
Phagocytes, such as neutrophils and monocytes, carry out phagocytosis. They detect pathogens, engulf them into a vesicle, and then fuse the vesicle with lysosomes containing enzymes that digest the pathogen. This is a non‑specific defence mechanism.
吞噬细胞（如中性粒细胞和单核细胞）进行吞噬作用。它们识别病原体，将其吞入囊泡，再将囊泡与含酶的溶酶体融合，消化病原体。这是非特异性防御机制。
Lymphocytes provide specific immunity. B lymphocytes produce antibodies that bind to antigens and mark pathogens for destruction. T lymphocytes destroy infected body cells. Some lymphocytes become memory cells, giving long‑lasting immunity against a specific disease.
淋巴细胞提供特异性免疫。B 淋巴细胞产生抗体，与抗原结合，标记病原体供清除。T 淋巴细胞破坏被感染的体细胞。部分淋巴细胞成为记忆细胞，提供针对特定疾病的长期免疫力。
8. Role of Platelets in Blood Clotting | 血小板在血液凝固中的作用
Blood clotting is a complex series of reactions that turns liquid blood into a gel. Platelets are essential at the initial stage. They aggregate at the wound site and release factors that help convert the soluble plasma protein fibrinogen into insoluble fibrin threads.
血液凝固是一系列将液体血液转化为凝胶的复杂反应。血小板在初始阶段必不可少。它们在伤口处聚集，并释放因子，帮助将可溶性血浆蛋白纤维蛋白原转化为不溶性纤维蛋白丝。
Fibrin forms a mesh that traps more platelets and red blood cells, creating a stable clot that seals the wound and prevents pathogen entry. Calcium ions and vitamin K are also required for some of the reactions.
纤维蛋白形成网状结构，网住更多血小板和红细胞，形成稳固的血凝块，封闭伤口并阻止病原体进入。某些反应还需要钙离子和维生素 K 参与。
Without sufficient platelets, clotting is impaired, which can lead to excessive bleeding and bruising.
若血小板不足，凝血功能会受损，导致出血倾向增加和容易瘀伤。
9. Substances Transported by Plasma | 血浆运输的物质
Plasma is the transport medium for many substances around the body. It carries soluble products of digestion from the small intestine to all cells, including glucose, amino acids, vitamins and minerals. It also transports hormones from endocrine glands to target organs.
血浆是全身众多物质的运输介质。它将可溶性消化产物（如葡萄糖、氨基酸、维生素和矿物质）从小肠运送到所有细胞，并将激素从内分泌腺运送至靶器官。
Waste substances are removed via plasma: carbon dioxide is carried in plasma as hydrogencarbonate ions, and urea is transported from the liver to the kidneys for excretion. Plasma also distributes heat, helping regulate body temperature.
代谢废物也通过血浆清除：二氧化碳以碳酸氢根离子的形式在血浆中运输，尿素从肝脏运至肾脏以便排泄。血浆还分配热量，帮助调节体温。
10. Summary Comparison of Blood Components | 血液成分比较总结
The following table summarises the key features of the four main blood components. Understanding these differences is vital for Edexcel IGCSE Biology examinations.
下表总结了四种主要血液成分的关键特征，理解这些差异对 Edexcel IGCSE 生物学考试至关重要。
- Red blood cells – 红细胞: No nucleus; biconcave shape; contain haemoglobin; carry oxygen. 无细胞核；双凹圆盘状；含血红蛋白；运输氧气。
- White blood cells – 白细胞: Have a nucleus; larger; part of immune system; phagocytes engulf pathogens, lymphocytes produce antibodies. 有细胞核；较大；免疫系统部分；吞噬细胞吞噬病原体，淋巴细胞产生抗体。
- Platelets – 血小板: Cell fragments; no nucleus; small; trigger blood clotting. 细胞碎片；无细胞核；体积小；触发凝血。
- Plasma – 血浆: Liquid part; 90% water; transports cells, nutrients, waste, hormones, plasma proteins. 液体部分；90% 水；运输细胞、营养、废物、激素、血浆蛋白。
In addition, plasma proteins such as fibrinogen (clotting), albumin (osmotic balance) and globulins (antibodies) demonstrate the diversity of plasma functions.
此外，血浆蛋白如纤维蛋白原（凝血）、白蛋白（渗透压平衡）和球蛋白（抗体）展现了血浆功能的多样性。
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