📚 Role of Plasma in Transport | 血浆在运输中的作用
Blood is a specialised connective tissue that circulates continuously around the body, delivering essential substances to cells and removing waste products. While the cellular components of blood – red blood cells, white blood cells and platelets – are widely known, the liquid matrix in which they are suspended, blood plasma, plays an equally critical role. Without plasma, the transport of many dissolved substances, from nutrients to hormones and metabolic wastes, would be impossible. This article explores the composition of plasma and its key functions in the transport of carbon dioxide, digested food, urea, hormones and heat energy, as required by Edexcel IGCSE Biology (specification point 2.60).
血液是一种特殊的结缔组织，在全身不断循环，向细胞输送必需物质并带走废弃物。血液中的细胞成分——红细胞、白细胞和血小板——广为人知，但悬浮这些细胞的液体基质——血浆——同样起着至关重要的作用。没有血浆，许多溶解物质（从营养物质到激素和代谢废物）的运输将无法实现。本文根据Edexcel IGCSE生物学考纲（考点2.60）的要求，探讨血浆的成分及其在运输二氧化碳、消化产物、尿素、激素和热能方面的关键功能。
1. What is Blood Plasma? | 什么是血浆？
Blood plasma is the straw-coloured liquid portion of blood that remains after the cellular components have been removed. It makes up approximately 55% of total blood volume. Plasma serves as the transport medium that carries blood cells, platelets, dissolved nutrients, hormones, waste products and gases throughout the circulatory system.
血浆是血液除去细胞成分后剩下的淡黄色液体部分，约占全血容积的55%。血浆作为运输介质，将血细胞、血小板、溶解的营养物质、激素、废弃物和气体通过循环系统输送到全身。
Because plasma is composed mainly of water, it is an excellent solvent. Many substances required by cells, such as glucose and amino acids, are soluble in water and can therefore be easily transported in solution. The fluid nature of plasma also allows it to flow easily through blood vessels, facilitating rapid distribution and collection of materials.
由于血浆的主要成分是水，它是一种优良的溶剂。细胞所需的许多物质（如葡萄糖和氨基酸）可溶于水，因此能够以溶液形式轻松运输。血浆的流动性也使其能够顺利通过血管，便于物质的快速分配和收集。
2. Composition of Plasma Supporting Transport | 支持运输的血浆成分
Plasma is roughly 90–92% water, which provides the fluidity and solvent capacity needed for transport. The remaining 8–10% consists of dissolved substances, including plasma proteins (albumins, globulins and fibrinogen), nutrients, electrolytes, waste products, gases and hormones.
血浆大约90–92%是水，这为运输提供了所需的流动性和溶剂能力。剩余的8–10%由溶解物质组成，包括血浆蛋白（白蛋白、球蛋白和纤维蛋白原）、营养物质、电解质、废弃物、气体和激素。
Albumins help maintain the osmotic pressure of blood, preventing excessive water loss from the circulation and ensuring that blood volume is sustained for efficient transport. Globulins include antibodies that are involved in immune defence, while fibrinogen is essential for blood clotting. These plasma proteins are synthesised mainly by the liver and contribute to the plasma’s ability to transport other molecules and maintain homeostasis.
白蛋白有助于维持血液渗透压，防止水分过多从循环中流失，并确保血容量足以支持有效的运输。球蛋白包括参与免疫防御的抗体，而纤维蛋白原是血液凝固所必需的。这些血浆蛋白主要由肝脏合成，有助于血浆运输其他分子并维持稳态。
3. Transport of Carbon Dioxide | 二氧化碳的运输
Carbon dioxide (CO₂) is constantly produced by respiring cells as a waste product. It diffuses from the cells into the blood plasma and is transported to the lungs for exhalation. Only a small proportion (around 5–10%) of carbon dioxide dissolves directly in the plasma.
二氧化碳（CO₂）作为废物由呼吸细胞不断产生。它从细胞扩散进入血浆，并被运输到肺部排出。只有一小部分（约5–10%）的二氧化碳直接溶解在血浆中。
The majority of carbon dioxide (about 70%) is transported in the plasma in the form of hydrogencarbonate ions (HCO₃⁻). This process begins inside red blood cells, where carbon dioxide reacts with water to form carbonic acid (H₂CO₃), catalysed by the enzyme carbonic anhydrase. The carbonic acid then dissociates to produce hydrogen ions (H⁺) and hydrogencarbonate ions (HCO₃⁻).
大部分二氧化碳（约70%）在血浆中以碳酸氢根离子（HCO₃⁻）的形式运输。这一过程始于红细胞内部，二氧化碳与水反应生成碳酸（H₂CO₃），由碳酸酐酶催化。随后碳酸解离产生氢离子（H⁺）和碳酸氢根离子（HCO₃⁻）。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
The hydrogencarbonate ions diffuse out of the red blood cells into the plasma, and it is in the plasma that they are carried to the lungs. The remaining carbon dioxide (around 20%) binds to haemoglobin inside red blood cells but this does not involve plasma. Thus, plasma is the primary medium for carbon dioxide transport as hydrogencarbonate ions.
碳酸氢根离子从红细胞扩散进入血浆，并由血浆携带至肺部。剩余的二氧化碳（约20%）在红细胞内与血红蛋白结合，但这与血浆无关。因此，血浆是作为碳酸氢根离子运输二氧化碳的主要介质。
4. Transport of Digested Food Products | 消化产物的运输
After the digestion of carbohydrates, proteins and fats in the small intestine, the soluble products – mainly glucose, amino acids, vitamins and minerals – are absorbed into the bloodstream. These water-soluble nutrients dissolve directly in the blood plasma and are transported via the hepatic portal vein to the liver, and then distributed to all body cells.
在小肠中消化碳水化合物、蛋白质和脂肪之后，可溶性产物（主要是葡萄糖、氨基酸、维生素和矿物质）被吸收进入血液。这些水溶性营养物质直接溶解在血浆中，经肝门静脉运输到肝脏，进而分布到全身所有细胞。
Plasma is therefore responsible for delivering the substrates needed for cellular respiration, growth and repair. For example, glucose transported in plasma provides the primary respiratory fuel for most cells, while amino acids are used for protein synthesis. Water-soluble vitamins such as vitamin C and the B-group vitamins are also transported dissolved in plasma.
因此，血浆负责输送细胞呼吸、生长和修复所需的底物。例如，血浆中运输的葡萄糖为大多数细胞提供了主要的呼吸燃料，而氨基酸则用于蛋白质合成。水溶性维生素（如维生素C和B族维生素）也溶解在血浆中运输。
Lipids are absorbed differently: they enter the lymphatic system as chylomicrons and eventually reach the blood, where the plasma carries them associated with lipoproteins. Although the majority of lipid transport occurs within lipoproteins, plasma remains the final transport vehicle for these digested fats.
脂质的吸收途径不同：它们以乳糜微粒的形式进入淋巴系统，最终到达血液，在那里血浆以脂蛋白复合物的形式运输它们。虽然大部分脂质运输是在脂蛋白内进行的，但血浆仍然是这些消化后的脂肪的最终运输工具。
5. Transport of Urea | 尿素的运输
Urea is a nitrogenous waste product formed in the liver from the deamination of excess amino acids. The amino group (–NH₂) is removed and converted into ammonia, which is highly toxic. The liver promptly converts ammonia into the less harmful urea through the ornithine cycle.
尿素是一种含氮废物，由肝脏通过脱氨基作用处理过剩氨基酸而形成。氨基（–NH₂）被移除后转化为毒性较高的氨。肝脏通过鸟氨酸循环迅速将氨转化为危害较小的尿素。
Urea then diffuses from the liver cells into the blood and dissolves in the plasma. It is transported in solution to the kidneys, where it is filtered out of the blood and excreted in urine. The constant removal of urea by plasma transport is essential to prevent toxic accumulation and to maintain a stable internal environment.
随后尿素从肝细胞扩散进入血液并溶解在血浆中。它以溶液形式被运输到肾脏，在那里从血液中滤出并随尿液排出体外。通过血浆运输不断清除尿素，对于防止有毒物质积累和维持内环境稳定至关重要。
6. Transport of Hormones | 激素的运输
Hormones are chemical messengers produced by endocrine glands. They are secreted directly into the bloodstream and travel via the plasma to reach their target organs or cells, which may be distant from the site of secretion. Because hormones are effective in very small concentrations, the rapid distribution provided by plasma is vital for coordinating physiological responses.
激素是由内分泌腺产生的化学信使。它们直接分泌进入血液，并通过血浆运输到达靶器官或靶细胞，这些靶点可能远离分泌部位。由于激素在极低浓度下即可发挥作用，血浆提供的快速分布对于协调生理反应至关重要。
For example, insulin, released by the pancreas, is dissolved in plasma and transported to liver and muscle cells to regulate blood glucose levels. Adrenaline, released from the adrenal glands during stress, is carried in plasma to the heart, blood vessels and liver, triggering the 'fight or flight' response.
例如，胰腺释放的胰岛素溶解在血浆中，并被运输到肝细胞和肌细胞以调节血糖水平。肾上腺在应激状态下释放的肾上腺素由血浆携带至心脏、血管和肝脏，激发“战斗或逃跑”反应。
Without plasma as a transport medium, hormones would not be able to reach their destination quickly enough to maintain homeostasis and coordinate body functions.
如果没有血浆作为运输介质，激素将无法快速到达目的地以维持稳态和协调身体功能。
7. Transport of Heat Energy | 热能的运输
Plasma, being mostly water, has a high specific heat capacity. This means it can absorb, retain and carry a considerable amount of thermal energy with only a small rise in temperature. As blood circulates, plasma helps to redistribute heat generated by metabolic reactions, mainly in the liver and active muscles, to cooler parts of the body.
血浆主要成分是水，因此具有高比热容。这意味着它能够吸收、保留和携带大量热能而本身温度上升很小。随着血液循环，血浆帮助将由新陈代谢（主要在肝脏和活跃肌肉中）产生的热量重新分配到身体较冷的部位。
This function is crucial in thermoregulation. When the body needs to conserve heat, blood flow to the skin is reduced, limiting heat loss. Conversely, when the body needs to lose heat, more blood is directed to the skin surface, allowing plasma to transport heat to the periphery where it can be dissipated. Thus, plasma plays a central role in maintaining a constant core temperature of about 37 °C.
这一功能在体温调节中至关重要。当身体需要保存热量时，皮肤血流量减少，限制热量散失。相反，当需要散热时，更多血液流向皮肤表面，使血浆将热能运输至周边部位散发。因此，血浆在维持约37℃ 的恒定核心体温中发挥核心作用。
8. Plasma and the Removal of Other Waste Products | 血浆与其他废物的清除
In addition to urea, plasma transports other metabolic waste products to the excretory organs. For instance, excess water, mineral ions and metabolic acids are carried by plasma to the kidneys for filtration and elimination. Carbon dioxide, as discussed, is taken to the lungs. This constant clearance of wastes by the plasma prevents the internal environment from becoming toxic and plays a key role in homeostasis.
除了尿素，血浆还将其他代谢废物运输至排泄器官。例如，过剩的水分、矿物质离子和代谢酸由血浆携带至肾脏进行滤过和排出。如前所述，二氧化碳被输送到肺部。血浆不断清除这些废物，防止体内环境变得有毒，并在稳态中发挥关键作用。
9. Distinguishing Plasma Functions from Cellular Blood Components | 血浆功能与血细胞成分的区别
A common mistake in IGCSE Biology is to attribute all transport functions to red blood cells. It is important to recognise which substances are transported by plasma and which are carried by cells.
IGCSE生物学中一个常见错误是将所有运输功能都归于红细胞。重要的是要识别哪些物质由血浆运输，哪些由细胞携带。
Red blood cells specialise in transporting oxygen, bound to the protein haemoglobin. They also contribute to carbon dioxide transport, carrying about 20% bound to haemoglobin. White blood cells are involved in immune responses, and platelets in blood clotting. Neither of these cellular components carries dissolved nutrients, urea, hormones or significant amounts of carbon dioxide.
红细胞专门运输氧气，氧气与血红蛋白结合。它们也参与二氧化碳的运输，约20%的二氧化碳与血红蛋白结合。白细胞参与免疫反应，血小板参与血液凝固。这些细胞成分都不负责运输溶解的营养物质、尿素、激素或大部分二氧化碳。
Plasma, in contrast, handles the bulk transport of carbon dioxide (as hydrogencarbonate ions), all digested food products, urea, hormones and heat. This division of labour ensures that the blood can meet all the body’s transport needs efficiently.
相比之下，血浆负责二氧化碳（以碳酸氢根离子形式）、全部消化产物、尿素、激素和热能的主要运输。这种分工确保了血液能够高效满足身体的所有运输需求。
10. Exam Focus: Specification Point 2.60 | 考试重点：考纲要点2.60
The Edexcel IGCSE specification requires you to "understand the role of plasma in the transport of carbon dioxide, digested food, urea, hormones and heat energy". In an exam, you could be asked to list these five roles, explain how plasma carries out each one, or compare the transport functions of plasma with those of red blood cells.
Edexcel IGCSE考纲要求你“理解血浆在二氧化碳、消化产物、尿素、激素和热能运输中的作用”。在考试中，你可能会被要求列出这五个作用，解释血浆如何执行每项功能，或者比较血浆与红细胞的运输功能。
To score full marks, be precise: for carbon dioxide, state that plasma transports it mainly as hydrogencarbonate ions; for digested food, name glucose and amino acids as key solutes; for urea, link it to deamination in the liver and excretion by the kidneys; for hormones, mention endocrine glands and target organs; for heat, connect it to the high specific heat capacity of water and thermoregulation.
要获得满分，表述必须精准：对于二氧化碳，应说明血浆主要以碳酸氢根离子形式运输它；对于消化产物，应指出葡萄糖和氨基酸是关键的溶质；对于尿素，要将其与肝脏的脱氨基作用和肾脏的排泄联系起来；对于激素，要提及内分泌腺和靶器官；对于热能，要联系水的高比热容和体温调节。
11. Common Misconceptions about Plasma Transport | 关于血浆运输的常见误解
Many students incorrectly believe that plasma transports oxygen. Oxygen is almost entirely transported by red blood cells, bound to haemoglobin. Plasma carries only a negligible amount of dissolved oxygen, and this is not part of the IGCSE specification focus.
许多学生误以为血浆运输氧气。事实上，氧气几乎完全由红细胞中的血红蛋白携带。血浆中仅溶解有极少量的氧气，并且这不是IGCSE考纲的重点。
Another misconception is that all waste products are carried only by red blood cells, or that digested food is transported by white blood cells. Remember: plasma is the liquid transport medium for dissolved substances, while blood cells have specialised, separate functions.
另一个误解是认为所有废物只由红细胞运输，或者消化产物由白细胞运输。记住：血浆是溶解物质的液体运输介质，而血细胞具有专门且不同的功能。
Clarifying these points will prevent you from losing marks on questions that require distinguishing between the roles of blood components.
澄清这些要点能避免你在需要区分血液成分作用的题目中失分。
12. Summary: Plasma as the Body’s Liquid Highway | 总结：血浆——身体的液体高速公路
Blood plasma is far more than just the fluid in which blood cells float. It is the body’s principal medium for transporting a vast array of essential substances. It carries carbon dioxide (as hydrogencarbonate ions) from tissues to the lungs, delivers water-soluble products of digestion from the small intestine to all cells, removes nitrogenous waste in the form of urea, distributes hormones to coordinate body functions, and helps regulate body temperature by moving heat energy. Understanding these roles is fundamental not only for success in IGCSE Biology but also for appreciating how the circulatory system maintains the internal balance necessary for life.
血浆远不止是血细胞漂浮其中的液体。它是身体运输大量必需物质的主要介质。它将二氧化碳（以碳酸氢根离子形式）从组织运送到肺部，将小肠消化的水溶性产物输送到所有细胞，以尿素的形式清除含氮废物，分配激素以协调身体功能，并通过移动热能帮助调节体温。理解这些角色不仅对在IGCSE生物学中取得优异成绩至关重要，也有助于理解循环系统如何维持生命所必需的内部平衡。
Published by TutorHao | Biology Revision Series | aleveler.com
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