📚 The Composition and Interrelationship of Plasma and Tissue Fluid | 血浆与组织液的成分及关系
Blood is a specialized connective tissue composed of formed elements (red blood cells, white blood cells, platelets) suspended in a liquid matrix called plasma. Tissue fluid, also known as interstitial fluid, is the fluid that directly bathes cells. Understanding the precise composition of plasma and tissue fluid, and how they are dynamically linked through capillary exchange, is a fundamental concept in CIE A-Level Biology.
血液是一种特化的结缔组织，由悬浮在称为血浆的液体基质中的有形成分（红细胞、白细胞、血小板）组成。组织液，又称细胞间液，是直接浸润细胞的液体。理解血浆和组织液的精确成分，以及它们如何通过毛细血管交换动态地联系起来，是 CIE A-Level 生物学中的一个基础概念。
1. Composition of Plasma | 血浆的组成
Plasma is the straw-colored liquid component of blood, making up about 55% of total blood volume. It acts as a transport medium, carrying essential substances and aiding in the maintenance of homeostasis.
血浆是血液中呈淡黄色的液体成分，约占血液总体积的 55%。它充当运输介质，携带必需物质并有助于维持稳态。
Key constituents include water (approximately 90-92%), plasma proteins (e.g., albumin, globulins, fibrinogen), inorganic ions (e.g., Na⁺, Cl⁻, Ca²⁺, HCO₃⁻), and a variety of dissolved organic molecules such as glucose, amino acids, and urea.
主要成分包括水（约 90-92%）、血浆蛋白（例如白蛋白、球蛋白、纤维蛋白原）、无机离子（例如 Na⁺、Cl⁻、Ca²⁺、HCO₃⁻），以及多种溶解的有机分子，如葡萄糖、氨基酸和尿素。
2. Composition of Tissue Fluid | 组织液的组成
Tissue fluid is the fluid that surrounds the cells of tissues, forming an intermediate medium between the blood and the cells. Its composition is largely similar to that of plasma, but with a critical difference.
组织液是包围组织细胞的液体，在血液和细胞之间形成中间介质。其成分在很大程度上与血浆相似，但有一个关键的区别。
Because tissue fluid is an ultrafiltrate of plasma, it contains water, dissolved ions, glucose, amino acids, and waste products like carbon dioxide. However, it contains significantly less protein than plasma, as large plasma proteins such as albumin and fibrinogen cannot easily pass through the capillary walls.
由于组织液是血浆的超滤液，它含有水、溶解的离子、葡萄糖、氨基酸以及二氧化碳等废物。然而，它所含的蛋白质明显少于血浆，因为白蛋白和纤维蛋白原等大分子血浆蛋白不易穿过毛细血管壁。
3. The Relationship Between Plasma and Tissue Fluid | 血浆与组织液的关系
The relationship between plasma and tissue fluid is dynamic and bidirectional, primarily occurring across the walls of capillaries. The capillary wall acts as a semi-permeable barrier, allowing water and small solutes to pass while restricting the passage of large plasma proteins and blood cells.
血浆与组织液的关系是动态且双向的，主要发生在毛细血管壁两侧。毛细血管壁充当半透膜屏障，允许水和小的溶质通过，同时限制大的血浆蛋白和血细胞通过。
This exchange is driven by two opposing forces: hydrostatic pressure (pushing fluid out) and oncotic pressure (pulling fluid in). At the arterial end of a capillary, net filtration occurs, forming tissue fluid. At the venous end, net reabsorption occurs, returning most fluid to the plasma.
这种交换由两种相反的力驱动：流体静压（将液体推出）和胶体渗透压（将液体吸入）。在毛细血管的动脉端，发生净滤过，形成组织液。在静脉端，发生净重吸收，将大部分液体返回血浆。
4. Net Filtration and Net Reabsorption | 净滤过与净重吸收
At the arterial end, the capillary hydrostatic pressure (pushing fluid outwards) is approximately 4.3 kPa. This pressure pushes fluid and small solutes out through the gaps between endothelial cells.
在动脉端，毛细血管流体静压（推动液体向外）约为 4.3 kPa。这种压力将液体和小溶质通过内皮细胞之间的间隙推出。
The plasma oncotic pressure (pulling water inwards) is approximately 3.3 kPa. This is due to the presence of dissolved plasma proteins. Since tissue fluid has very little protein, its oncotic pressure is negligible.
血浆胶体渗透压（吸引水分向内）约为 3.3 kPa。这是由溶解的血浆蛋白引起的。由于组织液的蛋白质含量极低，其胶体渗透压可忽略不计。
Therefore, the net pressure driving filtration at the arterial end is:
因此，推动动脉端滤过的净压强为：
Net pressure (arterial end) = 4.3 kPa – 3.3 kPa = +1.0 kPa
净压强（动脉端）= 4.3 kPa – 3.3 kPa = +1.0 kPa
At the venous end, the hydrostatic pressure falls to about 1.3 kPa due to resistance encountered in the narrow capillaries, whereas oncotic pressure remains at 3.3 kPa.
在静脉端，由于在狭窄的毛细血管中遇到阻力，流体静压降至约 1.3 kPa，而胶体渗透压保持在 3.3 kPa。
The net pressure favoring reabsorption is now:
现在有利于重吸收的净压强为：
Net pressure (venous end) = 1.3 kPa – 3.3 kPa = -2.0 kPa
净压强（静脉端）= 1.3 kPa – 3.3 kPa = -2.0 kPa
5. The Role of Oncotic Pressure | 胶体渗透压的作用
The oncotic pressure, also called colloid osmotic pressure, is specifically created by plasma proteins that are too large to cross the capillary wall. These proteins retain water within the blood vessels by exerting an osmotic pulling force.
胶体渗透压，也叫胶体渗透压，是由太大而无法穿过毛细血管壁的血浆蛋白产生的。这些蛋白质通过施加渗透性吸引力，将水保留在血管内。
If the balance between hydrostatic and oncotic pressure is disturbed, edema (excess accumulation of fluid in tissues) can occur. For instance, if diet lacks protein (low plasma protein concentration), oncotic pressure decreases, reducing reabsorption and causing fluid to accumulate in tissues.
如果流体静压和胶体渗透压之间的平衡受到干扰，就会发生水肿（组织内液体过多积聚）。例如，如果饮食缺乏蛋白质（血浆蛋白浓度低），胶体渗透压降低，重吸收减少，导致液体在组织中积聚。
6. The Role of the Lymphatic System | 淋巴系统的作用
Despite the elegant balance of pressures, not all tissue fluid is reabsorbed directly into the capillaries. Approximately 10% of the tissue fluid enters the lymphatic capillaries, forming lymph.
尽管压力达到平衡，但并非所有组织液都被直接重吸收回毛细血管。大约 10% 的组织液进入淋巴毛细管，形成淋巴。
The lymphatic system acts as a drainage system, returning this fluid, along with leaked plasma proteins and cellular debris, back into the bloodstream at the subclavian veins. This prevents tissue fluid from accumulating and maintains plasma volume.
淋巴系统充当引流系统，将此液体连同渗漏的血浆蛋白和细胞碎片，通过锁骨下静脉送回血液。这可以防止组织液积聚并维持血浆容量。
7. Comparative Table: Plasma vs Tissue Fluid | 比较表格：血浆与组织液
The following table summarizes the key differences and similarities between plasma and tissue fluid, which is a highly tested area in CIE exams.
下表总结了血浆和组织液之间的关键异同，这是 CIE 考试中高频考查的区域。
| Feature | Plasma | Tissue Fluid |
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