📚 Blood Plasma and Tissue Fluid | 血浆与组织液
Blood plasma and tissue fluid are two closely related extracellular fluids essential for transport, exchange and homeostasis in the human body. Plasma is the liquid matrix of blood, carrying cells, nutrients, and wastes, while tissue fluid (interstitial fluid) bathes cells, allowing direct exchange of substances across capillary walls. This article explains their composition, formation, and the physiological mechanisms that maintain fluid balance.
血浆与组织液是两种密切相关的人体细胞外液，在运输、物质交换和稳态维持中不可或缺。血浆是血液的液态基质，运载血细胞、营养及代谢废物；而组织液（间质液）则浸润细胞，通过毛细血管壁与血液进行物质交换。本文将阐述它们的组成、组织液的形成以及维持体液平衡的生理机制。
1. Blood Plasma: An Overview | 血浆：概述
Blood plasma is the pale yellow liquid component of blood that remains after removing all blood cells and platelets. It makes up approximately 55% of total blood volume and serves as the principal transport medium for cells, nutrients, hormones and metabolic wastes.
血浆是血液去除所有血细胞和血小板后残留的淡黄色液体组分。它约占全血容量的55%，是运载血细胞、营养、激素和代谢废物的主要介质。
Plasma is mainly water (90–92%), but it contains a rich mixture of dissolved solutes: plasma proteins, electrolytes, nutrients, respiratory gases, and waste products. Its composition is carefully regulated to maintain osmotic balance, pH and the distribution of body fluids.
血浆的主要成分是水（90–92%），但其中溶解了丰富的溶质：血浆蛋白、电解质、营养物质、呼吸气体和代谢废物。血浆成分受到精密调控，以维持渗透平衡、pH 和体液的分布。
2. Composition of Blood Plasma | 血浆的成分
The table below summarises the major constituents of human plasma and their approximate proportions.
下表概括人血浆的主要成分及其大致比例。
| Component | Approx. proportion | Key examples / role |
|---|---|---|
| Water | 90–92% | Universal solvent; temperature buffer |
| Plasma proteins | 7–8% | Albumin, globulins (α, β, γ), fibrinogen |
| Electrolytes | ~1% | Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, HCO₃⁻ |
| Nutrients | Variable | Glucose, amino acids, lipids, vitamins |
| Respiratory gases | Dissolved | O₂ (small amount), CO₂ (mainly as HCO₃⁻) |
| Waste products | Trace | Urea, creatinine, uric acid |
Albumin is the most abundant plasma protein (~55–60% of total plasma protein) and is largely responsible for the colloid osmotic (oncotic) pressure that draws water into the blood. Globulins include transport proteins and immunoglobulins (antibodies). Fibrinogen is a soluble precursor of fibrin, essential for blood clotting.
白蛋白是最丰富的血浆蛋白（约占血浆总蛋白的55–60%），主要负责维持将水分拉入血液的胶体渗透压。球蛋白包括运输蛋白和免疫球蛋白（抗体）。纤维蛋白原是纤维蛋白的可溶性前体，对凝血至关重要。
3. Key Functions of Blood Plasma | 血浆的主要功能
Plasma performs multiple vital roles beyond simple transport. It maintains blood pressure and volume because the plasma proteins, particularly albumin, exert oncotic pressure that counteracts the hydrostatic pressure pushing fluid out of the capillaries. Plasma also buffers pH (HCO₃⁻ and proteins), distributes heat, and transports hormones and clotting factors.
血浆除了简单的运输外还承担多种生命功能。它维持血压和血容量，因为血浆蛋白（尤其是白蛋白）产生的胶体渗透压可对抗将液体推出毛细血管的静水压。血浆还缓冲pH（HCO₃⁻ 和蛋白质）、分布热量，并运输激素与凝血因子。
Immunoglobulins circulating in plasma provide humoral immunity, while fibrinogen and other clotting factors enable haemostasis. The electrolyte composition of plasma closely mirrors that of interstitial fluid, which is essential for the normal function of excitable cells.
血浆中循环的免疫球蛋白提供体液免疫，而纤维蛋白原和其他凝血因子则实现止血。血浆的电解质组成与组织间液高度相似，这对可兴奋细胞的正常功能至关重要。
4. Capillaries: The Exchange Vessels | 毛细血管：交换血管
Capillaries are the smallest blood vessels, with walls consisting of a single layer of endothelial cells on a basement membrane. This thin barrier (typically 0.5–1 μm thick) permits rapid diffusion and filtration of water, gases, and small solutes between plasma and tissue fluid. Narrow intercellular clefts and, in some capillaries, fenestrations allow the passage of water and small solutes but restrict large plasma proteins.
毛细血管是最小的血管，管壁由单层内皮细胞及其基底膜构成。这层极薄的屏障（通常厚0.5–1 μm）允许水、气体和小分子溶质在血浆与组织液之间快速扩散与滤过。狭窄的细胞间隙以及某些毛细血管上的窗孔可让水和小溶质通过，但限制大分子血浆蛋白。
The total surface area of the capillary bed is enormous (estimated at ~600–1000 m² in an adult), maximising exchange efficiency. The balance of hydrostatic and oncotic pressures across the capillary wall determines the direction and rate of fluid movement – a concept central to the formation of tissue fluid.
毛细血管床的总表面积非常大（成人估计约600–1000 m²），最大限度地提高了交换效率。跨毛细血管壁的静水压和渗透压的平衡决定了液体流动的方向和速率——这是组织液形成的核心概念。
5. What Is Tissue Fluid? | 什么是组织液？
Tissue fluid, also called interstitial fluid, is the fluid that fills the spaces between cells. It is derived from blood plasma by filtration through the capillary wall and has a composition similar to plasma but with far less protein. This fluid provides a constant environment for cells, delivering oxygen and nutrients and receiving carbon dioxide and metabolic wastes.
组织液又称间质液，是填充细胞间隙的液体。它由血浆经毛细血管壁滤过而来，组成与血浆相似但蛋白含量低得多。该液体为细胞提供稳定的环境，输送氧气和营养，并接收二氧化碳和代谢废物。
About 20 litres of fluid filters out of the capillaries each day, yet most is immediately reabsorbed. The small excess (~3 litres per day) is returned to the circulation via the lymphatic system. This continuous circulation of tissue fluid ensures that no cell is more than a few micrometres away from a fresh supply of nutrients.
每天约有20升液体从毛细血管滤出，但大部分立即被重吸收。少量多余液体（每天约3升）通过淋巴系统回到血液。组织液的持续循环确保每个细胞距离新鲜营养供给不超过几微米。
6. Ultrafiltration: Formation of Tissue Fluid | 超滤：组织液的形成
At the arteriolar end of a capillary, the blood hydrostatic pressure (Pc) is relatively high – about 35 mmHg at the beginning of a typical systemic capillary, falling to around 25 mmHg as blood moves through. This pressure forces water and small solutes (ions, glucose, amino acids, urea) out through the capillary wall, leaving behind most plasma proteins and blood cells. This process is called ultrafiltration.
在毛细血管动脉端，血液静水压（Pc）相对较高——在典型的体循环毛细血管起始处约35 mmHg，随血液流动降至约25 mmHg。该压力将水和小溶质（离子、葡萄糖、氨基酸、尿素）推出毛细血管壁，绝大多数血浆蛋白和血细胞则被截留。这一过程称为超滤。
The filtrate that escapes forms tissue fluid. The endothelial barrier acts as a molecular sieve: small molecules pass freely, but proteins larger than about 70,000 Da (such as albumin) are largely retained. Consequently, tissue fluid contains only a tiny fraction of the protein found in plasma.
滤出的液体形成组织液。内皮屏障起到分子筛的作用：小分子自由通过，但大于约70000 Da的蛋白质（如白蛋白）基本被截留。因此，组织液所含蛋白质仅为血浆中的极小一部分。
7. Starling Forces in Detail | 详解斯塔林力
The net movement of fluid across the capillary wall is governed by the balance of four Starling forces: capillary hydrostatic pressure (Pc), interstitial hydrostatic pressure (Pi), plasma oncotic pressure (πc), and interstitial oncotic pressure (πi). The combined effect can be expressed as:
跨毛细血管壁的液体净移动由四种斯塔林力的平衡决定：毛细血管静水压（Pc）、组织液静水压（Pi）、血浆胶体渗透压（πc）和组织液胶体渗透压（πi）。其综合效果可表达为：
Net filtration pressure = (Pc − Pi) − (πc − πi)
Hydrostatic pressure is the physical pressure exerted by a fluid. In the capillary, Pc pushes fluid outward, while Pi is usually slightly negative (∼ −1 to −3 mmHg), which also favours outward flow. Oncotic pressure is the osmotic pressure generated by plasma proteins (mainly albumin); πc (~25–28 mmHg) pulls water into the capillary, whereas πi (~0.1–5 mmHg) is negligible because tissue fluid has very little protein.
静水压是液体施加的物理压力。在毛细血管中，Pc将液体向外推出，而Pi通常略为负值（约−1至−3 mmHg），也倾向于向外流动。胶体渗透压是由血浆蛋白（主要是白蛋白）产生的渗透压；πc（约25–28 mmHg）将水拉入毛细血管，而πi（约0.1–5 mmHg）可忽略不计，因为组织液蛋白极少。
Net filtration pressure ≈ (35 − 0) − (26 − 1) = 35 − 25 = +10 mmHg, favouring filtration.
在动脉端：Pc ≈ 35 mmHg，Pi ≈ 0 mmHg，πc ≈ 26 mmHg，πi ≈ 1 mmHg。净滤过压≈ (35 − 0) − (26 − 1) = 35 − 25 = +10 mmHg，促进滤出。
At the venular end: Pc drops to ~ et filtration pressure ≈ (15 − 0) − (26 − 1) = 15 − 25 = −10 mmHg, favouring reabsorption.
在静脉端：Pc降至约15 mmHg，而胶体渗透压几乎不变。净滤过压≈ (15 − 0) − (26 − 1) = 15 − 25 = −10 mmHg，促进重吸收。
8. Reabsorption of Tissue Fluid | 组织液的重吸收
As blood flows towards the venous end, the loss of fluid concentrates plasma proteins, so πc may actually rise slightly. The falling capillary hydrostatic pressure reverses the pressure gradient, and the oncotic pull of plasma proteins draws water, along with dissolved wastes and CO₂, back into the capillary. Approximately 85–90% of the filtered fluid is reabsorbed in this way.
随着血液流向静脉端，液体丢失使血浆蛋白浓缩，πc 实际可轻微上升。下降的毛细血管静水压逆转了压力梯度，血浆蛋白的胶体渗透拉力将水连同溶解的废物和CO₂拉回毛细血管。大约85–90%的滤出液以此方式被重吸收。
Reabsorption is critical for maintaining blood volume and for clearing metabolic wastes from the interstitial space. Any imbalance between filtration and reabsorption can lead to accumulation of tissue fluid, a condition known as oedema.
重吸收对于维持血容量和清除组织间隙中的代谢废物至关重要。滤出与重吸收之间的任何失衡都可导致组织液积聚，即水肿。
9. The Lymphatic System and Excess Fluid | 淋巴系统与多余液体
The daily net filtration usually exceeds reabsorption by a small amount – roughly 2–4 litres. This surplus tissue fluid is picked up by blind-ended lymphatic capillaries that permeate almost all tissues. Lymphatic capillaries are highly permeable, with overlapping endothelial cells that act as one-way valves, allowing fluid and large molecules (including escaped proteins) to enter but not escape.
每天净滤出通常比净重吸收略多——约2至4升。这些多余的组织液被遍布几乎所有组织的盲端淋巴毛细管收集。淋巴毛细管通透性极高，其内皮细胞重叠形成单向活瓣，允许液体和大分子（包括渗出的蛋白质）进入但无法逸出。
Once inside the lymphatic vessels, the fluid is called lymph. It is transported through lymph nodes (where pathogens and debris are filtered) and eventually returned to the blood via the thoracic duct and right lymphatic duct, which empty into the subclavian veins. This recovery pathway prevents the accumulation of fluid and protein in the tissues, maintaining interstitial homeostasis.
进入淋巴管后，该液体称为淋巴。淋巴流经淋巴结（病原体和碎片在此被过滤），最终通过胸导管和右淋巴导管注入锁骨下静脉，回到血流。这一回收途径防止液体和蛋白质在组织中积聚，维持组织间隙的稳态。
10. Differences Between Plasma and Tissue Fluid | 血浆与组织液的差异
The table below highlights the compositional differences between blood plasma and tissue fluid.
下表突出显示了血浆与组织液的成分差异。
| Feature | Blood Plasma | Tissue Fluid |
|---|---|---|
| Protein content | High (~7–9 g/100 mL); albumin, globulins, fibrinogen | Very low (~0.1–0.5 g/100 mL); mainly small peptides and a few escaped albumin molecules |
| Cellular elements | Erythrocytes, leucocytes, platelets (suspended) | Essentially acellular; occasional migrating leucocytes |
| Clotting factors | Present (fibrinogen, etc.) | Absent or negligible; tissue fluid does not clot |
| Dissolved gases | O₂, CO₂ transported in plasma and cells | Lower O₂, higher CO₂ near active cells |
| Location | Inside blood vessels | Interstitial spaces, lymph capillaries |
The most functionally significant difference is the protein concentration. The high protein content of plasma generates strong oncotic pressure, which is the primary force drawing fluid back into the capillary at the venous end. The virtual absence of proteins in tissue fluid means that its oncotic pressure is negligible, making hydrostatic pressure the dominant outward force.
功能上最显著的差异在于蛋白质浓度。血浆的高蛋白含量产生强大的胶体渗透压，这是静脉端将液体拉回毛细血管的主要力量。组织液中几乎不含蛋白质，意味着其胶体渗透压可忽略，使得静水压成为主要外向动力。
11. Plasma Proteins and Oncotic Pressure | 血浆蛋白与胶体渗透压
Albumin accounts for about 75–80% of the plasma oncotic pressure because it is the most abundant plasma protein and has a relatively low molecular weight (66.5 kDa), contributing more molecules per gram
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