📚 Why 270 Million? The Biology of Red Blood Cells | 为何是2.7亿？红细胞的生物学
Every time you breathe in, the oxygen you take up must be carried from your lungs to every cell in your body. That journey depends on a single tiny cell: the red blood cell. In each one of your red blood cells, there are approximately 270 million molecules of haemoglobin. Understanding this number – and the structure that makes it possible – is a core idea in IGCSE Biology.
每一次你吸气，吸入的氧气都必须从肺运送到身体的每个细胞。这段旅程依赖于一个微小细胞：红细胞。在你的每个红细胞中，约有2.7亿个血红蛋白分子。理解这个数字，以及使其成为可能的细胞结构，是IGCSE生物学的核心内容之一。
1. The Blood and Its Components | 血液及其组成
Blood is a tissue made up of red blood cells, white blood cells, platelets and plasma. Red blood cells (erythrocytes) are the most numerous type of blood cell; a cubic millimetre of blood contains about 5 million red cells in a healthy adult. Their primary job is gas transport: carrying oxygen from the lungs to the tissues, and some carbon dioxide from the tissues back to the lungs.
血液是一种组织，由红细胞、白细胞、血小板和血浆组成。红细胞（erythrocytes）是数量最多的血细胞；健康成人的每立方毫米血液中约有500万个红细胞。它们的主要任务是气体运输：将氧气从肺部运至组织，并将部分二氧化碳从组织运回肺部。
The table below summarises the main functions of the blood components.
下表总结了血液各成分的主要功能。
| Component | Function |
| Red blood cells | Transport oxygen; help carry carbon dioxide |
| White blood cells | Defence: phagocytosis and antibody production |
| Platelets | Blood clotting |
| Plasma | Carries cells, nutrients, waste, hormones, heat |
2. The Adapted Structure of Red Blood Cells | 红细胞的结构适应
Red blood cells are highly specialised. Their shape is a biconcave disc – like a squashed doughnut with a dimple on each side. This shape gives a large surface area-to-volume ratio, which speeds up the diffusion of oxygen across the cell membrane. In addition, mature red blood cells have no nucleus and few organelles, leaving more space for haemoglobin. They are also extremely flexible, allowing them to bend and slide through narrow capillaries that are only as wide as their own diameter.
红细胞高度特化。它们的形状是双凹圆盘状——就像一个两面凹陷的压扁甜甜圈。这种形状提供了较大的表面积与体积之比，从而加速氧气穿过细胞膜的扩散。此外，成熟红细胞没有细胞核，细胞器也很少，为血红蛋白留出了更大空间。它们还非常柔韧，能够弯曲并穿过与其直径相当的狭窄毛细血管。
- Biconcave shape / 双凹形 – large surface area for faster diffusion | 大表面积，加快扩散
- No nucleus / 无核 – extra space for haemoglobin | 为血红蛋白提供额外空间
- Flexible membrane / 膜柔韧 – can pass through narrow capillaries | 能通过狭窄的毛细血管
- Haemoglobin / 血红蛋白 – binds oxygen reversibly | 可逆性结合氧气
3. Haemoglobin: The Secret of 270 Million | 血红蛋白：2.7亿的秘密
Haemoglobin is a red pigment protein made of four polypeptide chains, each with an iron-containing haem group. The iron atom can bind to oxygen reversibly. A single red blood cell contains about 270 million haemoglobin molecules. Since each haemoglobin molecule can carry up to four oxygen molecules, one red blood cell can transport more than 1 billion oxygen molecules at a time.
血红蛋白是一种红色色素蛋白，由四条多肽链组成，每条链都有一个含铁的亚铁血红素基团。铁原子能够可逆地结合氧气。一个红细胞中约有2.7亿个血红蛋白分子。由于每个血红蛋白分子最多能携带四个氧分子，因此一个红细胞一次能运输超过10亿个氧分子。
The reversible reaction can be written simply as:
可逆反应可简写为：
Hb + 4O₂ ⇌ Hb(O₂)₄
In the lungs, the high oxygen concentration pushes the equilibrium to the right, forming oxyhaemoglobin. In respiring tissues, where oxygen concentration is low, the equilibrium shifts to the left and oxygen is released.
在肺部，高氧浓度使平衡向右移动，生成氧合血红蛋白。在呼吸组织中，氧浓度低，平衡向左移动，释放氧气。
4. Oxygen Transport and the Dissociation Curve | 氧气运输与解离曲线
Haemoglobin does not simply load and unload oxygen in a linear way. Its affinity for oxygen changes depending on the partial pressure of oxygen (PO₂). This relationship is shown by the oxygen dissociation curve, which has a characteristic S (sigmoid) shape. At high PO₂ (around 13 kPa in the alveoli), haemoglobin is almost fully saturated. At low PO₂ (around 4 kPa in active muscles), it releases a large amount of oxygen.
血红蛋白并不是以线性方式装载和卸载氧气。它对氧气的亲和力随氧分压（PO₂）而变化。这种关系用氧解离曲线表示，呈典型的S形（乙状）曲线。在PO₂高时（肺泡中约13 kPa），血红蛋白几乎完全饱和。在PO₂低时（活跃肌肉中约4 kPa），它会释放大量氧气。
Carbon dioxide can influence this curve. The Bohr effect states that an increase in CO₂ concentration lowers the pH of the blood, reducing haemoglobin’s affinity for oxygen. This encourages haemoglobin to release more oxygen where respiration is active.
二氧化碳会影响这条曲线。玻尔效应表明，CO₂浓度升高会降低血液pH，使血红蛋白对氧的亲和力下降。这促使血红蛋白在呼吸活跃的组织中释放更多氧气。
Simple saturation values at a standard body temperature (37 °C):
标准体温（37 °C）下的简单饱和度值：
| Oxygen partial pressure (kPa) | Haemoglobin saturation (%) |
| 2 | 35 |
| 4 | 75 |
| 6 | 90 |
| 10 | 97 |
| 13 | 99 |
5. Carbon Dioxide Transport: Beyond Oxygen | 二氧化碳运输：超越氧气
Red blood cells also help to remove carbon dioxide. In the tissues, CO₂ diffuses into the red blood cells, where the enzyme carbonic anhydrase catalyses its reaction with water:
红细胞也帮助清除二氧化碳。在组织中，CO₂扩散进入红细胞，在碳酸酐酶的催化下与水反应：
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
The hydrogen carbonate ions (HCO₃⁻) diffuse out of the red blood cells into the plasma, where they are carried to the lungs. To maintain electrical balance, chloride ions diffuse into the red blood cells (the chloride shift). Meanwhile, some CO₂ combines directly with the amino groups of haemoglobin to form carbaminohaemoglobin. About 70% of carbon dioxide is transported as HCO₃⁻, about 20% bound to haemoglobin, and a small amount dissolved in plasma.
碳酸氢根离子（HCO₃⁻）扩散出红细胞进入血浆，并被携带到肺部。为维持电平衡，氯离子扩散进入红细胞（氯离子转移）。与此同时，部分CO₂直接与血红蛋白上的氨基结合，形成氨甲酰血红蛋白。约70%的二氧化碳以HCO₃⁻形式运输，约20%与血红蛋白结合，少量溶解在血浆中。
6. The Life Cycle of Red Blood Cells | 红细胞的寿命周期
Red blood cells are produced in the red bone marrow found in the ribs, sternum, vertebrae and pelvis. Each second, millions of new red blood cells are released into the bloodstream. A typical red blood cell lives for about 120 days. As they age, their membranes begin to stiffen and the cells become less flexible.
红细胞产生于肋骨、胸骨、椎骨和骨盆中的红骨髓。每秒都有数百万个新红细胞被释放到血流中。典型红细胞的寿命约为120天。随着衰老，它们的膜变硬，细胞变得不那么柔韧。
Worn-out red blood cells are broken down by macrophages in the spleen and liver. The iron from haemoglobin is recycled to the bone marrow, while the remaining parts are converted into bilirubin, a yellow-green pigment that gives bile its colour and faeces its brown colour.
衰老的红细胞被脾脏和肝脏中的巨噬细胞分解。血红蛋白中的铁被回收至骨髓，其余部分转化为胆红素，这是一种黄绿色色素，使胆汁呈黄色，粪便呈棕色。
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