📚 Transport in Animals: Circulatory Systems, Heart and Exchange | 动物体内运输：循环系统、心脏与物质交换
All living cells need a constant supply of oxygen and nutrients, and they must remove carbon dioxide and other metabolic wastes. In small or thin organisms, diffusion alone is enough because no cell is far from the external environment. However, larger animals need a specialised transport system to move substances rapidly over long distances. This article covers the key ideas in Topic 3.2 Transport in Animals, including circulatory systems, blood vessels, the heart, blood components, gas transport and tissue fluid.
所有活细胞都需要持续获得氧气和营养物质，同时必须排出二氧化碳和其他代谢废物。对于小型或扁平动物，仅靠扩散就足够了，因为没有细胞离外界环境太远。然而，较大的动物需要专门的运输系统来快速长距离运送物质。本文涵盖生物 3.2 动物体内运输的核心内容，包括循环系统、血管、心脏、血液成分、气体运输和组织液。
1. Why Multicellular Animals Need Transport Systems | 为什么多细胞动物需要运输系统
Small organisms such as Amoeba can exchange gases and waste products directly across their body surface because their surface area to volume ratio is large and diffusion distances are very short.
像变形虫这样的小型生物可以直接通过体表交换气体和废物，因为它们的表面积与体积之比大，扩散距离非常短。
In larger multicellular animals, the surface area to volume ratio decreases and most cells are too far from the external environment for diffusion to supply oxygen and remove carbon dioxide quickly enough.
在较大的多细胞动物中，表面积与体积之比下降，大多数细胞离外部环境太远，仅靠扩散无法足够快地供应氧气和排出二氧化碳。
Therefore, animals need a mass transport system containing a pump, vessels and a transport medium to move substances efficiently between exchange surfaces and body cells.
因此，动物需要一个包含泵、管道和运输介质的大规模运输系统，在交换表面和体细胞之间高效地运输物质。
2. Open and Closed Circulatory Systems | 开放与封闭循环系统
In an open circulatory system, blood is pumped into open body cavities called haemocoels, where it comes into direct contact with tissues before returning slowly to the heart. This system is found in insects and many molluscs.
在开放循环系统中，血液被泵入称为血腔的开放体腔，在体腔中与组织直接接触，然后缓慢流回心脏。昆虫和许多软体动物具有这种系统。
An open system is less efficient for delivering oxygen at high speed because blood pressure is low and flow is slow. However, it works well for small animals with lower metabolic demands.
开放系统在高速输送氧气方面效率较低，因为血压低、流速慢。但对于代谢需求较低的小型动物来说，它已经足够。
In a closed circulatory system, blood remains within vessels and is kept separate from tissue fluid. Exchange occurs across capillary walls. This allows higher pressure, faster flow and more precise delivery of blood to specific organs, as seen in mammals, fish and birds.
在封闭循环系统中，血液始终在血管内流动，与组织液分离。物质交换通过毛细血管壁进行。这样能产生更高的压力、更快的流速，并将血液更精确地输送到特定器官，如哺乳动物、鱼类和鸟类。
3. Single and Double Circulation | 单循环与双循环
Fish have a single circulation. The heart pumps deoxygenated blood to the gills, where it is oxygenated, and then the blood travels to the rest of the body before returning to the heart.
鱼类具有单循环。心脏将缺氧血泵入鳃，在鳃处获得氧气，然后血液流向全身各处，最后回到心脏。
One disadvantage of single circulation is that blood pressure drops significantly after passing through the gill capillaries, so oxygen delivery to body tissues is relatively slow and limits metabolic rate.
单循环的一个缺点是血液经过鳃毛细血管后压力显著下降，因此向身体组织输送氧气的速度相对较慢，限制了代谢率。
Mammals have a double circulation. The right side of the heart sends blood to the lungs through the pulmonary circulation, and the left side sends oxygenated blood to the rest of the body through the systemic circulation. This maintains high blood pressure and allows efficient oxygen delivery.
哺乳动物具有双循环。心脏右侧通过肺循环将血液送往肺部，左侧通过体循环将含氧血送往全身。双循环能维持较高的血压，并实现高效的氧气输送。
4. Blood Vessels: Arteries, Capillaries and Veins | 血管：动脉、毛细血管和静脉
Arteries carry blood away from the heart. They have thick muscular walls, abundant elastic tissue and a relatively narrow lumen to withstand and maintain the high pressure generated by the heart.
动脉将血液从心脏运出。它们有较厚的肌肉壁、丰富的弹性组织和相对较窄的管腔，以承受并维持心脏产生的高压。
Arterioles are smaller branches of arteries that control blood flow into capillary beds by vasoconstriction and vasodilation, helping to regulate blood pressure and distribution.
小动脉是动脉的较小分支，通过血管收缩和血管舒张控制进入毛细血管床的血流量，有助于调节血压和血液分配。
Capillaries are the site of exchange between blood and tissues. Their walls are only one endothelial cell thick, giving a very short diffusion distance, and their narrow lumen brings red blood cells close to tissue cells.
毛细血管是血液与组织之间进行物质交换的场所。其管壁仅由一层内皮细胞组成，扩散距离极短；狭窄的管腔使红细胞靠近组织细胞。
Venules and veins return blood to the heart. Veins have thinner muscular walls, a wide lumen and valves that prevent backflow of low-pressure blood. The surrounding skeletal muscles also help squeeze blood towards the heart.
小静脉和静脉将血液送回心脏。静脉的肌肉壁较薄、管腔较宽，并具有瓣膜防止低压血液倒流。周围的骨骼肌也有助于将血液挤向心脏。
| Vessel 血管 | Structure 结构 | Function 功能 |
|---|---|---|
| Artery 动脉 | Thick muscle and elastic wall, narrow lumen | Carries blood away from heart at high pressure |
| Capillary 毛细血管 | One-cell-thick wall, very narrow lumen | Allows exchange of gases, nutrients and wastes |
| Vein 静脉 | Thin wall, wide lumen, valves | Returns blood to heart at low pressure |
5. The Mammalian Heart and Cardiac Cycle | 哺乳动物心脏与心动周期
The mammalian heart has four chambers: left and right atria receive blood, and left and right ventricles pump blood out. The left ventricle has a much thicker muscular wall than the right ventricle because it must generate higher pressure to pump blood around the whole body.
哺乳动物心脏有四个腔室：左、右心房接收血液，左、右心室将血液泵出。左心室的肌肉壁比右心室厚得多，因为它必须产生更高的压力，将血液泵送到全身。
Atrioventricular valves separate the atria from the ventricles. The tricuspid valve is on the right side and the bicuspid or mitral valve is on the left side. Semilunar valves are found at the base of the aorta and pulmonary artery. These valves prevent backflow of blood.
房室瓣将心房与心室分开。右侧为三尖瓣，左侧为二尖瓣。主动脉和肺动脉基部有半月瓣。这些瓣膜可防止血液倒流。
During the cardiac cycle, atrial systole pushes blood into the ventricles. Ventricular systole then forces blood into the aorta and pulmonary artery while the atrioventricular valves close. Diastole is the relaxation phase when the heart refills with blood.
在心动周期中，心房收缩将血液推入心室。随后心室收缩将血液压入主动脉和肺动脉，同时房室瓣关闭。舒张期是心脏放松并重新充盈血液的阶段。
Cardiac output = Stroke volume × Heart rate
心输出量 = 每搏输出量 × 心率
6. Control of the Heartbeat | 心跳的控制
The sinoatrial node (SAN) in the wall of the right atrium acts as the pacemaker. It initiates an electrical impulse that spreads across both atria, causing atrial contraction.
位于右心房壁的窦房结（SAN）起起搏器作用。它发出电冲动，传导至两侧心房，引起心房收缩。
The impulse reaches the atrioventricular node (AVN), where it is delayed briefly. This delay allows the ventricles to fill completely before they contract. The impulse then travels down the bundle of His and Purkyne fibres, causing ventricular systole from the apex upwards.
冲动到达房室结（AVN）后会短暂延迟。这个延迟使心室在收缩前完全充盈。随后冲动沿希氏束和浦肯野纤维向下传导，从心尖向上引起心室收缩。
An electrocardiogram (ECG) records these electrical changes. The P wave represents atrial contraction, the QRS complex represents ventricular contraction, and the T wave represents ventricular recovery.
心电图（ECG）记录这些电变化。P 波代表心房收缩，QRS 波群代表心室收缩，T 波代表心室恢复。
7. Blood Components and Haemoglobin | 血液成分与血红蛋白
Blood consists of plasma, red blood cells, white blood cells and platelets. Plasma transports glucose, amino acids, hormones, urea, carbon dioxide and ions such as sodium and chloride.
血液由血浆、红细胞、白细胞和血小板组成。血浆运输葡萄糖、氨基酸、激素、尿素、二氧化碳以及钠离子和氯离子等。
Red blood cells are biconcave discs with no nucleus in mammals. This shape increases the surface area for oxygen uptake, allows rapid diffusion of oxygen, and makes the cells flexible enough to pass through narrow capillaries.
哺乳动物的红细胞为双凹圆盘状，无细胞核。这种形状增加了吸收氧气的表面积，有利于氧气的快速扩散，并使细胞具有足够柔性以通过狭窄的毛细血管。
Haemoglobin is a quaternary protein with four polypeptide chains, each containing a haem group with an iron ion. Each haemoglobin molecule can bind up to four oxygen molecules cooperatively.
血红蛋白是一种具有四条多肽链的四级结构蛋白，每条链含有一个带铁离子的血红素基。每个血红蛋白分子最多可协同结合四个氧分子。
8. Oxygen Dissociation Curves and the Bohr Effect | 氧解离曲线与玻尔效应
The oxygen dissociation curve for adult haemoglobin is S-shaped. Binding of the first oxygen molecule changes the shape of haemoglobin, making it easier for further oxygen molecules to bind. This is called cooperative binding.
成人血红蛋白的氧解离曲线呈 S 形。第一个氧分子的结合改变了血红蛋白的形状，使后续氧分子更容易结合。这称为协同结合。
At high partial pressures of oxygen in the lungs, haemoglobin becomes almost fully saturated with oxygen. At lower partial pressures in respiring tissues, oxygen is released from haemoglobin.
在肺部氧分压较高时，血红蛋白几乎被氧完全饱和。在呼吸组织的氧分压较低时，氧气从血红蛋白中释放出来。
The Bohr effect states that increased carbon dioxide concentration and increased acidity reduce haemoglobin’s affinity for oxygen, shifting the curve to the right. This promotes oxygen release in actively respiring tissues.
玻尔效应表明，二氧化碳浓度升高和酸性增加会降低血红蛋白对氧的亲和力，使曲线右移。这促进氧气在活跃呼吸的组织中释放。
9. Carbon Dioxide Transport | 二氧化碳的运输
Carbon dioxide is transported in three ways: about 5% dissolves directly in plasma, about 10% binds to haemoglobin as carbaminohaemoglobin, and about 85% is carried as hydrogen carbonate ions in the plasma.
二氧化碳以三种方式运输：约 5% 直接溶解在血浆中，约 10% 以氨基甲酰血红蛋白形式与血红蛋白结合，约 85% 以碳酸氢根离子形式在血浆中运输。
Inside red blood cells, carbon dioxide reacts with water under the enzyme carbonic anhydrase to form carbonic acid. This dissociates into hydrogen ions and hydrogen carbonate ions. The hydrogen carbonate ions diffuse out into the plasma, and chloride ions move into the red blood cells to maintain charge balance. This is called the chloride shift.
在红细胞内，二氧化碳在碳酸酐酶催化下与水反应生成碳酸。碳酸解离为氢离子和碳酸氢根离子。碳酸氢根离子扩散到血浆中，氯离子移入红细胞以维持电荷平衡。这称为氯转移。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
10. Tissue Fluid and Lymph Formation | 组织液与淋巴的形成
At the arterial end of a capillary, the hydrostatic pressure of the blood is higher than the oncotic pressure exerted by plasma proteins. This forces water and small solutes out through gaps between endothelial cells, forming tissue fluid.
在毛细血管动脉端，血液的静水压高于血浆蛋白产生的胶体渗透压。这使水和小分子溶质通过内皮细胞间隙被压出，形成组织液。
At the venous end, hydrostatic pressure falls and the oncotic pressure of plasma proteins draws most of the fluid back into the capillary. Excess tissue fluid drains into lymph capillaries and becomes lymph, which is eventually returned to the blood.
在静脉端，静水压下降，血浆蛋白的胶体渗透压将大部分液体重新吸回毛细血管。多余的组织液流入淋巴毛细管，成为淋巴，最终返回血液。
Tissue fluid bathes the cells directly, allowing exchange of oxygen, nutrients and metabolic waste between blood and body cells because the capillary wall and tissue fluid provide only a short diffusion distance.
组织液直接浸泡细胞，使血液与体细胞之间能够交换氧气、营养物质和代谢废物，因为毛细血管壁和组织液只提供了很短的扩散距离。
11. Fetal Haemoglobin and Altitude Adaptation | 胎儿血红蛋白与高海拔适应
Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. Its oxygen dissociation curve lies to the left, allowing the fetus to obtain oxygen from maternal blood in the placenta where the partial pressure of oxygen is relatively low.
胎儿血红蛋白对氧的亲和力高于成人血红蛋白。其氧解离曲线偏左，使胎儿能够在胎盘氧分压相对较低的情况下从母体血液中获取氧气。
Animals living at high altitude may also have haemoglobin with a higher oxygen affinity or a higher red blood cell count. These adaptations improve oxygen uptake and transport in low-oxygen conditions.
生活在高海拔地区的动物也可能具有氧亲和力更高的血红蛋白或更高的红细胞数量。这些适应有助于在低氧条件下改善氧气的摄取和运输。
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