📚 Transport Systems in Animals | 动物体内的物质运输系统
All living cells require a constant supply of oxygen and nutrients, and must remove metabolic waste products. In small organisms, diffusion alone is sufficient. However, as animals increase in size and complexity, their surface area to volume ratio decreases, making diffusion inadequate — hence the evolution of specialised transport systems.
所有活细胞都需要持续获得氧气和营养物质，同时必须排出代谢废物。在小型生物体中，仅靠扩散就足够了。然而，随着动物体积增大、结构变复杂，其表面积与体积之比下降，使得扩散不再足够——因此演化出了特化的运输系统。
1. Why Do Animals Need Transport Systems? | 动物为何需要运输系统？
The key factor is the surface area to volume (SA:V) ratio. As an organism grows, volume increases with the cube, while surface area increases only with the square. A large animal such as a mammal has a very low SA:V ratio, so diffusion across the outer surface cannot meet cellular demands.
关键因素是表面积与体积之比（SA:V）。随着生物体生长，体积按三次方增加，而表面积仅按平方增加。像哺乳动物这样的大型动物，其SA:V比很低，因此通过外表面进行的扩散无法满足细胞的需求。
Additionally, mammals are endothermic (warm-blooded) and maintain a high metabolic rate. Their cells consume oxygen and glucose rapidly, requiring an efficient bulk transport system. The circulatory system provides this by moving blood rapidly around the body.
此外，哺乳动物是恒温动物（温血动物），保持高代谢率。其细胞快速消耗氧气和葡萄糖，需要高效的整体运输系统。循环系统通过使血液在体内快速流动来提供这种运输。
Three features are essential for any effective transport system: a fluid medium (blood), a pumping mechanism (heart), and a network of tubes (blood vessels). All three work together to deliver materials to tissues and remove waste.
任何有效的运输系统都必须具备三个特征：流体介质（血液）、泵送机制（心脏）和管道网络（血管）。三者协同工作，将物质输送到组织并清除废物。
2. Open and Closed Circulatory Systems | 开管式与闭管式循环系统
Arthropods such as insects have an open circulatory system. Blood (haemolymph) is pumped by a tubular heart into a cavity called the haemocoel, where it directly bathes organs. There are no capillaries or veins — blood returns to the heart through open pores called ostia.
节肢动物（如昆虫）具有开管式循环系统。血液（血淋巴）由管状心脏泵入称为血腔的体腔，直接浸浴器官。系统中没有毛细血管或静脉——血液通过称为心孔的开放孔隙返回心脏。
Mammals have a closed circulatory system: blood is always contained within vessels (arteries, capillaries, veins). This allows higher blood pressure, more rapid and efficient delivery of oxygen, and independent control of blood flow to different organs.
哺乳动物具有闭管式循环系统：血液始终封闭在血管（动脉、毛细血管、静脉）内。这种系统允许更高的血压、更快速高效的氧气输送，并能独立控制不同器官的血流量。
The closed system is more efficient for animals with high metabolic demands. The open system is energetically cheaper but limits the size and activity level of the organism — which is why insects tend to be small.
闭管式系统更适合高代谢需求的动物。开管式系统在能量上更经济，但限制了生物体的大小和活动水平——这就是昆虫往往较小的原因。
3. Single and Double Circulation | 单循环与双循环
Fish have a single circulation: blood passes through the heart once per complete circuit. The heart pumps deoxygenated blood to the gills, where it is oxygenated, then continues to the rest of the body before returning to the heart. This system is relatively slow because blood pressure drops as blood passes through the gill capillaries.
鱼类具有单循环：血液在每个完整回路中只经过心脏一次。心脏将脱氧血液泵入鳃，在鳃中完成氧合，然后流向身体其余部分，再返回心脏。该系统的速度相对较慢，因为血液经过鳃毛细血管时血压会下降。
Mammals have a double circulation: blood passes through the heart twice in one complete circuit. The right side of the heart pumps blood to the lungs (pulmonary circulation), while the left side pumps blood to the rest of the body (systemic circulation).
哺乳动物具有双循环：血液在一个完整回路中两次经过心脏。心脏右侧将血液泵入肺（肺循环），而心脏左侧将血液泵向身体其余部分（体循环）。
Double circulation has two major advantages: blood is pumped at higher pressure, and oxygenated and deoxygenated blood are completely separated. This ensures tissues receive a rapid, oxygen-rich blood supply, supporting the high metabolic demands of endotherms.
双循环有两个主要优点：血液以更高的压力泵出，且含氧血与脱氧血完全分离。这确保组织获得快速、富含氧气的血液供应，支持恒温动物高代谢需求。
4. Structure of the Mammalian Heart | 哺乳动物心脏的结构
The mammalian heart has four chambers: two atria (thin-walled, receive blood) and two ventricles (thick-walled, pump blood out). The left and right sides are separated by a septum, preventing mixing of oxygenated and deoxygenated blood.
哺乳动物心脏有四个腔室：两个心房（壁薄，接收血液）和两个心室（壁厚，泵出血液）。左右两侧由中隔分开，防止含氧血与脱氧血混合。
The left ventricle has a much thicker muscular wall than the right ventricle because it must pump blood to the entire body at high pressure, whereas the right ventricle only pumps blood to the nearby lungs.
左心室的肌壁比右心室厚得多，因为它必须将血液以高压泵向全身，而右心室只需将血液泵往邻近的肺部。
Four valves ensure one-way flow of blood. The atrioventricular (AV) valves — the tricuspid on the right and bicuspid (mitral) on the left — lie between atria and ventricles. The semilunar valves — pulmonary and aortic — sit at the exits of the ventricles. The coronary arteries supply the heart muscle itself with oxygenated blood.
四个瓣膜确保血液单向流动。房室瓣（AV瓣）——右侧的三尖瓣和左侧的二尖瓣（僧帽瓣）——位于心房与心室之间。半月瓣——肺动脉瓣和主动脉瓣——位于心室出口处。冠状动脉为心肌本身提供含氧血液。
Right side: vena cava → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery
Left side: pulmonary vein → left atrium → bicuspid valve → left ventricle → aortic valve → aorta
右侧：腔静脉 → 右心房 → 三尖瓣 → 右心室 → 肺动脉瓣 → 肺动脉
左侧：肺静脉 → 左心房 → 二尖瓣 → 左心室 → 主动脉瓣 → 主动脉
5. The Cardiac Cycle | 心动周期
The cardiac cycle consists of three phases. Cardiac diastole: the heart relaxes, blood fills the atria, and pressure in the atria rises until it exceeds ventricular pressure, forcing the AV valves open and allowing blood to pass into the ventricles.
心动周期包括三个阶段。心舒期：心脏舒张，血液充盈心房，心房压力升高直至超过心室压力，迫使房室瓣开放，使血液进入心室。
Atrial systole: the atria contract, pushing the remaining blood into the ventricles. Ventricular systole: the ventricles contract violently, ventricular pressure rises sharply, the AV valves close (producing the ‘lub’ sound), then the semilunar valves open and blood is ejected into the pulmonary artery and aorta.
心房收缩期：心房收缩，将剩余血液推入心室。心室收缩期：心室猛烈收缩，心室压力急剧升高，房室瓣关闭（产生“lub”音），随后半月瓣开放，血液被射入肺动脉和主动脉。
When ventricular pressure falls below arterial pressure, the semilunar valves snap shut (producing the ‘dub’ sound). The pressure changes in the left ventricle drive the opening and closing of valves in a precisely timed sequence.
当心室压力降至动脉压力以下时，半月瓣猛然关闭（产生“dub”音）。左心室内的压力变化以精确的时间顺序驱动瓣膜的开闭。
Ventricular systole ≈ 120 mmHg (aorta) | Atrial pressure ≈ 5–8 mmHg | Valve closure prevents backflow
心室收缩压 ≈ 120 mmHg（主动脉）| 心房压 ≈ 5–8 mmHg | 瓣膜关闭防止倒流
6. Blood Vessels | 血管
Arteries carry blood away from the heart under high pressure. They have thick walls composed of three layers: tough elastic tissue, smooth muscle, and endothelium. The elastic tissue allows arteries to stretch and recoil, smoothing out the pulsatile flow generated by the heart. The smooth muscle permits vasoconstriction and vasodilation to regulate blood flow.
动脉将血液从心脏输向全身，承受高压。其管壁厚，由三层结构组成：坚韧的弹性组织、平滑肌和内皮。弹性组织使动脉能伸展和回缩，缓冲心脏产生的脉冲式血流。平滑肌允许血管收缩和舒张以调节血流量。
Arterioles are smaller branches of arteries with a relatively larger proportion of smooth muscle. They act as control valves, directing blood to specific capillary beds according to local metabolic need.
小动脉是动脉的较小分支，平滑肌比例相对较高。它们起到控制阀门的作用，根据局部代谢需求将血液分配到特定的毛细血管床。
Capillaries are microscopic vessels with walls only one cell thick (endothelium). They have no elastic or muscle tissue. Their thin walls, large total cross-sectional area, and slow blood flow maximise the exchange of oxygen, carbon dioxide, nutrients, and waste between blood and tissues. Some capillaries have fenestrations (pores) to speed up exchange.
毛细血管是显微镜下可见的微小血管，管壁仅有一层内皮细胞厚度，没有弹性组织或肌肉组织。其壁薄、总截面积大、血流缓慢，最大限度促进血液与组织之间氧气、二氧化碳、营养物质和废物的交换。部分毛细血管具有窗孔（孔隙）以加速交换。
Veins return blood to the heart under low pressure. They have thinner walls, a wider lumen, and contain semilunar valves that prevent backflow of blood. The skeletal muscle pump — contraction of surrounding skeletal muscles — squeezes veins and assists blood return; this is why leg movement helps venous return.
静脉在低压下将血液送回心脏。其管壁较薄、管腔较大，内含半月瓣以防止血液倒流。骨骼肌泵——周围骨骼肌的收缩——挤压静脉辅助血液回流；这就是腿部活动有助于静脉回流的原因。
| Feature | Artery | Vein | Capillary |
| Wall thickness | Thick | Thin | One cell thick |
| Lumen | Small | Large | Very small |
| Valves | No | Yes | No |
| Blood pressure | High | Low | Intermediate, slow flow |
7. Blood and Its Components | 血液及其组成成分
Blood is a specialised connective tissue composed of plasma and formed elements. Plasma (~55% of blood volume) is a pale yellow fluid consisting mostly of water, containing dissolved plasma proteins (albumin, globulins, fibrinogen), glucose, amino acids, hormones, mineral ions, and waste products such as urea.
血液是一种特化的结缔组织，由血浆和有形成分组成。血浆（约占血液体积的55%）是一种淡黄色液体，主要成分为水，含有溶解的血浆蛋白（白蛋白、球蛋白、纤维蛋白原）、葡萄糖、氨基酸、激素、无机盐离子以及尿素等废物。
Red blood cells (erythrocytes) are biconcave discs with no nucleus or mitochondria. Their shape increases the surface area for gas diffusion, and their flexibility allows them to squeeze through narrow capillaries. Each red blood cell contains approximately 250 million haemoglobin molecules, enabling efficient oxygen transport.
红细胞呈双凹圆盘状，无细胞核和线粒体。其形状增加了气体扩散的表面积，而灵活性使其能挤过狭窄的毛细血管。每个红细胞含有约2.5亿个血红蛋白分子，从而实现高效的氧气运输。
White blood cells (leukocytes) defend the body against infection. Phagocytes (neutrophils and macrophages) engulf and destroy pathogens by phagocytosis. Lymphocytes (B and T cells) produce antibodies and coordinate the immune response. Platelets are small cell fragments involved in blood clotting — they release clotting factors that convert fibrinogen to fibrin, forming a mesh that traps red blood cells.
白细胞防御身体免受感染。吞噬细胞（中性粒细胞和巨噬细胞）通过吞噬作用吞没并消灭病原体。淋巴细胞（B细胞和T细胞）产生抗体并协调免疫应答。血小板是参与血液凝固的小细胞碎片——它们释放凝血因子，将纤维蛋白原转化为纤维蛋白，形成网罗红细胞的网状结构。
8. Tissue Fluid and Lymph | 组织液与淋巴
Tissue fluid is the fluid that bathes cells, formed by filtration of blood plasma through capillary walls. At the arterial end of a capillary, hydrostatic pressure (blood pressure forcing fluid out) is about 4.6 kPa, which exceeds the osmotic (oncotic) pressure exerted by plasma proteins (about 3.3 kPa). The net pressure of about 1.3 kPa pushes fluid out of the capillary.
组织液是浸浴细胞的液体，由血浆经毛细血管壁过滤形成。在毛细血管动脉端，静水压（推动液体外出的血压）约为4.6 kPa，超过血浆蛋白施加的渗透压（胶体渗透压，约3.3 kPa）。约1.3 kPa的净压力将液体推出毛细血管。
At the venous end, the hydrostatic pressure falls to about 2.3 kPa, while the oncotic pressure remains at 3.3 kPa. The net pressure now forces fluid back into the capillary (about 1 kPa inward). This balance of pressures is described by the Starling forces and explains why most fluid is reabsorbed.
在静脉端，静水压降至约2.3 kPa，而胶体渗透压仍为3.3 kPa。净压力现在使液体返回毛细血管（向内约1 kPa）。这一压力平衡被称为斯塔林力，解释了为什么大部分液体被重吸收。
Not all fluid is reabsorbed — approximately 10% remains in the tissues as excess tissue fluid. This is collected by blind-ended lymphatic vessels and returned to the blood as lymph. Lymph nodes filter lymph and house lymphocytes, contributing to immune defence.
并非所有液体都被重吸收——大约10%残留在组织中成为多余的组织液。这部分液体由盲端的淋巴管收集，以淋巴形式返回血液。淋巴结过滤淋巴并容纳淋巴细胞，参与免疫防御。
9. Oxygen Transport | 氧气的运输
Haemoglobin is a globular protein with a quaternary structure containing four polypeptide chains, each with an iron-containing haem group. Each haem group can bind one oxygen molecule, so one haemoglobin molecule can carry up to four O₂ molecules.
血红蛋白是一种具有四级结构的球状蛋白，含有四条多肽链，每条链上有一个含铁的血红素基团。每个血红素基团可结合一个氧分子，因此一个血红蛋白分子最多可携带四个O₂分子。
Oxygen combines with haemoglobin to form oxyhaemoglobin. The binding of the first oxygen molecule causes a conformational change that increases the affinity of the remaining haem groups for oxygen — this is cooperative binding. This explains the sigmoid (S-shaped) shape of the oxygen dissociation curve.
氧与血红蛋白结合形成氧合血红蛋白。第一个氧分子的结合引起构象变化，使其余血红素基团对氧的亲和力增加——这就是协同结合。这解释了氧解离曲线的S形（乙状）形态。
Hb + 4O₂ ⇌ Hb(O₂)₄
The affinity of haemoglobin for oxygen is affected by partial pressure of carbon dioxide. In actively respiring tissues, high CO₂ concentration lowers blood pH, reducing haemoglobin’s affinity for oxygen — this is the Bohr effect. It ensures oxygen is released more readily in tissues where it is needed most.
血红蛋白对氧的亲和力受二氧化碳分压影响。在呼吸旺盛的组织中，高CO₂浓度降低血液pH值，降低血红蛋白对氧的亲和力——这就是玻尔效应。它确保氧在最需要的组织中被更容易地释放出来。
10. Carbon Dioxide Transport | 二氧化碳的运输
Carbon dioxide is transported in the blood in three forms. Approximately 5% is dissolved directly in plasma. About 10–20% binds to amino groups of haemoglobin to form carbaminohaemoglobin. The remaining 75–85% is transported as hydrogen carbonate (bicarbonate) ions.
二氧化碳以三种形式在血液中运输。约5%直接溶解在血浆中。约10–20%与血红蛋白的氨基结合形成氨基甲酰血红蛋白。其余75–85%以碳酸氢根离子形式运输。
Inside red blood cells, carbon dioxide reacts with water to form carbonic acid, catalysed by the enzyme carbonic anhydrase. Carbonic acid dissociates into hydrogen ions (H⁺) and hydrogen carbonate ions (HCO₃⁻). The H⁺ ions bind to haemoglobin (buffering, preventing pH change), and the HCO₃⁻ ions diffuse out of the cell into the plasma.
在红细胞内部，二氧化碳与水反应生成碳酸，该反应由碳酸酐酶催化。碳酸解离为氢离子（H⁺）和碳酸氢根离子（HCO₃⁻）。H⁺离子与血红蛋白结合（起缓冲作用，防止pH值变化），HCO₃⁻离子扩散出细胞进入血浆。
To maintain electrical neutrality, chloride ions (Cl⁻) diffuse into the red blood cell from the plasma — this is the chloride shift. In the lungs, these reactions are reversed: HCO₃⁻ re-enters red blood cells, combines with H⁺ to form CO₂, which is exhaled.
为维持电中性，氯离子（Cl⁻）从血浆扩散进入红细胞——这就是氯离子转移。在肺中，这些反应被逆转：HCO₃⁻重新进入红细胞，与H⁺结合生成CO₂，后者被呼出体外。
11. Transport of Nutrients and Waste | 营养物质与废物的运输
The circulatory system transports absorbed nutrients from the small intestine to all body tissues. Monosaccharides (glucose) and amino acids are carried dissolved in plasma. Lipids and fat-soluble vitamins are transported in the form of chylomicrons — lipoprotein particles that travel via the lymphatic system before entering the bloodstream.
循环系统将小肠吸收的营养物质运送到全身各组织。单糖（葡萄糖）和氨基酸以溶解状态在血浆中运输。脂质和脂溶性维生素以乳糜微粒形式运输——这些脂蛋白颗粒先经淋巴系统，再进入血液循环。
Metabolic waste products are also transported by the blood. Urea, produced in the liver from excess amino acids, is carried in plasma to the kidneys for excretion. Carbon dioxide is transported as described above and removed by the lungs. Lactic acid produced during anaerobic respiration is carried to the liver for conversion back to glucose or pyruvate.
代谢废物也由血液运输。尿素在肝脏中由多余氨基酸产生，以血浆为载体运至肾脏排出。二氧化碳按前述方式运输并由肺排出。无氧呼吸产生的乳酸被运至肝脏，转化为葡萄糖或丙酮酸。
Hormones, such as insulin and adrenaline, are also transported in the blood from endocrine glands to their target tissues. This allows rapid, coordinated communication between different parts of the body — a key function of the transport system beyond simple nutrient delivery.
胰岛素、肾上腺素等激素也通过血液从内分泌腺运输到靶组织。这实现了身体不同部位之间快速、协调的通讯——这是运输系统超越简单营养输送的关键功能。
In summary, the animal transport system — a pump, a network of vessels, and a specialised fluid — is indispensable for delivering oxygen and nutrients, removing waste, and maintaining homeostasis. Understanding each component, from the cardiac cycle to gas transport, is essential for mastering A-Level Biology and interpreting real-world problems such as hypertension and oedema.
总而言之，动物运输系统——泵、血管网络和特化的液体——对于输送氧气和营养物质、清除废物以及维持内环境稳定不可或缺。理解每一个组成部分，从心动周期到气体运输，是掌握A-Level生物学和解读高血压、水肿等实际问题的关键。
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