📚 Transport Systems in Animals | 动物运输系统
In large multicellular organisms, diffusion alone cannot meet the metabolic demands of all cells. Nutrients and oxygen must be delivered efficiently, while waste products such as carbon dioxide and urea must be removed. Animals have evolved complex transport systems, typically featuring a pump, a circulating fluid and a network of vessels. This article focuses on the mammalian circulatory system, including the structure and function of the heart, blood vessels, blood components, and the mechanisms for transporting oxygen and carbon dioxide.
在大型多细胞生物中，仅靠扩散无法满足所有细胞的代谢需求。营养物质和氧气必须高效送达，而二氧化碳和尿素等废物则必须被清除。动物进化出了复杂的运输系统，通常包含一个泵、循环流体和血管网络。本文重点介绍哺乳动物的循环系统，包括心脏的结构与功能、血管、血液成分以及氧气和二氧化碳的运输机制。
1. The Need for Transport Systems | 运输系统的必要性
As organisms increase in size, their surface area to volume ratio decreases. This limits the rate of diffusion for nutrients and gases. Unicellular organisms and simple multicellular organisms rely on diffusion, but larger animals require a mass transport system to move substances over long distances, maintain a steep concentration gradient, and link exchange surfaces with every cell.
随着生物体体积增大，其表面积与体积之比减小，限制了营养物质和气体的扩散速率。单细胞生物和简单的多细胞生物依赖扩散，但较大的动物需要物质运输系统，以长距离输送物质、维持陡峭的浓度梯度，并将交换表面与每个细胞连接起来。
2. The Mammalian Circulatory System: Double Circulation | 哺乳动物循环系统：双循环
Mammals have a closed, double circulatory system. The heart pumps blood to the lungs (pulmonary circulation) where gas exchange occurs, and then separately pumps oxygenated blood to the rest of the body (systemic circulation). This separation ensures that blood flowing to the body is fully oxygenated and delivered at high pressure, enabling efficient oxygen delivery to tissues.
哺乳动物具有封闭式双循环系统。心脏将血液泵入肺（肺循环）进行气体交换，然后分别将含氧血液泵至全身（体循环）。这种分隔确保流向身体的血液充分含氧并以较高压力输送，从而高效地向组织供氧。
3. Structure of the Heart | 心脏结构
The mammalian heart is composed of four chambers: right atrium, right ventricle, left atrium and left ventricle. The atria receive blood returning to the heart, while the ventricles pump blood out. Atrioventricular valves (tricuspid on the right, bicuspid/mitral on the left) prevent backflow into the atria, and semilunar valves guard the exits to the pulmonary artery and aorta. The left ventricular wall is much thicker than the right because it must generate sufficient pressure to propel blood through the systemic circuit.
哺乳动物心脏由四个腔室组成：右心房、右心室、左心房和左心室。心房接收回心血液，心室将血液泵出。房室瓣（右侧为三尖瓣，左侧为二尖瓣）防止血液回流至心房，半月瓣守护肺动脉和主动脉的出口。左心室壁比右心室壁厚得多，因为它必须产生足够的压力推动血液流经体循环。
4. The Cardiac Cycle and Control of Heart Rate | 心动周期与心率控制
The cardiac cycle consists of atrial systole (contraction), ventricular systole and diastole (relaxation). During atrial systole, blood is forced into the ventricles. Ventricular systole then pushes blood into the arteries, while the atrioventricular valves snap shut, producing the first heart sound. Diastole follows, allowing the chambers to fill. The second heart sound is caused by closure of the semilunar valves.
心动周期包括心房收缩期、心室收缩期和舒张期。在心房收缩期间，血液被压入心室。随后心室收缩将血液推入动脉，同时房室瓣猛然关闭，产生第一心音。接着是舒张期，腔室重新充盈。第二心音由半月瓣关闭引起。
Heart rate is myogenic, initiated by the sinoatrial node (SAN), the heart’s natural pacemaker. The electrical impulse spreads over the atria, causing contraction, and is then delayed at the atrioventricular node (AVN) before travelling down the bundle of His and Purkinje fibres to the ventricles. Heart rate is modulated by the autonomic nervous system (sympathetic nerves increase rate, parasympathetic via the vagus nerve decrease rate) and by the hormone adrenaline.
心率具有肌源性，由窦房结（SAN）——心脏的天然起搏点——发起。电信号扩散至心房引起收缩，然后在房室结（AVN）稍有延迟，再经希氏束和浦肯野纤维传至心室。心率受自主神经系统调节（交感神经加速，迷走神经的副交感作用减慢），也受激素肾上腺素的影响。
5. Blood Vessels: Arteries, Veins and Capillaries | 血管：动脉、静脉与毛细血管
Blood vessels are structurally adapted to their functions. The following table summarises the key differences.
血管在结构上适应其功能。下表概括了主要差异。
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Wall thickness | Thick, with smooth muscle and elastic fibres | Thinner, less muscle and elastic tissue | Extremely thin (one cell thick) |
| Lumen | Relatively narrow | Wide, often collapsed | Very narrow, just wide enough for red blood cells |
| Valves | Absent (except for semilunar valves at the heart exit) | Present to prevent backflow | Absent |
| Function | Carry blood away from the heart at high pressure | Carry blood back to the heart at low pressure | Exchange of nutrients, gases and waste with tissues |
Arteries have thick elastic walls that can withstand and maintain pressure; their smooth muscle allows vasoconstriction and vasodilation. Veins contain valves and are located between skeletal muscles, which help return blood to the heart. Capillaries form extensive networks, providing a large surface area and short diffusion distance for efficient exchange.
动脉壁厚且富有弹性，能承受并维持血压；平滑肌可实现血管收缩与舒张。静脉含有瓣膜，位于骨骼肌之间，有助于血液回流心脏。毛细血管形成广阔网络，提供巨大表面积和极短的扩散距离，实现高效物质交换。
6. Composition and Functions of Blood | 血液组成与功能
Blood is a fluid connective tissue comprising plasma, red blood cells (erythrocytes), white blood cells (leucocytes) and platelets (thrombocytes). Plasma (55% of blood volume) is primarily water, carrying dissolved substances such as nutrients, hormones, plasma proteins, carbon dioxide and urea.
血液是一种液态结缔组织，由血浆、红细胞（红血球）、白细胞（白血球）和血小板（凝血细胞）组成。血浆（占血容量的55%）主要成分是水，运载溶解的营养物质、激素、血浆蛋白、二氧化碳和尿素等。
Red blood cells are biconcave discs lacking a nucleus and most organelles, maximising space for haemoglobin. They transport oxygen and a small amount of carbon dioxide. White blood cells are part of the immune system, defending against pathogens. Platelets are cell fragments involved in blood clotting, which prevents excess bleeding and pathogen entry.
红细胞呈双面凹的圆盘状，无细胞核和大多数细胞器，以最大程度容纳血红蛋白。它们运输氧气和少量二氧化碳。白细胞是免疫系统的组成部分，抵御病原体。血小板是细胞碎片，参与凝血过程，防止过量出血和病原体侵入。
7. Transport of Oxygen: Haemoglobin and Bohr Effect | 氧气运输：血红蛋白与波尔效应
Haemoglobin is a quaternary protein composed of four polypeptide chains, each containing a haem group with an iron ion that reversibly binds one oxygen molecule. In the lungs, where partial pressure of oxygen (pO₂) is high, haemoglobin becomes saturated with oxygen to form oxyhaemoglobin. In respiring tissues, lower pO₂ promotes dissociation and oxygen release.
血红蛋白是一种四级结构蛋白，由四条多肽链组成，每条链含有一个血红素基团，其中的铁离子可逆地结合一个氧分子。在肺部，氧分压（pO₂）高，血红蛋白与氧结合饱和，形成氧合血红蛋白。在呼吸组织中，较低的pO₂促进解离并释放氧气。
The oxygen dissociation curve for haemoglobin is sigmoidal (S‑shaped), demonstrating cooperative binding: binding of the first oxygen molecule causes a conformational change that makes subsequent binding easier. The Bohr effect describes how increased carbon dioxide concentration and decreased pH (more H⁺) shift the curve to the right, enhancing oxygen unloading in active tissues. Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, shifting its curve to the left, which aids oxygen transfer across the placenta.
血红蛋白的氧解离曲线呈S形，体现协同结合：第一个氧分子的结合引起构象变化，使后续氧分子更易结合。波尔效应描述二氧化碳浓度升高和pH降低（H⁺增多）如何使曲线右移，促进活跃组织中氧的卸载。胎儿血红蛋白对氧的亲和力高于成人血红蛋白，曲线左移，有助于氧气经胎盘转运。
8. Transport of Carbon Dioxide | 二氧化碳运输
Carbon dioxide is transported in the blood in three ways: about 5% dissolves directly in plasma; around 10% binds to haemoglobin to form carbaminohaemoglobin; and the majority (about 85%) is converted to hydrogencarbonate ions (HCO₃⁻) in red blood cells. The enzyme carbonic anhydrase catalyses the reaction:
二氧化碳以三种方式在血液中运输：约5%直接溶解在血浆中；约10%与血红蛋白结合形成氨甲酰血红蛋白；绝大多数（约85%）在红细胞内转化为碳酸氢根离子（HCO₃⁻）。碳酸酐酶催化该反应：
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
The hydrogencarbonate ions diffuse out of the red blood cell into the plasma in exchange for chloride ions – the chloride shift – to maintain electrical neutrality. Hydrogen ions released are buffered by haemoglobin, preventing large changes in blood pH. In the lungs, the process reverses: HCO₃⁻ re‑enters the red blood cell, combines with H⁺, and the CO₂ regenerated diffuses into the alveoli.
碳酸氢根离子从红细胞扩散至血浆，同时氯离子进入红细胞——即氯转移——以维持电荷平衡。释放的氢离子被血红蛋白缓冲，防止血液pH大幅波动。在肺部，该过程逆转：HCO₃⁻重新进入红细胞，与H⁺结合，再生成的CO₂扩散至肺泡。
9. Tissue Fluid and Lymphatic System | 组织液与淋巴系统
At the arterial end of capillaries, high hydrostatic pressure forces fluid out through the capillary walls, forming tissue fluid (interstitial fluid). This fluid bathes the cells, allowing exchange of nutrients, oxygen and waste. At the venous end, the hydrostatic pressure is lower and oncotic pressure (due to plasma proteins) draws most of the fluid back into the capillary. The small amount of fluid that remains is collected by the lymphatic system.
在毛细血管的动脉端，高静水压将液体挤出管壁，形成组织液（细胞间液）。该液体浸润细胞，进行营养物质、氧气和废物的交换。在静脉端，静水压下降，而胶体渗透压（由血浆蛋白产生）将大部分液体吸回毛细血管。剩余的少量液体由淋巴系统收集。
Lymph is transported through lymphatic vessels, which contain valves and eventually return the fluid to the blood via the subclavian veins. Along the lymphatic vessels, lymph nodes filter the lymph and house lymphocytes, playing a key role in immune defence. The lymphatic system also transports fats absorbed in the small intestine via lacteals.
淋巴通过淋巴管输送，淋巴管含有瓣膜，最终经锁骨下静脉将液体送回血液。淋巴管沿线的淋巴结过滤淋巴并容纳淋巴细胞，在免疫防御中发挥关键作用。淋巴系统还通过乳糜管运输小肠吸收的脂肪。
10. Cardiovascular Diseases | 心血管疾病
Coronary heart disease (CHD) occurs when the coronary arteries, which supply blood to the heart muscle, become narrowed or blocked. This is commonly due to atherosclerosis – the build‑up of fatty deposits (plaques) under the artery lining. Plaques can rupture, triggering the formation of a blood clot (thrombus) that may completely block the artery, leading to myocardial infarction (heart attack).
冠心病（CHD）发生在供应心肌血液的冠状动脉变窄或阻塞时。这通常由动脉粥样硬化引起——即动脉内膜下脂质沉积（斑块）的积聚。斑块可能破裂，引发血栓形成，完全阻塞血管，导致心肌梗死（心脏病发作）。
Risk factors include high blood pressure, high LDL cholesterol, smoking, obesity, lack of exercise and genetic predisposition. Preventive measures involve lifestyle changes such as a balanced diet low in saturated fats, regular physical activity, not smoking, and medication to control blood pressure and cholesterol. Understanding the structure and function of the transport system helps in diagnosing and managing these conditions.
风险因素包括高血压、高LDL胆固醇、吸烟、肥胖、缺乏运动和遗传易感性。预防措施包括改变生活方式，如低饱和脂肪的均衡饮食、定期体育活动、不吸烟，以及使用药物控制血压和胆固醇。了解运输系统的结构与功能有助于诊断和处理这些病症。
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