📚 Transport in Mammals | 哺乳动物的运输
Mammals rely on a highly efficient transport system to maintain the internal environment of their complex, active bodies. The circulatory system – consisting of a muscular heart, a network of blood vessels and blood – ensures that every living cell receives oxygen and nutrients while metabolic wastes are swiftly removed. This article covers the essential A-level concepts of mammalian transport, from the structure of the heart to the molecular basis of gas exchange.
哺乳动物依赖高效的运输系统来维持复杂而活跃的身体内环境。由肌肉质心脏、血管网络和血液组成的循环系统确保每个活细胞都能获得氧气和营养，同时代谢废物被迅速带走。本文涵盖了哺乳动物运输的核心A-level概念，从心脏的结构到气体交换的分子基础。
1. The Need for Transport Systems in Mammals | 哺乳动物对运输系统的需求
Mammals are large, multicellular organisms with a small surface area to volume ratio. Oxygen and nutrients cannot reach all cells by simple diffusion alone, as diffusion is slow over distances greater than a few millimetres. A dedicated mass transport system is therefore essential for overcoming the limitations of diffusion.
哺乳动物是大型多细胞生物，表面积与体积之比较小。氧气和营养物质无法仅靠简单扩散到达所有细胞，因为扩散在超过几毫米的距离上非常缓慢。因此，专门的集束运输系统对于克服扩散的局限性至关重要。
Mammals also have a high metabolic rate, which generates a large and constant demand for oxygen and glucose, and produces waste products such as carbon dioxide and urea at a rapid pace. The transport system circulates fluids quickly throughout the body, delivering these substances to and from respiring tissues.
哺乳动物的代谢率很高，对氧气和葡萄糖的需求巨大而持续，同时以极快的速度产生二氧化碳和尿素等废物。运输系统将流体快速循环至全身，向呼吸组织供应这些物质并带走废物。
2. Structure of the Mammalian Heart | 哺乳动物心脏的结构
The mammalian heart is a double pump with four chambers: two thin-walled atria that receive blood, and two thick-walled ventricles that pump blood out of the heart. A muscular wall called the septum separates the left and right sides, preventing the mixing of oxygenated and deoxygenated blood.
哺乳动物心脏是一个具有四个腔室的双泵：两个壁薄的心房接收血液，两个壁厚的心室将血液泵出心脏。被称为中隔的肌壁分隔左右两侧，防止含氧血和缺氧血混合。
Blood enters the right atrium from the vena cavae and passes into the right ventricle through the tricuspid valve. The right ventricle pumps blood to the lungs via the pulmonary artery. Oxygenated blood returns to the left atrium through the pulmonary veins, flows past the bicuspid (mitral) valve into the left ventricle, and is then ejected into the aorta to be distributed to the whole body. Semilunar valves at the exits of the ventricles prevent backflow into the heart.
血液从上腔静脉和下腔静脉进入右心房，经过三尖瓣流入右心室。右心室将血液通过肺动脉泵入肺部。含氧血经肺静脉返回左心房，通过二尖瓣流入左心室，随后被射入主动脉分配至全身。心室出口处的半月瓣防止血液倒流回心脏。
The left ventricle has a much thicker muscular wall than the right because it must generate sufficient pressure to propel blood through the extensive systemic circulation, whereas the right ventricle only needs to pump blood to the nearby lungs.
左心室的肌壁比右心室厚得多，因为它必须产生足够的压力推动血液流经庞大的体循环，而右心室只需将血液泵到附近的肺部。
3. The Cardiac Cycle and Control of Heartbeat | 心动周期与心跳控制
The cardiac cycle consists of three main stages: atrial systole, ventricular systole and diastole. During atrial systole, the atria contract, forcing blood into the ventricles. Ventricular systole follows, causing a rapid rise in ventricular pressure that closes the atrioventricular valves and opens the semilunar valves, ejecting blood into the arteries. In diastole, both chambers relax, pressure falls, and the heart refills.
心动周期由三个主要阶段组成：心房收缩期、心室收缩期和舒张期。在心房收缩期，心房收缩，将血液挤入心室。接着是心室收缩期，引起心室内压急剧升高，关闭房室瓣并打开半月瓣，将血液射入动脉。在舒张期，两个腔室均舒张，压力下降，心脏重新充盈。
The heartbeat is initiated by the sinoatrial node (SAN), a group of specialised cells in the wall of the right atrium. The SAN acts as a pacemaker, generating electrical impulses that spread across the atria, triggering atrial contraction. The impulses are then delayed at the atrioventricular node (AVN) to allow complete ventricular filling, before travelling down the bundle of His and along Purkinje fibres to stimulate ventricular contraction from the apex upwards.
心跳由位于右心房壁的一组特化细胞——窦房结（SAN）启动。SAN起搏点作用，产生电脉冲并扩散通过心房，触发心房收缩。脉冲随后在房室结（AVN）处延迟，以确保心室完全充盈，之后经希氏束和浦肯野纤维从心尖向上传导，刺激心室收缩。
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 walls rich in smooth muscle and elastic fibres to withstand high pressure. The elasticity allows them to stretch during ventricular systole and recoil during diastole, maintaining blood flow. Arterioles are smaller branches that control blood flow to capillary beds by vasoconstriction and vasodilation.
动脉将血液从心脏运送出去。它们的管壁厚，富含平滑肌和弹性纤维，以承受高压。弹性使其在心室收缩时扩张，在舒张时回缩，从而维持血流。小动脉是更小的分支，通过血管收缩和舒张控制流入毛细血管床的血量。
Capillaries are the site of exchange between blood and tissues. Their walls consist of a single layer of endothelial cells, minimising diffusion distance. Narrow lumens force red blood cells to pass in single file, slowing flow and maximising contact time for exchange of gases, nutrients and wastes.
毛细血管是血液与组织进行物质交换的场所。管壁仅由单层内皮细胞构成，最大限度地缩短了扩散距离。狭窄的管腔迫使红细胞单行通过，减缓流速，最大化气体、营养物质和废物交换的接触时间。
Veins return blood to the heart. They have thinner walls, a larger lumen and contain valves to prevent backflow as blood travels at low pressure. The contraction of surrounding skeletal muscles (the skeletal muscle pump) helps propel venous blood towards the heart.
静脉将血液送回心脏。它们管壁较薄、管腔较大，并含有瓣膜，以防止低压下血液倒流。周围骨骼肌的收缩（骨骼肌泵）有助于推动静脉血向心脏回流。
| Feature | Artery | Capillary | Vein |
|---|---|---|---|
| Wall thickness | Thick | Very thin (one cell) | Thin |
| Elastic/muscle fibres | Many | None | Few |
| Lumen | Narrow | Very narrow | Wide |
| Valves | None (except aortic & pulmonary semilunar) | None | Present |
特征 | 动脉 | 毛细血管 | 静脉：壁厚度 – 厚； 极薄（单细胞）； 薄。弹性/肌纤维 – 多； 无； 少。管腔 – 狭窄； 极窄； 宽阔。瓣膜 – 无（除主/肺动脉半月瓣）； 无； 有。
5. Blood Composition and Function | 血液的组成和功能
Blood is composed of plasma (about 55%) and formed elements – red blood cells, white blood cells and platelets. Plasma is a pale yellow liquid consisting of water, dissolved proteins (such as albumin, globulins, fibrinogen), hormones, nutrients, gases and waste products. It acts as a solvent and transport medium.
血液由血浆（约55%）和有形成分——红细胞、白细胞和血小板组成。血浆是淡黄色液体，包含水、溶解的蛋白质（如白蛋白、球蛋白、纤维蛋白原）、激素、营养物质、气体和废物。血浆充当溶剂和运输介质。
Red blood cells (erythrocytes) are biconcave, disc-shaped cells without a nucleus or mitochondria. They contain haemoglobin, which binds and transports oxygen. Their shape increases surface area for diffusion, and their flexibility allows them to squeeze through narrow capillaries. White blood cells (leucocytes) are part of the immune system, while platelets are cell fragments involved in blood clotting.
红细胞（红血球）为双凹碟形，无细胞核和线粒体。它们含有血红蛋白，用于结合和运输氧气。其形状增大了扩散表面积，其柔韧性使它们能挤过狭窄的毛细血管。白细胞是免疫系统的一部分，而血小板是参与血液凝固的细胞碎片。
6. Haemoglobin and Oxygen Transport | 血红蛋白与氧气的运输
Haemoglobin is a quaternary-structured protein found in red blood cells. It consists of four polypeptide subunits, each containing a haem group with an iron ion (Fe²⁺) that can reversibly bind one molecule of oxygen. This allows each haemoglobin molecule to carry up to four O₂ molecules, forming oxyhaemoglobin (Hb(O₂)₄).
血红蛋白是一种存在于红细胞中的四级结构蛋白质。它由四个多肽亚基组成，每个亚基包含一个含亚铁离子（Fe²⁺）的血红素辅基，可逆地结合一分子氧气。这使得每个血红蛋白分子最多可携带四个O₂分子，形成氧合血红蛋白Hb(O₂)₄。
Oxygen binds to haemoglobin cooperatively: the binding of the first O₂ molecule changes the shape of the haemoglobin, making it easier for subsequent O₂ molecules to bind. Conversely, when one O₂ dissociates, it becomes easier for the remaining O₂ to be released – a phenomenon called positive cooperativity.
氧气与血红蛋白以协同方式结合：第一个O₂分子的结合会改变血红蛋白的形状，使后续O₂更容易结合。反之，当一个O₂解离时，其余氧分子更容易被释放——这一现象称为正协同效应。
7. Oxygen Dissociation Curves and the Bohr Effect | 氧解离曲线与玻尔效应
The oxygen dissociation curve shows the relationship between the partial pressure of oxygen (pO₂) and the percentage saturation of haemoglobin with oxygen. It has a sigmoid (S-shaped) curve due to cooperative binding. At low pO₂ in respiring tissues, haemoglobin readily unloads oxygen; at high pO₂ in the lungs, it becomes fully saturated.
氧解离曲线显示了氧分压（pO₂）与血红蛋白氧饱和度之间的关系。由于协同结合，曲线呈S形。在呼吸组织的低pO₂下，血红蛋白容易卸载氧气；在肺部的高pO₂下，血红蛋白变得完全饱和。
| Partial pressure O₂ / kPa | Saturation / % |
|---|---|
| 0 | 0 |
| 2 | 20 |
| 4 | 50 |
| 6 | 78 |
| 8 | 93 |
| 12 | 98 |
氧分压 (kPa) | 饱和度 (%)：0 – 0；2 – 20；4 – 50；6 – 78；8 – 93；12 – 98。
The Bohr effect describes the shift of the oxygen dissociation curve to the right in the presence of increased carbon dioxide concentration (or decreased pH). This shift enhances oxygen unloading in tissues where CO₂ and acidity are high – an adaptation matching supply to demand. Higher temperatures and the presence of 2,3-bisphosphoglycerate (2,3-BPG) also shift the curve to the right.
玻尔效应描述了在二氧化碳浓度升高（或pH降低）时氧解离曲线向右偏移的现象。这种偏移增强了在CO₂和酸度高的组织中的氧气卸载——这是一种使供氧与需氧匹配的适应。较高的温度和2,3-二磷酸甘油酸（2,3-BPG）的存在也会使曲线右移。
8. Carbon Dioxide Transport | 二氧化碳的运输
Carbon dioxide produced by respiring cells is transported in the blood in three forms. About 5–7% dissolves directly in the plasma. A further 20–25% binds reversibly to haemoglobin, forming carbaminohaemoglobin (HbCO₂). The majority – around 70% – is converted into hydrogencarbonate ions (HCO₃⁻) inside red blood cells.
呼吸细胞产生的二氧化碳以三种形式在血液中运输。约5–7%直接溶解在血浆中。另有20–25%可逆地与血红蛋白结合，形成碳氨基血红蛋白（HbCO₂）。大部分——约70%——在红细胞内转化为碳酸氢根离子（HCO₃⁻）。
Inside red blood cells, the enzyme carbonic anhydrase catalyses the reaction:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
The hydrogencarbonate ions diffuse out of the cell into the plasma, while chloride ions (Cl⁻) move in to maintain electrochemical neutrality – the chloride shift. The H⁺ ions are buffered by haemoglobin, forming haemoglobinic acid, which prevents large pH changes in the blood.在红细胞内，碳酸酐酶催化下列反应：CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻。碳酸氢根离子扩散出细胞进入血浆，同时氯离子（Cl⁻）移入以维持电化学中性——即氯离子转移。H⁺离子被血红蛋白缓冲，形成血红蛋白酸，防止血液pH大幅变化。
9. Tissue Fluid and Lymph Formation | 组织液和淋巴的形成
At the arterial end of a capillary, higher hydrostatic pressure forces water and small solutes out through the capillary wall into the spaces between cells, forming tissue fluid. Large plasma proteins and blood cells remain in the capillary because the endothelium acts as a filter.
在毛细血管的动脉端，较高的静水压迫使水和小分子溶质穿过毛细血管壁进入细胞间隙，形成组织液。大分子血浆蛋白和血细胞则因内皮层的过滤作用而留在毛细血管内。
As the blood moves towards the venous end, the hydrostatic pressure drops. The osmotic effect of plasma proteins (colloid osmotic pressure) pulls about 90% of the tissue fluid back into the capillary. The remaining 10% drains into blind-ended lymph capillaries and is now called lymph. The lymphatic system returns this fluid to the blood via the thoracic duct, which joins the subclavian veins near the heart.
当血液流向静脉端时，静水压下降。血浆蛋白的渗透效应（胶体渗透压）将约90%的组织液拉回毛细血管。剩下的10%排入盲端的淋巴毛细管，此时称为淋巴。淋巴系统通过胸导管将这些液体送回血液，胸导管在心脏附近汇入锁骨下静脉。
Tissue fluid formation provides the environment for direct exchange of nutrients, oxygen and wastes between blood capillaries and cells. Without lymphatic drainage, fluid would accumulate in tissues, causing oedema.
组织液的形成提供了毛细血管与细胞之间直接交换营养、氧气和废物的环境。如果没有淋巴引流，液体会在组织中积聚，导致水肿。
10. Transport of Nutrients, Hormones, Waste Products and Heat | 营养物质、激素、废物与热量的运输
Blood plasma is the primary carrier of the products of digestion. Glucose and amino acids, absorbed from the small intestine, are transported to the liver via the hepatic portal vein and then distributed to body cells. Lipids are transported in the form of lipoproteins, while fat-soluble vitamins are carried by carrier proteins or in chylomicrons.
血浆是消化产物的主要载体。从小肠吸收的葡萄糖和氨基酸经肝门静脉运往肝脏，然后分配至全身细胞。脂质以脂蛋白形式运输，脂溶性维生素则由载体蛋白或乳糜微粒携带。
Hormones secreted by endocrine glands are carried by the blood to their target organs. For instance, insulin and glucagon travel from the pancreas to the liver and muscles to regulate blood glucose concentration. Urea, produced by the liver during the breakdown of excess amino acids, is transported in the plasma to the kidneys for excretion.
内分泌腺分泌的激素通过血液运送至靶器官。例如，胰岛素和胰高血糖素从胰腺运往肝脏
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