📚 3.2 Transport in Animals | 动物体内运输 考点突破
In this topic we cover the essential concepts of circulatory systems, heart function, blood vessels, oxygen and carbon dioxide transport, and tissue fluid formation. Master these points to tackle transport in animals questions confidently.
本主题涵盖了循环系统、心脏功能、血管、氧气与二氧化碳运输以及组织液形成等核心概念。掌握这些要点，即可从容应对动物体内运输相关考题。
1. Circulatory Systems Overview | 循环系统概览
Animals possess either an open circulatory system (e.g. in insects) where blood flows freely within the body cavity, or a closed circulatory system (e.g. in mammals) where blood is confined to vessels throughout its journey. The mammalian closed system is a double circulation: the pulmonary circuit carries blood to the lungs for gas exchange, while the systemic circuit delivers oxygenated blood to body tissues. This separation ensures high pressure and efficient oxygen delivery.
动物拥有开放式循环系统（如昆虫，血液在体腔内自由流动）或闭管式循环系统（如哺乳动物，血液始终在血管内流动）。哺乳动物的闭管式循环为双循环：肺循环将血液运送至肺部进行气体交换，体循环则将含氧血输送到全身组织。这种分隔保证了高压与高效的氧气输送。
Key features of mammalian transport include a muscular heart with chambers, valves to prevent backflow, and distinct arteries, veins and capillaries. The double circulation prevents mixing of oxygenated and deoxygenated blood, maximising oxygen supply to respiring cells.
哺乳动物运输系统的关键特征包括具有腔室的肌肉质心脏、防止回流的瓣膜以及明确的动脉、静脉和毛细血管。双循环防止了含氧血与缺氧血的混合，最大程度为呼吸细胞供应氧气。
2. The Mammalian Heart | 哺乳动物心脏结构
The heart has four chambers: two thin-walled atria that receive blood and two thick-walled ventricles that pump blood out. The left ventricle wall is thicker than the right because it must generate high pressure to propel blood through the entire systemic circuit. The right ventricle pumps only to the lungs, a shorter and lower-resistance path.
心脏有四腔：两个壁较薄的心房负责接纳血液，两个壁厚的心室负责泵血。左心室壁比右心室更厚，因为它需要产生高压将血液推向整个体循环；右心室仅将血液泵向距离短、阻力小的肺部。
- Atrioventricular (AV) valves (tricuspid on the right, bicuspid/mitral on the left) lie between atria and ventricles, anchored by chordae tendineae and papillary muscles to prevent inversion.
- 房室瓣（右侧三尖瓣，左侧二尖瓣）位于心房与心室之间，通过腱索和乳头肌固定，防止瓣膜翻转。
- Semilunar valves guard the exits to the pulmonary artery and aorta, preventing backflow from the arteries into the ventricles.
- 半月瓣分别守在肺动脉与主动脉出口，阻止血液从动脉倒流回心室。
The septum separates the left and right sides completely, crucial for keeping oxygenated and deoxygenated blood apart.
房间隔和室间隔将左右两侧完全分隔，这对于隔离含氧血与缺氧血至关重要。
3. Cardiac Cycle and ECG | 心动周期与心电图
The cardiac cycle consists of three main phases: atrial systole, ventricular systole and diastole. During atrial systole, the atria contract, pushing remaining blood into the ventricles. Ventricular systole follows; the ventricles contract, closing AV valves (producing the first heart sound ‘lub’) and opening semilunar valves to eject blood. In diastole, the heart muscle relaxes, semilunar valves close (second sound ‘dub’) and the atria start to fill again.
心动周期包括三个主要阶段：心房收缩期、心室收缩期和舒张期。心房收缩时，心房将残余血液压入心室；随后心室收缩，房室瓣关闭（产生第一心音“lub”），半月瓣打开，血液射出。舒张期心肌放松，半月瓣关闭（第二心音“dub”），心房重新开始充血。
The coordinated contraction originates from the sinoatrial node (SAN) – the pacemaker. Excitation spreads across the atria, then converges at the atrioventricular node (AVN), which delays the impulse before it travels down the Bundle of His and Purkinje fibres to cause ventricular contraction. An electrocardiogram (ECG) traces these electrical events: the P wave represents atrial depolarisation, the QRS complex ventricular depolarisation, and the T wave ventricular repolarisation.
协调的收缩源自窦房结（SAN，起搏点）。兴奋波传遍心房，汇至房室结（AVN），在此稍有延迟后沿希氏束和浦肯野纤维下传，引发心室收缩。心电图（ECG）记录这些电事件：P波代表心房除极，QRS波群代表心室除极，T波表示心室复极。
4. Control of Heart Rate | 心率调控
Heart rate is modulated by the autonomic nervous system and hormones. The cardiovascular centre in the medulla oblongata sends impulses via sympathetic nerves (releasing noradrenaline) to increase rate and force, while parasympathetic vagus nerves (releasing acetylcholine) decrease heart rate. Adrenaline from the adrenal glands also stimulates the SAN to raise heart rate, preparing the body for ‘fight or flight’.
心率受自主神经系统和激素调节。延髓的心血管中枢通过交感神经（释放去甲肾上腺素）提高心率与收缩力，而副交感迷走神经（释放乙酰胆碱）降低心率。肾上腺分泌的肾上腺素也会刺激窦房结，加快心率，为“战或逃”反应做准备。
Factors such as blood pressure, pH, CO₂ concentration and O₂ levels are detected by baroreceptors and chemoreceptors. For instance, during exercise, increased CO₂ causes a fall in pH, detected by chemoreceptors in the aorta and carotid arteries, leading to increased heart rate and ventilation to restore homeostasis.
血压、pH、CO₂浓度及O₂水平等因素被压力感受器和化学感受器感知。例如运动时CO₂升高导致pH下降，主动脉和颈动脉的化学感受器检测到此变化，促使心率加快、通气增强，以恢复稳态。
5. Blood Vessel Structure-Function | 血管结构与功能
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Wall thickness | Thick (elastic+muscle) | Thinner, less muscle | One cell thick |
| Valves | Absent | Present (prevent backflow) | Absent |
| Lumen diameter | Narrow, keeps high pressure | Wide, low resistance | Very narrow (one RBC) |
| Function | Carry blood away from heart at high pressure | Return blood to heart at low pressure | Exchange of substances |
Arteries have thick walls rich in elastin and smooth muscle to withstand and maintain high pressure. Elastin allows recoil after pulse, smoothing blood flow. Veins have thinner walls but wide lumens and pocket valves that prevent backflow aided by skeletal muscle contraction. Capillaries possess walls of a single layer of endothelial cells on a basement membrane, minimising diffusion distance and allowing exchange via pores.
动脉壁厚，富含弹性蛋白和平滑肌，以承受并维持高压；弹性回缩可平滑血流。静脉壁较薄，但管腔宽大，具有袋状瓣膜防止血液倒流，骨骼肌收缩可辅助推动。毛细血管壁仅由一层内皮细胞和基底膜构成，极大缩短了扩散距离，并通过微孔进行交换。
6. Blood Composition | 血液成分
Blood comprises plasma (~55%) and formed elements (~45%). Plasma consists mostly of water with dissolved proteins (albumin, fibrinogen, globulins), nutrients, waste products, hormones and ions. It serves as a transport medium and maintains osmotic balance. Red blood cells (erythrocytes) lack a nucleus and are biconcave discs, packed with the haemoglobin protein for oxygen transport – their shape provides a large surface area for gas exchange. White blood cells (leucocytes) are nucleated and defend against pathogens (phagocytes and lymphocytes). Platelets are small cell fragments that initiate blood clotting.
血液由约55%的血浆和45%的有形成分组成。血浆主要含水，并溶解了白蛋白、纤维蛋白原、球蛋白等蛋白质，以及养分、废物、激素和离子，充当运输介质并维持渗透平衡。红细胞（红细胞）无核，呈双凹圆盘状，内充满血红蛋白，这种形状为气体交换提供了较大的表面积。白细胞有核，负责免疫防御（吞噬细胞和淋巴细胞）。血小板是小的细胞碎片，参与凝血过程。
The production of red blood cells (erythropoiesis) is stimulated by the hormone erythropoietin (EPO) released from kidneys in response to low oxygen levels. At high altitudes, increased EPO boosts RBC count to improve oxygen carrying capacity.
红细胞的生成（红细胞生成）受促红细胞生成素（EPO）刺激，当氧气水平降低时肾脏分泌EPO。高海拔地区EPO增多，提高红细胞数量，增强携氧能力。
7. Oxygen Transport and Haemoglobin | 氧气运输与血红蛋白
Oxygen is carried in red blood cells reversibly bound to haemoglobin (Hb). Each Hb molecule contains four haem groups, each with an Fe²⁺ ion that can bind one O₂ molecule. This forms oxyhaemoglobin:
Hb + 4O₂ ⇌ Hb(O₂)₄
Oxygen binds in the lungs where partial pressure of oxygen (pO₂) is high, and dissociates in respiring tissues where pO₂ is low. The oxygen dissociation curve is S‑shaped (sigmoidal) due to cooperative binding – binding of the first O₂ facilitates subsequent binding. Conversely, as O₂ is released, it becomes progressively easier for the remaining O₂ to dissociate.
氧气在红细胞内与血红蛋白（Hb）可逆结合。每个Hb分子含四个血红素基团，每个基团有一个Fe²⁺离子可结合一个O₂分子，形成氧合血红蛋白。氧气在肺部高pO₂环境下结合，在呼吸组织低pO₂处解离。由于协同结合效应，氧解离曲线呈S形——第一个O₂的结合促进后续结合，同样地，随着O₂的解离，剩余O₂也更易释放。
The Bohr effect shifts the curve to the right: an increase in CO₂ concentration leads to a lower pH (more H⁺), which decreases haemoglobin’s affinity for oxygen, enhancing unloading at tissues. Other factors that favour unloading (right shift) include increased temperature and higher 2,3‑BPG concentration.
波尔效应使曲线右移：CO₂浓度升高导致pH下降（H⁺增多），降低了血红蛋白对氧气的亲和力，从而促进组织处的氧气释放。其他促进卸载的因素（曲线右移）还包括温度升高和2,3‑BPG浓度增加。
8. Carbon Dioxide Transport | 二氧化碳运输
Carbon dioxide produced by cells is transported in the blood in three ways: dissolved in plasma (~5%), bound to haemoglobin as carbaminohaemoglobin (~10%), and most importantly as bicarbonate ions (~85%). In red blood cells, the enzyme carbonic anhydrase catalyses the reaction:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
The H⁺ are buffered by haemoglobin, forming reduced haemoglobin (HHb), while bicarbonate ions diffuse out of the red blood cell into plasma, exchanged for chloride ions (chloride shift) to maintain electrical neutrality. In the lungs, the reverse reaction occurs: HCO₃⁻ re‑enters red cells, combines with H⁺ to form CO₂ and H₂O, and CO₂ diffuses out into alveoli to be exhaled.
细胞产生的二氧化碳通过三种方式在血液中运输：约5%直接溶于血浆，约10%与血红蛋白结合成碳氨甲血红蛋白，最主要的约85%转化成碳酸氢根离子。在红细胞内，碳酸酐酶催化反应：CO₂与H₂O生成碳酸，并迅速解离为H⁺和HCO₃⁻。H⁺被血红蛋白缓冲形成还原血红蛋白（HHb），而碳酸氢根扩散出红细胞进入血浆，同时氯离子内移（氯转移）以维持电中性。在肺部发生逆反应：HCO₃⁻重入红细胞，与H⁺结合生成CO₂和H₂O，CO₂扩散至肺泡呼出。
9. Tissue Fluid and Lymph Formation | 组织液与淋巴形成
At the arterial end of capillaries, a higher hydrostatic pressure (blood pressure) forces fluid out through capillary pores, forming tissue fluid that bathes cells. This fluid contains water, glucose, amino acids, oxygen and ions but lacks large plasma proteins and cells. At the venous end, hydrostatic pressure falls, and the osmotic pull of plasma proteins (colloid osmotic pressure) draws most of the fluid back into the capillary. The balance between filtration and reabsorption is described by Starling’s forces.
在毛细血管的动脉端，较高的流体静力压（血压）将液体压出毛细血管，形成组织液浸浴细胞。组织液含有水、葡萄糖、氨基酸、氧气和离子，但缺乏大分子血浆蛋白和细胞。在静脉端，流体静力压下降，血浆蛋白产生的胶体渗透压将大部分液体重新拉回毛细血管。滤过与重吸收的平衡由斯塔林力描述。
The small amount of tissue fluid that is not reabsorbed drains into blind‑ended lymph capillaries, forming lymph. Lymph is returned to the blood circulation via lymphatic vessels and the thoracic duct to the subclavian veins. Lymph nodes filter pathogens and house lymphocytes, playing a role in immune defence.
未被重吸收的少量组织液进入盲端的淋巴毛细管，形成淋巴。淋巴经淋巴管和胸导管回流至锁骨下静脉，回到血液循环。淋巴结过滤病原体并驻留淋巴细胞，发挥免疫防御作用。
10. Cardiovascular Disease Risk Factors | 心血管疾病危险因素
Cardiovascular diseases such as coronary heart disease and stroke are often linked to atherosclerosis – the build‑up of fatty plaques (atheroma) under the endothelium of arteries. Plaque formation narrows the lumen, restricts blood flow and increases the risk of thrombosis. If a coronary artery becomes blocked, the heart muscle is deprived of oxygen, leading to myocardial infarction (heart attack).
心血管疾病如冠心病和中风常与动脉粥样硬化相关——脂质斑块（粥样斑块）在动脉内膜下积聚。斑块使管腔变窄，血流受阻，增加血栓形成风险。若冠状动脉堵塞，心肌缺氧，导致心肌梗死（心脏病发作）。
Major risk factors include high blood cholesterol (especially LDL), hypertension, smoking, obesity, physical inactivity and diabetes. Lifestyle changes and medications (statins, anticoagulants) can reduce risk. Understanding these factors allows evaluation of epidemiological data.
主要危险因素包括高血胆固醇（尤其是LDL）、高血压、吸烟、肥胖、缺乏运动和糖尿病。改变生活方式及使用药物（他汀类、抗凝剂）可降低风险。理解这些因素有助于分析流行病学数据。
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