📚 The Mammalian Cardiovascular System | 哺乳动物心血管系统
Mammals have evolved a highly efficient circulatory system that delivers oxygen and nutrients to every cell while removing carbon dioxide and metabolic waste. Understanding its structure and function is central to A-Level Biology, as it links anatomy, physiology, gas exchange and disease.
哺乳动物进化出了高效的心血管系统，能够为每一个细胞输送氧气和营养物质，同时清除二氧化碳和代谢废物。理解其结构与功能是 A-Level 生物学的核心内容，因为它将解剖学、生理学、气体交换和疾病联系在一起。
1. Overview of the Mammalian Circulatory System | 哺乳动物循环系统概览
Mammals possess a closed, double circulatory system in which blood is confined to vessels and passes through the heart twice during one complete circuit of the body.
哺乳动物具有封闭式双循环系统，血液始终限制在血管内流动，并在一次完整的全身循环中两次经过心脏。
This arrangement keeps oxygenated and deoxygenated blood separate, maintaining a steep concentration gradient for efficient gas exchange in the lungs and tissues.
这种结构使含氧血和脱氧血保持分离，从而在肺部和组织中维持了较高的浓度梯度，有利于高效的气体交换。
The double circulation consists of the pulmonary circuit, which carries blood between the heart and lungs, and the systemic circuit, which carries blood between the heart and the rest of the body.
双循环由肺循环和体循环组成：肺循环在心脏与肺之间运输血液，体循环在心脏与全身其他组织之间运输血液。
2. Double Circulation: Pulmonary and Systemic Circuits | 双循环：肺循环与体循环
In the pulmonary circuit, deoxygenated blood leaves the right ventricle through the pulmonary artery and travels to the lungs. Here carbon dioxide diffuses out of the blood and oxygen diffuses in, so the blood becomes oxygenated.
在肺循环中，脱氧血从右心室经肺动脉流向肺部。在肺部，二氧化碳从血液中扩散出去，氧气扩散进入血液，因此血液转变为含氧血。
The oxygenated blood returns to the left atrium through the pulmonary vein. From the left atrium it enters the left ventricle, which pumps it into the aorta and around the systemic circuit to supply every organ.
含氧血通过肺静脉回到左心房，再从左心房进入左心室，左心室将血液泵入主动脉，并沿体循环供应全身各个器官。
After delivering oxygen to respiring tissues, the now deoxygenated blood returns to the right atrium through the vena cava, completing the double circuit.
在向进行呼吸作用的组织释放氧气后，血液变为脱氧血，通过腔静脉回到右心房，从而完成双循环。
Because the systemic circuit is much longer and offers greater resistance, the left ventricle has a thicker muscular wall than the right ventricle.
由于体循环路径更长、阻力更大，左心室的心肌壁比右心室更厚。
3. Structure of the Heart | 心脏的结构
The mammalian heart is a muscular organ with four chambers: the right atrium, right ventricle, left atrium and left ventricle. A central septum separates the left and right sides, preventing mixing of oxygenated and deoxygenated blood.
哺乳动物的心脏是一个肌肉器官，具有四个腔室：右心房、右心室、左心房和左心室。中央的室间隔将左右两侧分开，防止含氧血和脱氧血混合。
The atria are thin-walled chambers that receive blood returning to the heart, while the ventricles have thicker muscular walls because they must generate enough pressure to pump blood out of the heart.
心房壁较薄，负责接收回流到心脏的血液；心室壁较厚，因为心室必须产生足够的压力将血液泵出心脏。
The left ventricle wall is the thickest because it must overcome the high resistance of the whole systemic circulation, whereas the right ventricle only drives blood through the lower-resistance pulmonary circulation.
左心室壁最厚，因为它必须克服整个体循环的高阻力；而右心室仅需要推动血液通过阻力较低的肺循环。
Four valves ensure one-way flow of blood: the tricuspid valve between the right atrium and right ventricle, the bicuspid (mitral) valve between the left atrium and left ventricle, and two semilunar valves at the exits of the ventricles.
四个瓣膜确保血液单向流动：右心房与右心室之间的三尖瓣、左心房与左心室之间的二尖瓣，以及两个心室出口处的半月瓣。
4. Cardiac Cycle and Heart Valves | 心动周期与心脏瓣膜
The cardiac cycle is the sequence of events in one complete heartbeat. It consists of atrial systole, ventricular systole and diastole of the whole heart.
心动周期是一次完整心跳中发生的一系列事件，包括心房收缩、心室收缩和全心舒张。
During atrial systole, the atria contract and push blood into the relaxed ventricles through the open atrioventricular valves. During ventricular systole, the ventricles contract, causing the atrioventricular valves to close and the semilunar valves to open, ejecting blood into the arteries.
在心房收缩期，心房收缩，将血液通过开放的房室瓣推入舒张的心室。在心室收缩期，心室收缩，使房室瓣关闭、半月瓣打开，将血液射入动脉。
During diastole, the ventricles relax, the semilunar valves close to prevent backflow, and the atrioventricular valves open so the ventricles can fill again.
在舒张期，心室舒张，半月瓣关闭以防止血液倒流，同时房室瓣打开，使心室再次充盈。
Valve closure produces the characteristic heart sounds: the first sound is caused by closure of the atrioventricular valves, and the second sound by closure of the semilunar valves.
瓣膜关闭产生特征性的心音：第一心音由房室瓣关闭引起，第二心音由半月瓣关闭引起。
Cardiac cycle = atrial systole → ventricular systole → diastole
心动周期 = 心房收缩 → 心室收缩 → 舒张期
5. Coordination of the Heartbeat | 心跳的协调
The heartbeat is myogenic, meaning it originates within the heart muscle itself rather than from external nerves. The sinoatrial node (SAN) in the wall of the right atrium acts as the natural pacemaker.
心脏搏动是肌源性的，即搏动起源于心肌自身，而不是来自外部神经。右心房壁中的窦房结（SAN）发挥天然起搏器的作用。
The SAN initiates a wave of electrical excitation that spreads across both atria, causing them to contract. The excitation then reaches the atrioventricular node (AVN) located near the junction of the atria and ventricles.
窦房结发出电兴奋波，传遍两侧心房，引起心房收缩。随后兴奋传至位于心房与心室交界附近的房室结（AVN）。
From the AVN, the impulse travels along the bundle of His and Purkinje fibres down the interventricular septum and up the ventricle walls, ensuring coordinated ventricular contraction from the apex upwards.
冲动从房室结沿房室束和浦肯野纤维传导，经室间隔向下，再沿心室壁向上传导，确保心室从心尖向上协调收缩。
An electrocardiogram (ECG) records these electrical events. The P wave represents atrial depolarisation, the QRS complex represents ventricular depolarisation, and the T wave represents ventricular repolarisation.
心电图（ECG）记录这些电活动。P 波代表心房去极化，QRS 波群代表心室去极化，T 波代表心室复极化。
6. Cardiac Output and Its Control | 心输出量及其调控
Cardiac output is the volume of blood pumped by each ventricle per minute. It is calculated as the product of heart rate and stroke volume.
心输出量是每个心室每分钟泵出的血液体积，其计算公式为心率与每搏输出量的乘积。
CO = HR × SV
心输出量 = 心率 × 每搏输出量
A typical resting heart rate is about 70 beats per minute and a typical stroke volume is about 70 cm³, giving a cardiac output of approximately 5 dm³ min⁻¹.
静息心率通常约为每分钟 70 次，每搏输出量约为 70 cm³，因此心输出量约为 5 dm³ min⁻¹。
Cardiac output is controlled by the autonomic nervous system. Sympathetic stimulation increases heart rate and stroke volume, while parasympathetic stimulation via the vagus nerve decreases heart rate.
心输出量受自主神经系统调控。交感神经兴奋可提高心率和每搏输出量，而经迷走神经的副交感神经兴奋则降低心率。
The hormone adrenaline also increases heart rate and stroke volume by binding to receptors on cardiac muscle cells, preparing the body for increased activity.
激素肾上腺素也能通过结合心肌细胞上的受体来提高心率和每搏输出量，使身体为活动增强做好准备。
7. Blood Vessels: Arteries, Capillaries and Veins | 血管：动脉、毛细血管和静脉
Arteries carry blood away from the heart under high pressure. Their walls are thick and elastic, containing smooth muscle and collagen, which allows them to withstand and maintain pressure.
动脉将血液在高压下运离心脏。动脉壁厚且富有弹性，含有平滑肌和胶原蛋白，使其能够承受并维持血压。
Arterioles are smaller branches of arteries. They have proportionally more smooth muscle, which enables them to constrict or dilate and therefore regulate blood flow into capillary beds.
小动脉是动脉的较小分支，其平滑肌比例更高，能够收缩或舒张，从而调节进入毛细血管床的血流量。
Capillaries are microscopic vessels whose walls consist of a single layer of endothelial cells. This minimises diffusion distance and allows efficient exchange of gases, nutrients and waste between blood and tissue fluid.
毛细血管是管壁仅由单层内皮细胞构成的微小血管。这最大限度地缩短了扩散距离，使气体、营养物质和代谢废物能够在血液与组织液之间高效交换。
Veins carry blood back to the heart at lower pressure. They have thinner walls than arteries, a larger lumen, and many possess valves that prevent the backflow of blood.
静脉在较低压力下将血液送回心脏。其管壁比动脉薄，管腔较大，许多静脉具有防止血液倒流的瓣膜。
8. Blood Pressure and Its Regulation | 血压及其调节
Blood pressure is the force exerted by circulating blood on the walls of blood vessels. It is highest in the arteries during ventricular systole and lowest in the veins.
血压是循环血液对血管壁施加的力。心室收缩期动脉中的血压最高，静脉中的血压最低。
Arterial blood pressure is usually recorded as systolic pressure over diastolic pressure, for example 120/80 mm Hg in a healthy adult.
动脉血压通常记录为收缩压与舒张压之比，例如健康成年人的血压为 120/80 mm Hg。
Blood pressure depends mainly on cardiac output and total peripheral resistance. Arteriolar constriction increases peripheral resistance and raises blood pressure, while arteriolar dilation lowers it.
血压主要取决于心输出量和总外周阻力。小动脉收缩会增加外周阻力并升高血压，小动脉舒张则会降低血压。
Baroreceptors in the carotid sinus and aortic arch detect changes in blood pressure and send signals to the cardiovascular centre in the medulla oblongata, which adjusts heart rate and arteriolar diameter to restore normal pressure.
颈动脉窦和主动脉弓中的压力感受器检测血压变化，并将信号传至延髓的心血管中枢，后者通过调节心率和小动脉直径来恢复正常血压。
BP = CO × TPR
血压 = 心输出量 × 总外周阻力
9. Blood Composition and Functions | 血液成分与功能
Blood consists of plasma, red blood cells, white blood cells and platelets. Plasma makes up about 55% of blood volume and is mainly water containing dissolved solutes such as glucose, amino acids, ions, hormones and plasma proteins.
血液由血浆、红细胞、白细胞和血小板组成。血浆约占血液体积的 55%，主要是含有葡萄糖、氨基酸、离子、激素和血浆蛋白等溶质的水溶液。
Red blood cells, or erythrocytes, are specialised for oxygen transport. They are biconcave discs, lack a nucleus in mammals, and contain the pigment haemoglobin.
红细胞又称红血球，专门用于运输氧气。哺乳动物的红细胞呈双凹圆盘状，无细胞核，并含有血红蛋白色素。
White blood cells, or leucocytes, defend the body against infection. They include phagocytes such as neutrophils and monocytes, and lymphocytes involved in the specific immune response.
白细胞又称白血球，负责防御感染。它们包括中性粒细胞和单核细胞等吞噬细胞，以及参与特异性免疫应答的淋巴细胞。
Platelets are small cell fragments that play an essential role in blood clotting. When a blood vessel is damaged, platelets stick to the exposed collagen and release clotting factors, leading to the formation of a fibrin mesh.
血小板是参与血液凝固的小细胞碎片。当血管受损时，血小板黏附在暴露的胶原上并释放凝血因子，最终形成纤维蛋白网。
10. Transport of Oxygen and Carbon Dioxide | 氧气和二氧化碳的运输
Most oxygen in the blood is carried reversibly bound to haemoglobin inside red blood cells. Each haemoglobin molecule can bind up to four oxygen molecules to form oxyhaemoglobin.
血液中的大部分氧气以可逆方式与红细胞内的血红蛋白结合运输。每个血红蛋白分子最多可以结合四个氧分子，形成氧合血红蛋白。
Hb + 4O₂ ⇌ Hb(O₂)₄
血红蛋白 + 4O₂ ⇌ 氧合血红蛋白
The oxygen dissociation curve is S-shaped because binding of the first oxygen molecule causes a conformational change in haemoglobin, making it easier for the next oxygen molecules to bind. This is called cooperative binding.
氧解离曲线呈 S 形，因为第一个氧分子的结合会引起血红蛋白构象变化，使后续氧分子更容易结合。这称为协同结合。
In respiring tissues, high carbon dioxide concentration and low pH shift the curve to the right. This Bohr effect reduces the affinity of haemoglobin for oxygen, promoting oxygen release where it is needed most.
在呼吸组织内，高二氧化碳浓度和低 pH 使曲线右移。这种波尔效应降低了血红蛋白对氧的亲和力，从而促进氧气在最需要的部位释放。
Carbon dioxide is transported in three main ways: about 5–7% dissolved in plasma, about 10% combined with haemoglobin as carbaminohaemoglobin, and about 85% as hydrogen carbonate ions in plasma.
二氧化碳主要以三种方式运输：约 5–7% 溶解在血浆中，约 10% 与血红蛋白结合形成氨基甲酰血红蛋白，约 85% 以碳酸氢根离子形式存在于血浆中。
Inside red blood cells, the enzyme carbonic anhydrase catalyses the conversion of carbon dioxide and water to carbonic acid, which dissociates into hydrogen ions and hydrogen carbonate ions. Chloride shift maintains electrical neutrality as hydrogen carbonate ions leave the cell.
在红细胞内，碳酸酐酶催化二氧化碳与水生成碳酸，碳酸再解离为氢离子和碳酸氢根离子。当碳酸氢根离子离开细胞时，氯离子转移维持了电中性。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
二氧化碳 + 水 ⇌ 碳酸 ⇌ 氢离子 + 碳酸氢根离子
11. Tissue Fluid Formation and Return | 组织液的形成与回流
Tissue fluid is formed by filtration of plasma through capillary walls. At the arteriole end of a capillary, the hydrostatic pressure of the blood is greater than the osmotic pull of plasma proteins, so fluid is pushed out.
组织液由血浆经毛细血管壁滤过形成。在毛细血管动脉端，血液的静水压大于血浆蛋白的渗透压，因此液体被推出血管。
This outward flow carries oxygen, glucose and other nutrients to the cells. At the venule end, the hydrostatic pressure has fallen, while the osmotic pressure remains largely unchanged, so most fluid re-enters the capillary by osmosis.
这种向外流动将氧气、葡萄糖和其他营养物质带到细胞。在毛细血管静脉端，静水压已经降低，而渗透压基本不变，因此大部分液体通过渗透作用重新进入毛细血管。
Not all tissue fluid is reabsorbed. The remaining 10% drains into lymphatic capillaries and is returned to the blood through the lymphatic system, preventing the build-up of excess fluid.
并非所有组织液都被重吸收。剩余约 10% 的组织液进入淋巴毛细管，并通过淋巴系统返回血液，从而防止多余液体积聚。
Fick's law describes the rate of diffusion across capillary walls: it is proportional to surface area and concentration gradient, and inversely proportional to diffusion distance.
菲克定律描述了物质跨毛细血管壁扩散的速率：它与表面积和浓度梯度成正比，与扩散距离成反比。
Rate of diffusion ∝ (surface area × concentration gradient) ÷ diffusion distance
扩散速率 ∝ (表面积 × 浓度梯度) ÷ 扩散距离
12. Cardiovascular Disease and Risk Factors | 心血管疾病与风险因素
Cardiovascular disease is a major cause of death worldwide. It includes conditions such as coronary heart disease, stroke and hypertension, all of which are linked to damage or narrowing of blood vessels.
心血管疾病是全球主要死亡原因之一，包括冠心病、中风和高血压等，这些疾病都与血管损伤或狭窄有关。
Atherosclerosis is the build-up of fatty plaques, mainly cholesterol and inflammatory cells, in the walls of arteries. This narrows the lumen and reduces blood flow, while also making the plaque prone to rupture and thrombus formation.
动脉粥样硬化是动脉壁内形成脂肪斑块（主要是胆固醇和炎性细胞）的过程。它使管腔变窄、血流减少，同时使斑块易于破裂并形成血栓。
If a coronary artery becomes blocked by a thrombus, the supply of oxygen to heart muscle is reduced or stopped, causing myocardial infarction, commonly known as a heart attack.
如果冠状动脉被血栓堵塞，心肌供氧减少或中断，就会导致心肌梗死，即通常所说的心脏病发作。
Major risk factors include smoking, high blood cholesterol, high blood pressure, obesity, diabetes, physical inactivity and a diet high in saturated fats and salt. Genetic factors also influence cardiovascular risk.
主要风险因素包括吸烟、高血胆固醇、高血压、肥胖、糖尿病、缺乏运动以及高饱和脂肪和高盐饮食。遗传因素也会影响心血管疾病的风险。
Many of these risks can be reduced through lifestyle changes such as regular exercise, a balanced diet, stopping smoking and managing stress.
通过定期运动、均衡饮食、戒烟和控制压力等生活方式改变，可以降低其中许多风险。
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