IB Biology: Blood Circulation Key Points | IB 生物：血液循环考点精讲

📚 IB Biology: Blood Circulation Key Points | IB 生物：血液循环考点精讲

The circulatory system is a fundamental topic in IB Biology, linking structure to function at every level from molecular oxygen transport to whole-organism homeostasis. A strong grasp of the heart’s myogenic control, vessel adaptations, and the composition of blood is essential for high marks on Paper 1, Paper 2, and the internal assessment.

循环系统是 IB 生物的基础主题，它将从分子水平的氧气运输到整体稳态的各个层面的结构与功能联系在一起。牢固掌握心脏的肌源性控制、血管适应以及血液组成，对于在试卷一、试卷二和内部评估中取得高分至关重要。

1. Heart Structure and Function | 心脏结构与功能

The human heart is a four-chambered, double pump. The right atrium and right ventricle handle deoxygenated blood returning from the body, pumping it to the lungs via the pulmonary artery. The left atrium and left ventricle receive oxygenated blood from the lungs and pump it through the aorta to all systemic tissues. The left ventricular wall is significantly thicker than the right, because it must generate higher pressure to overcome total peripheral resistance in the systemic circuit.

人类心脏是一个四腔室的双泵。右心房和右心室处理从身体回流的缺氧血，通过肺动脉泵入肺部。左心房和左心室接收来自肺部的富氧血，并通过主动脉泵到所有体循环组织。左心室壁明显比右心室厚，因为它必须产生更高压力来克服体循环中的总外周阻力。

Four valves ensure unidirectional blood flow. The atrioventricular (AV) valves (tricuspid on the right, bicuspid/mitral on the left) close during ventricular contraction, preventing backflow into the atria. The semilunar valves (pulmonary and aortic) prevent blood from flowing back into the ventricles when the heart relaxes. The ‘lub-dup’ heart sounds correspond to the closing of these valves.

四个瓣膜确保血液单向流动。房室瓣（右侧三尖瓣，左侧二尖瓣）在心室收缩时关闭，防止血液回流心房。半月瓣（肺动脉瓣和主动脉瓣）防止心脏舒张时血液流回心室。”lub-dup” 心音正是这些瓣膜关闭的声音。

2. The Cardiac Cycle | 心动周期

One complete heartbeat consists of atrial systole, ventricular systole, and diastole. During atrial systole, the atria contract, forcing remaining blood into the ventricles. Then ventricular systole begins: the ventricles contract, AV valves snap shut (first heart sound), and semilunar valves open as pressure exceeds that in the arteries. In diastole, the whole heart relaxes, semilunar valves close (second heart sound), and the atria passively fill, preparing for the next cycle.

一次完整心跳包括心房收缩期、心室收缩期和舒张期。在心房收缩期，心房收缩，将残余血液挤入心室。然后心室收缩期开始：心室收缩，房室瓣猛然关闭（第一心音），当心室内压力超过动脉压力时半月瓣打开。在舒张期，整个心脏舒张，半月瓣关闭（第二心音），心房被动充盈，为下一个周期做准备。

Stroke volume (SV) is the volume of blood ejected by each ventricle per beat, and heart rate (HR) is beats per minute. Their product gives cardiac output:

每搏输出量 (SV) 是每个心室每次搏动射出的血量，心率 (HR) 是每分钟心跳次数。它们的乘积就是心输出量：

Cardiac Output = Stroke Volume × Heart Rate

心输出量 = 每搏输出量 × 心率

3. Myogenic Muscle and the Pacemaker | 肌源性肌肉与起搏器

Cardiac muscle is myogenic — it contracts on its own without nervous stimulation. The pacemaker is the sinoatrial node (SAN) located in the wall of the right atrium. The SAN spontaneously generates electrical impulses that spread across both atria, causing atrial systole. The signal then passes to the atrioventricular node (AVN), which delays it briefly to allow complete ventricular filling. Impulses then travel down the Bundle of His and Purkinje fibres, triggering coordinated ventricular contraction from the apex upward.

心肌是肌源性的——它无需神经刺激就能自主收缩。起搏器是位于右心房壁的窦房结 (SAN)。SAN 自发产生电冲动，传遍两个心房，引起心房收缩。信号随后传到房室结 (AVN)，在此处短暂延迟以确保心室完全充盈。冲动再沿希氏束和浦肯野纤维传导，从心尖向上引发协调的心室收缩。

Although the heart beats autonomously, the sympathetic and parasympathetic nervous systems modulate rate. Sympathetic stimulation (via noradrenaline) increases heart rate and force, while parasympathetic stimulation (via the vagus nerve releasing acetylcholine) slows it down. This autonomic control is vital during exercise and rest.

尽管心脏自主跳动，交感神经系统和副交感神经系统会调节心率。交感刺激（通过去甲肾上腺素）提高心率和收缩力，副交感刺激（通过迷走神经释放乙酰胆碱）则使其减慢。这种自主控制在运动和休息时至关重要。

4. Blood Vessels: Arteries, Capillaries, Veins | 血管：动脉、毛细血管、静脉

Arteries carry blood away from the heart under high pressure. Their walls are thick with smooth muscle and elastic fibres, allowing them to stretch during systole and recoil during diastole, maintaining a relatively constant flow. Arterioles, small branches, control blood distribution through vasoconstriction and vasodilation.

动脉将血液在高压下输离心脏。其管壁厚，富含平滑肌和弹性纤维，能在心脏收缩期扩张、舒张期回弹，维持相对稳定的血流。小动脉通过血管收缩和血管舒张来控制血液分配。

Capillaries are the site of exchange. Their walls consist of a single layer of endothelial cells, providing a very short diffusion distance. Narrow diameter (about 7–8 µm) forces red blood cells to pass in single file, maximising surface contact and diffusion efficiency. Tissue fluid formation occurs at the arterial end of capillaries, where hydrostatic pressure exceeds osmotic pressure.

毛细血管是物质交换的场所。它们的管壁仅由单层内皮细胞构成，提供了极短的扩散距离。管径窄（约 7–8 µm），迫使红细胞成单行通过，最大化表面接触和扩散效率。组织液在毛细血管动脉端形成，此处静水压超过渗透压。

Veins return blood to the heart at low pressure. They have thinner walls, a larger lumen, and valves to prevent backflow. Skeletal muscle contraction compresses veins, pushing blood upward; this is the skeletal muscle pump. Venous return is crucial for refilling the atria.

静脉在低压下将血液送回心脏。它们的管壁较薄、管腔较大，并有瓣膜防止回流。骨骼肌收缩挤压静脉，将血液向上推送；这便是骨骼肌泵。静脉回流对心房再充盈至关重要。

5. Blood Composition | 血液组成

Blood is a connective tissue made of plasma (about 55%) and formed elements (about 45%). Plasma is roughly 90% water and contains dissolved proteins (albumin, globulins, fibrinogen), glucose, amino acids, ions, hormones, and waste products. Formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).

血液是一种结缔组织，由血浆（约 55%）和有形成分（约 45%）组成。血浆约 90% 是水，含有溶解的蛋白质（白蛋白、球蛋白、纤维蛋白原）、葡萄糖、氨基酸、离子、激素和废物。有形成分包括红细胞、白细胞和血小板。

Erythrocytes are biconcave discs without a nucleus when mature, maximising surface area for oxygen transport. They contain haemoglobin, which binds and releases oxygen. Leukocytes form part of the immune system; some are phagocytic, others produce antibodies. Platelets are small cell fragments that release clotting factors when a vessel is damaged, initiating the clotting cascade to convert fibrinogen into fibrin.

红细胞是双凹圆盘状，成熟后无细胞核，最大化氧气运输的表面积。它们含有血红蛋白，能结合和释放氧气。白细胞是免疫系统的一部分；有些具吞噬作用，有些产生抗体。血小板是小的细胞碎片，当血管受损时释放凝血因子，启动凝血级联反应，将纤维蛋白原转化为纤维蛋白。

6. Transport of Oxygen and Carbon Dioxide | 氧气与二氧化碳的运输

Oxygen is transported in two forms: about 1.5% dissolved in plasma and 98.5% bound to haemoglobin as oxyhaemoglobin. Each haemoglobin molecule can bind up to four O₂ molecules cooperatively; the binding of one O₂ increases the affinity for the next (the Bohr effect describes how increased CO₂ and lower pH reduce haemoglobin’s affinity for oxygen, promoting release at respiring tissues).

氧气以两种形式运输：约 1.5% 溶解在血浆中，98.5% 与血红蛋白结合形成氧合血红蛋白。每个血红蛋白分子最多可协同结合四个 O₂ 分子；第一个 O₂ 的结合会提高对后续 O₂ 的亲和力（波尔效应描述了 CO₂ 增加和 pH 降低如何降低血红蛋白对氧的亲和力，促进在呼吸组织处的释放）。

Carbon dioxide is carried in three ways: about 7% dissolved in plasma, 23% bound to haemoglobin as carbaminohaemoglobin, and 70% converted to hydrogen carbonate ions (HCO₃⁻) inside red blood cells. The enzyme carbonic anhydrase catalyses the reaction CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. The chloride shift maintains electrical neutrality as HCO₃⁻ moves out of the cell in exchange for Cl⁻.

二氧化碳以三种方式运输：约 7% 溶解在血浆中，23% 与血红蛋白结合为氨基甲酰血红蛋白，70% 在红细胞内转化为碳酸氢根离子 (HCO₃⁻)。碳酸酐酶催化反应 CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻。氯离子转移维持电中性，HCO₃⁻ 离开细胞，Cl⁻ 进入交换。

7. The Lymphatic System | 淋巴系统

The lymphatic system is a network of vessels that returns excess tissue fluid (now called lymph) to the bloodstream. Lymph capillaries are blind-ended, permeable vessels that collect fluid, proteins, and fats (as chylomicrons) from the interstitial space. Lymph passes through lymph nodes, where pathogens are filtered and immune responses are activated by lymphocytes. Eventually, lymph drains into the subclavian veins, re-entering blood circulation.

淋巴系统是一个将过剩组织液（此时称为淋巴）送回血液的血管网络。淋巴毛细管是盲端、通透的血管，从组织间隙收集液体、蛋白质和脂肪（以乳糜微粒形式）。淋巴流经淋巴结，在这里病原体被过滤，淋巴细胞启动免疫应答。最终，淋巴汇入锁骨下静脉，重新进入血液循环。

Unlike blood, lymph is not actively pumped; it moves via smooth muscle contractions in larger lymph vessels, one-way valves, and the skeletal muscle pump. The return of interstitial fluid is critical to maintain blood volume and prevent oedema.

与血液不同，淋巴不是主动泵送的；它通过较大淋巴管中的平滑肌收缩、单向瓣膜以及骨骼肌泵来流动。组织液的回流对维持血容量和防止水肿至关重要。

8. Blood Pressure and Its Regulation | 血压及其调节

Blood pressure (BP) is the force exerted by blood against vessel walls, typically measured in the brachial artery as systolic pressure over diastolic pressure (e.g., 120/80 mmHg). Systolic pressure occurs during ventricular contraction; diastolic pressure is the residual pressure when the ventricles relax. Mean arterial pressure is the driving force for tissue perfusion.

血压 (BP) 是血液对血管壁施加的压力，通常在肱动脉测量，表示为收缩压/舒张压（例如 120/80 mmHg）。收缩压出现在心室收缩期间；舒张压是心室舒张时的残余压力。平均动脉压是组织灌注的驱动力。

Baroreceptors in the carotid sinus and aortic arch detect stretch caused by pressure changes. When BP rises, they send more impulses to the medulla oblongata, which triggers parasympathetic activation (slows heart rate) and reduces sympathetic output, leading to vasodilation and lower BP. When BP drops, reflex sympathetic stimulation increases heart rate, stroke volume, and vasoconstriction, raising BP back to normal.

颈动脉窦和主动脉弓中的压力感受器检测压力变化引起的牵张。当血压升高，它们向延髓发送更多冲动，触发副交感激活（减慢心率）并减少交感输出，导致血管舒张和血压降低。当血压下降，反射性交感刺激提高心率、每搏输出量并引起血管收缩，将血压回升至正常。

9. Coronary Circulation and Heart Disease | 冠状循环与心脏病

The heart has its own blood supply via the coronary arteries, which branch off the aorta and encircle the heart. These vessels deliver oxygen and nutrients to the thick cardiac muscle. If a coronary artery becomes narrowed or blocked by atherosclerosis — the build-up of fatty plaques — the downstream myocardium receives insufficient oxygen, leading to chest pain (angina) or a heart attack (myocardial infarction) if tissue dies.

心脏通过冠状动脉获得自身血液供应，冠状动脉从主动脉分支并环绕心脏。这些血管将氧气和营养物质输送到厚实的心肌。如果冠状动脉因动脉粥样硬化（脂肪斑块堆积）变窄或阻塞，下游心肌就会缺氧，导致胸痛（心绞痛），若组织坏死则引发心脏病发作（心肌梗死）。

Risk factors for coronary heart disease include high blood cholesterol (especially LDL), hypertension, smoking, diabetes, obesity, and physical inactivity. A diet high in saturated fats and trans fats raises LDL levels, while a Mediterranean-style diet rich in unsaturated fats and fibre can lower risk. Lifestyle changes and medications like statins are used to manage the condition.

冠心病的风险因素包括高血脂胆固醇（尤其是低密度脂蛋白）、高血压、吸烟、糖尿病、肥胖和缺乏运动。富含饱和脂肪和反式脂肪的饮食会升高 LDL 水平，而富含不饱和脂肪和纤维的地中海式饮食可降低风险。通过改变生活方式和他汀类药物等来管理此症。

10. Fetal Circulation and Adaptations at Birth | 胎儿循环与出生时的适应

The fetal circulatory system is adapted for a liquid environment where the lungs are non-functional. The placenta provides oxygenated blood via the umbilical vein. To bypass the lungs, two shunts exist: the foramen ovale, an opening between the right and left atria, and the ductus arteriosus, a vessel connecting the pulmonary artery to the aorta. The ductus venosus bypasses the fetal liver, directing blood from the umbilical vein toward the heart.

胎儿循环系统适应肺部不工作的液体环境。胎盘通过脐静脉提供富氧血。为绕过肺部，存在两个分流：卵圆孔（右心房与左心房之间的开口）和动脉导管（连接肺动脉与主动脉的血管）。静脉导管绕过胎儿肝脏，将脐静脉血液导向心脏。

At birth, the first breaths inflate the lungs, drastically reducing pulmonary resistance and increasing oxygen levels. The foramen ovale closes due to pressure reversal (higher pressure in the left atrium than the right), becoming the fossa ovalis. The ductus arteriosus constricts in response to rising oxygen tension, eventually degenerating into the ligamentum arteriosum. The ductus venosus closes and becomes the ligamentum venosum. These changes establish the mature, independent circulatory pattern.

出生时，第一口呼吸使肺部扩张，大幅降低肺循环阻力并提高血氧水平。卵圆孔因压力逆转（左心房压力高于右心房）而关闭，变成卵圆窝。动脉导管因氧分压升高而收缩，最终退化为动脉韧带。静脉导管关闭并变为静脉韧带。这些变化建立起成熟、独立的循环模式。

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IB Biology: Blood Circulation Key Points | IB 生物：血液循环 考点精讲