📚 Control of Heart Beat | 心脏跳动的控制
The human heart beats about 100,000 times a day, pumping blood continuously without any conscious effort. This rhythmic contraction is not triggered by external nerves but arises from within the cardiac muscle itself, a property known as myogenicity. However, the intrinsic rhythm is constantly fine-tuned by the autonomic nervous system and circulating hormones to meet the body's changing demands. Understanding the control of the heartbeat integrates the study of specialised conducting tissues, ion movements, reflex arcs, and chemical signalling, all of which are key topics in A-Level Biology.
人类心脏每天跳动约十万次，无需意识控制便能持续泵血。这种节律性收缩并非由外部神经触发，而是源于心肌本身的特性，即肌源性。然而，内在节律会持续受到自主神经系统和循环激素的精细调节，以满足身体不断变化的需求。理解心跳的控制需要综合学习特化的传导组织、离子运动、反射弧以及化学信号，这些都是A-Level生物学中的重要主题。
1. Myogenic Heart and Pacemaker Potential | 心脏的肌源性与起搏器电位
Cardiac muscle is myogenic, meaning the impulse for contraction originates within the muscle cells themselves, not from motor neurones. This is demonstrated by the sinoatrial node (SAN), a patch of specialised cells in the wall of the right atrium. The SAN cells spontaneously generate action potentials due to an unstable resting membrane potential, which drifts slowly upwards to a threshold level. This gradual depolarisation, known as the pacemaker potential, is mainly caused by the opening of funny (If) channels that allow slow Na⁺ influx, followed by the opening of T-type Ca²⁺ channels, leading to a full action potential once threshold is reached.
心肌具有肌源性，也就是说收缩的冲动起源于肌肉细胞自身，而非运动神经元。位于右心房壁上的一小团特化细胞——窦房结（SAN）便是这一特性的体现。SAN细胞由于静息膜电位不稳定，会缓慢向阈值水平漂移，从而自发产生动作电位。这种逐步的去极化称为起搏器电位，主要由奇特的（If）通道开放、允许缓慢的Na⁺内流所引发，随后T型Ca²⁺通道开放，使膜电位在达到阈值后爆发完整的动作电位。
The natural rate of SAN discharge sets the resting heart rate, typically around 60–80 beats per minute (bpm) in a healthy adult. If all neural and hormonal influences are removed, the heart will continue to beat at this intrinsic rate, providing clear evidence of myogenicity. This autonomous rhythm ensures that cardiac muscle always contracts as a functional syncytium, with atrial and ventricular chambers working in a coordinated sequence.
SAN自身放电的频率设定了静息心率，健康成人通常约为每分钟60–80次。如果去除所有神经和激素的影响，心脏仍会以这一固有频率跳动，这为肌源性提供了明确证据。这种自主节律保证了心肌始终以功能性合胞体的形式收缩，心房和心室按协调的顺序工作。
Ion fluxes around the SAN are central to its rhythmicity. During the pacemaker potential, Na⁺ enters through HCN channels, and later Ca²⁺ enters through voltage-gated channels. The rising phase of the SAN action potential is carried by Ca²⁺ influx (unlike most neurones, which use fast Na⁺ channels), and repolarisation occurs when K⁺ channels open. This interplay of ions produces a self-sustaining oscillation that can be modified by extracellular signals.
SAN周围的离子流动是其节律性的核心。在起搏器电位期间，Na⁺通过HCN通道进入，随后Ca²⁺通过电压门控通道进入。SAN动作电位的上升支主要由Ca²⁺内流承载（不同于大多数神经元使用快速的Na⁺通道），而当K⁺通道开放时发生复极化。这些离子的协同作用产生了一种可自我维持的振荡，并能被细胞外信号修饰。
2. The Sinoatrial Node as the Primary Pacemaker | 窦房结——主导起搏点
The SAN is commonly called the heart's natural pacemaker. It initiates excitation that spreads across the right and left atria through gap junctions in the intercalated discs, causing both atria to contract almost simultaneously. A wave of depolarisation then converges on the atrioventricular node (AVN), the only electrical connection between the atria and ventricles in normal hearts. The fibrous tissue of the atrioventricular septum insulates the ventricles, preventing direct spread of the impulse from the atria.
SAN通常被称为心脏的天然起搏器。它发起的兴奋通过闰盘中的缝隙连接在左右心房传播，引起两个心房几乎同时收缩。然后，去极化波汇聚于房室结（AVN），后者是正常心脏中房室之间唯一的电学连接。房室隔的纤维组织将心室绝缘，阻止了冲动从心房的直接传播。
If the SAN fails, subordinate pacemakers in the AVN or even the ventricles can take over, albeit at a slower inherent rate (40–60 bpm for AVN, 20–40 bpm for Purkinje fibres). This hierarchical backup system protects the heart from immediate standstill, though the slower rhythm may not be sufficient to sustain vigorous exercise. The SAN's dominance illustrates the concept of rhythmicity and overdrive suppression, where the fastest pacemaker normally imposes its frequency on slower ones.
如果SAN失效，位于AVN甚至心室的下级起搏点可以接替工作，尽管其固有速率较慢（AVN为40–60次/分，浦肯野纤维为20–40次/分）。这种等级化的备用系统可防止心脏立即停跳，但较慢的节律可能不足以支持激烈运动。SAN的主导作用体现了节律性和超速抑制的概念，即最快的起搏点通常将其频率强加于较慢的起搏点。
The spread of excitation from the SAN is recorded on an electrocardiogram (ECG). The P wave reflects atrial depolarisation initiated by the SAN. A healthy sinus rhythm shows consistent P waves and a regular interval between QRS complexes. Pathologies such as atrial fibrillation may arise if the SAN's control is disrupted and chaotic electrical activity takes over, underlining the crucial role of the SAN in maintaining an orderly heartbeat.
SAN引发的兴奋传播可记录在心电图（ECG）上。P波反映由SAN启动的心房去极化。正常的窦性心律显示一致的P波及QRS波群间规则的间隔。如果SAN的控制被破坏，并由混乱的电活动取代，就可能出现心房颤动等病理状态，这突显了SAN在维持有序心跳中的关键作用。
3. The Atrioventricular Node and the Delay | 房室结与延搁
The atrioventricular node (AVN) sits at the base of the right atrium near the interventricular septum. Its cells are smaller in diameter and have a lower density of gap junctions compared to atrial muscle, causing the impulse to conduct much more slowly. This AVN delay of about 0.1 seconds is essential for heart function: it allows the atria to complete their contraction and empty blood into the ventricles before the ventricles begin to contract. Without this delay, atrial and ventricular systole would overlap, reducing filling efficiency.
房室结（AVN）位于右心房底部，靠近室间隔。与心房肌相比，其细胞直径较小，缝隙连接密度也较低，这使得冲动传导速度大大减慢。AVN约0.1秒的延搁对心脏功能至关重要：它能使心房在心室开始收缩之前完成收缩并将血液排入心室。如果没有这一延搁，心房与心室收缩将会重叠，降低充盈效率。
From the AVN, the excitation wave passes into the bundle of His, a tract of conducting fibres that penetrates the fibrous septum and divides into right and left bundle branches along the interventricular septum. The bundle of His and its branches represent the only route for the impulse to travel from the atria to the ventricular myocardium. This limited pathway ensures an organised, apex-to-base contraction of the ventricles rather than random depolarisation.
从AVN开始，兴奋波传入房室束（希氏束），这是一束穿透纤维隔的传导纤维，并沿着室间隔分为右束支和左束支。希氏束及其分支是冲动从心房传到心室心肌的唯一通路。这种受限制的通路确保了心室有序地自心尖向心底收缩，而非杂乱的去极化。
The AVN also acts as a gatekeeper for excessive atrial impulses. In conditions like atrial flutter or fibrillation, the AVN filters out many rapid signals, protecting the ventricles from dangerously fast rates. This filtering capacity can be influenced by autonomic nerves, adding another layer of regulation to the heartbeat. The intentional pharmacological or ablative block of the AVN is sometimes used to treat certain arrhythmias.
AVN还充当着对过度心房冲动的“守门人”。在心房扑动或颤动等情况下，AVN会滤去许多过快信号，保护心室免于危险的高速频率。这种过滤能力可受自主神经影响，为心跳增添了又一层调节。有时治疗特定心律失常会故意用药物或消融术阻断AVN。
4. Purkinje Fibres and Ventricular Systole | 浦肯野纤维与心室收缩
After the bundle branches, the impulse is rapidly distributed through a network of Purkinje fibres, which are large, pale-staining cells with abundant glycogen and fast-conducting properties. These fibres extend throughout the inner ventricular walls and papillary muscles, ensuring that the depolarisation wave reaches the apex first. Consequently, contraction starts at the apex and pushes blood upward toward the outflow tracts, a mechanism that optimises ejection of blood into the pulmonary artery and aorta.
经过束支后，冲动通过浦肯野纤维网迅速分布。浦肯野纤维是一种大型、浅染色的细胞，富含糖原且传导速度快。这些纤维遍布心室内壁和乳头肌，确保去极化波首先抵达心尖。因此，收缩从心尖开始，将血液向上推向流出道，这一机制优化了血液向肺动脉和主动脉的射血。
The speed of conduction in Purkinje fibres is remarkable, with velocities of 2–4 m s⁻¹, compared with about 0.3–0.5 m s⁻¹ in ordinary ventricular muscle. This rapid transmission allows all parts of the ventricles to be depolarised within about 0.1 seconds, creating a coordinated and powerful systole. On the ECG, the QRS complex represents this rapid ventricular depolarisation, and a prolonged QRS duration often indicates conduction delay within the ventricular network.
浦肯野纤维的传导速度非常显著，可达2–4 m s⁻¹，而普通心室肌仅约0.3–0.5 m s⁻¹。这种快速传递使得整个心室在约0.1秒内完成去极化，产生协调而有力的收缩。在心电图上，QRS波群即代表这种快速的心室去极化，QRS时限延长常提示心室网络内的传导延迟。
Purkinje fibres also exhibit a latent pacemaker capability, as mentioned earlier. Although their intrinsic rhythm is much slower than the SAN, they can initiate escape beats if higher pacemakers fail. The extended refractory period of Purkinje and ventricular muscle cells prevents tetanic contractions and ensures the heart relaxes between beats, which is crucial for adequate ventricular filling. A triggered extra-systole can arise from a Purkinje fibre if it becomes irritable due to ischaemia or electrolyte imbalance.
如前所述，浦肯野纤维还具有潜在的起搏能力。尽管其固有节律远慢于SAN，但若上级起搏点失效，它们能发出逸搏。浦肯野纤维和心室肌细胞较长的有效不应期可防止强直收缩，并确保心脏在每次搏动间舒张，这对心室充分充盈至关重要。缺血或电解质紊乱可使浦肯野纤维应激性增高，从而引发早搏。
5. The Electrocardiogram (ECG) and the Cardiac Cycle | 心电图与心动周期
An ECG records the electrical activity of the heart, providing a convenient means to link the control of heartbeat with the mechanical events of the cardiac cycle. The P wave corresponds to atrial depolarisation triggered by the SAN; the short PR segment represents the AVN delay; the QRS complex marks ventricular depolarisation; and the T wave indicates ventricular repolarisation. Atrial repolarisation is normally hidden within the QRS complex. By analysing these waves, clinicians can infer the integrity of the heart's conduction system.
心电图记录心脏的电活动，为将心跳控制与心动周期中的机械事件联系起来提供了便捷方法。P波对应SAN触发的心房去极化；短的PR段代表AVN延搁；QRS波群标志心室去极化；T波表示心室复极化。心房复极通常被掩埋在QRS波群中。通过分析这些波，临床医生可以推断心脏传导系统的完整性。
The cardiac cycle consists of systole (contraction) and diastole (relaxation). Atrial systole follows the P wave and completes ventricular filling; the AVN delay ensures that the atrial kick finishes before the ventricles contract. Ventricular systole begins during the QRS complex and causes a sharp rise in intraventricular pressure, closing the atrioventricular valves and then opening the semilunar valves. The T wave heralds ventricular diastole, during which pressure falls, the semilunar valves close, and the ventricles fill passively from the atria.
心动周期由收缩期和舒张期组成。心房收缩紧随P波之后，完成心室充盈；AVN延搁保证了心房收缩在心室开始收缩前结束。心室收缩在QRS波群期间开始，引起心室内压迅速上升，先关闭房室瓣，随后打开半月瓣。T波预示着心室舒张，此时压力下降，半月瓣关闭，心室被动地从心房接受充盈。
Heart sounds and pressure changes are direct consequences of the electrical orchestration. The first heart sound ('lub') coincides with the closure of the AV valves at the start of ventricular systole, while the second sound ('dub') occurs with closure of the semilunar valves at the beginning of diastole. An aberrant conduction sequence, such as a ventricular ectopic beat, will disrupt the normal pressure–sound relationships and reduce cardiac output.
心音和压力变化是电编排的直接结果。第一心音（“lub”）与心室收缩开始时房室瓣的关闭重合，第二心音（“dub”）与舒张期开始时半月瓣的关闭重合。异常的传导顺序，如室性早搏，会打乱正常的压力–心音关系并降低心输出量。
6. Autonomic Nervous Control of Heart Rate | 自主神经对心率的影响
Although the heart beats intrinsically, the autonomic nervous system modulates the rate and strength of contraction to meet metabolic demands. The cardiovascular centre in the medulla oblongata integrates sensory information and sends efferent signals through sympathetic and parasympathetic pathways. This dual innervation provides a push–pull mechanism that can rapidly adjust cardiac output from as low as 40 bpm to over 200 bpm in moments of intense stress or exercise.
尽管心脏有其固有的节律，自主神经系统仍会调节心率和收缩强度以满足代谢需求。延髓中的心血管中枢整合感觉信息，并通过交感和副交感通路发送传出信号。这种双重神经支配提供了一种推挽机制，能在强烈应激或运动时瞬间将心输出量的心率从低至40次/分调整到200次/分以上。
Parasympathetic fibres reach the heart via the vagus nerves. They primarily innervate the SAN, AVN, and to a lesser extent the atria, but they have very little direct effect on the ventricles. The postganglionic neurones release acetylcholine (ACh), which binds to muscarinic M₂ receptors on the pacemaker cells. This opens potassium channels (Kᴬᶜʰ) and slows the rate of pacemaker depolarisation, thereby decreasing the heart rate – an effect known as negative chronotropy. Additionally, the AVN delay is prolonged, giving a slight negative dromotropic effect.
副交感纤维通过迷走神经到达心脏。它们主要支配SAN和AVN，并在较小程度上支配心房，但对心室的直接影响极小。节后神经元释放乙酰胆碱（ACh），与起搏细胞上的M₂毒蕈碱受体结合。这会开放钾离子通道（Kᴬᶜʰ），减慢起搏器去极化速率，从而降低心率——这是一种负性变时效应。此外，AVN延搁延长，产生轻微的负性变传导效应。
7. Sympathetic Input and the Accelerator Nerve | 交感输入与心加速神经
Sympathetic postganglionic neurones from the sympathetic chain and the accelerator nerve (cardiac nerve) release noradrenaline, which binds to β₁-adrenergic receptors on the SAN, AVN, and the ventricular myocardium. Noradrenaline increases the slope of the pacemaker potential by enhancing the If current and the Ca²⁺ influx, leading to a faster depolarisation rate and thus a higher heart rate (positive chronotropy). It also raises the force of ventricular contraction (positive inotropy) and speeds up conduction through the AVN (positive dromotropy).
来自交感链和心加速神经（心神经）的交感节后神经元释放去甲肾上腺素，后者与SAN、AVN和心室肌上的β₁-肾上腺素受体结合。去甲肾上腺素通过增强If电流和Ca²⁺内流，增加了起搏器电位的斜率，导致去极化速率加快，从而使心率升高（正性变时效应）。它还能增加心室收缩力（正性变力效应）并加快AVN传导（正性变传导效应）。
The sympathetic influence on ventricular muscle is important because it increases cardiac output not only by raising the heart rate but also by enhancing contractility. During physical exertion, increased sympathetic outflow raises the end-systolic emptying, reducing the end-systolic volume. This mobilises the Frank–Starling mechanism at a higher level of performance, all without fluid loading. The β₁-receptor antagonist drugs (beta blockers) reduce heart rate and blood pressure by precisely opposing this pathway, illustrating the clinical relevance of sympathetic control.
交感神经对心室肌的影响非常重要，因为它不仅通过提高心率，还通过增强收缩力来增加心输出量。在体力消耗期间，增加的交感传出使收缩末排血量增多，减少了收缩末期容积。这在无需液体负荷的情况下，调动了更高水平的Frank–Starling机制。β₁受体拮抗剂（β受体阻滞剂）正是通过对抗这一通路来降低心率和血压，体现了交感控制的临床意义。
8. Parasympathetic Dominance at Rest and Vagal Tone | 静息时的副交感优势与迷走张力
At rest, the heart is under continuous parasympathetic influence, often termed vagal tone. If the vagus nerves are blocked (for example, with atropine), the heart rate typically rises from about 70 bpm to over 100 bpm, revealing the tonic inhibitory effect the parasympathetic system exerts on the SAN. This chronic brake is beneficial because it conserves energy and maintains a low resting rate, allowing a wide dynamic range for sympathetic activation when required.
在静息状态下，心脏持续受副交感的影响，常被称为迷走张力。如果迷走神经被阻断（例如使用阿托品），心率通常会从约70次/分升至100次/分以上，这揭示了副交感系统对SAN的紧张性抑制作用。这种长期的“刹车”效应有益于节约能量并维持较低的静息率，从而在需要时为交感激活留下宽广的动态范围。
Vagal stimulation also slows AVN conduction. An increased vagal tone during sleep, or in highly trained endurance athletes, can result in resting heart rates as low as 40–50 bpm – a physiological sinus bradycardia. The strong vagal influence can transiently reduce cardiac output, but it rarely causes symptoms because the trained heart has a larger stroke volume. Athletes illustrate the extreme adaptability of autonomic heart control.
迷走刺激还会减慢AVN传导。睡眠期间或训练有素的耐力运动员的迷走张力增高，可导致静息心率低至40–50次/分——这是一种生理性窦性心动过缓。强烈的迷走影响可暂时降低心输出量，但很少引起症状，因为训练有素的心脏拥有更大的每搏输出量。运动员体现了自主心脏控制的极端适应能力。
The interplay between sympathetic and parasympathetic systems is antagonistic at the level of the SAN. Sympathetic stimulation accelerates the heart rate, whereas parasympathetic stimulation slows it. During the transition from rest to exercise, a withdrawal of vagal tone is rapidly followed by an increase in sympathetic drive, a sequence that provides a smooth and efficient upshift in cardiac performance. This coordination is orchestrated by the medullary cardiovascular centre.
在SAN水平上，交感与副交感系统相互拮抗。交感刺激加速心率，而副交感刺激减慢心率。在从休息到运动的过渡期间，迷走张力的撤除紧接着交感驱动增强，这一序列为心脏性能提供了平稳高效的上调。这种协调由延髓心血管中枢掌控。
9. Hormonal Control by Adrenaline | 肾上腺素的激素调控
In addition to neural regulation, the hormone adrenaline (epinephrine) is released from the adrenal medulla into the bloodstream under stress or during exercise. Adrenaline binds to the same β₁-adrenergic receptors as noradrenaline on the heart, producing a rise in heart rate, an increase in contractility, and faster AVN conduction. Because the circulatory route delivers adrenaline to all cardiac cells, its effects are more prolonged and diffuse than those of sympathetic nerve stimulation.
除神经调节外，在应激或运动期间，肾上腺髓质将肾上腺素释放到血流中。肾上腺素与去甲肾上腺素一样，作用于心脏的β₁-肾上腺素受体，引起心率上升、收缩力增强以及AVN传导加快。由于循环途径将肾上腺素输送到所有心脏细胞，其效应比交感神经刺激更持久且弥散。
Adrenaline also binds to α and β₂ receptors in blood vessels, causing vasoconstriction in some beds and vasodilation in others, thus redistributing blood flow to skeletal muscles. The integrated effect of increased cardiac output and targeted vasodilation enhances oxygen delivery to active tissues. This hormonal mechanism is a key part of the 'fight or flight' response and can be mimicked by drugs such as dobutamine and isoprenaline in clinical settings.
肾上腺素还会与血管中的α和β₂受体结合，在某些血管床引起收缩，在另一些引起舒张，从而重新分配流向骨骼肌的血流。心输出量增加与靶向血管舒张的综合效应提高了对活动组织的氧供。这一激素机制是“战斗或逃跑”反应的关键组成部分，临床上可用多巴酚丁胺和异丙肾上腺素等药物来模拟。
Long-term stress or phaeochromocytoma (a tumour of the adrenal medulla) can cause a persistently high adrenaline tone, leading to tachycardia, hypertension, and increased risk of arrhythmias. This demonstrates that while hormonal control is essential for acute adaptation, its chronic overactivity can be detrimental. The concept of hormonal modulation reinforces the idea that the control of the heartbeat is a multisystem homeostatic variable.
长期应激或嗜铬细胞瘤（肾上腺髓质肿瘤）可导致持续的肾上腺素高张力，引起心动过速、高血压和心律失常风险增加。这表明虽然激素调控对急性适应至关重要，但其慢性过度活跃可能有害。激素调节的概念强化了这样的观念：心跳控制是一个多系统的稳态变量。
10. Baroreceptors and the Reflex Control of Blood Pressure | 压力感受器与血压的反射控制
Baroreceptors are stretch-sensitive nerve endings located in the walls of the carotid sinus (at the bifurcation of the common carotid artery) and the aortic arch. They monitor arterial blood pressure and send afferent signals via the glossopharyngeal (carotid sinus) and vagus (aortic arch) nerves to the cardiovascular centre in the medulla. An increase in blood pressure stretches the arterial walls, increasing the firing rate of baroreceptors. This triggers a reflex bradycardia – the heart rate slows via enhanced vagal output and reduced sympathetic drive, lowering cardiac output and returning blood pressure toward normal.
压力感受器是位于颈动脉窦（颈总动脉分叉处）和主动脉弓管壁上的牵张敏感神经末梢。它们监测动脉血压，并通过舌咽神经（颈动脉窦）和迷走神经（主动脉弓）将传入信号发送至延髓的心血管中枢。血压升高会牵张动脉壁，增加压力感受器的放电频率。这会触发反射性心动过缓——通过增强迷走传出和减弱交感驱动使心率减慢，降低心输出量，从而使血压恢复正常。
Conversely, when blood pressure falls, the decreased baroreceptor firing rate disinhibits sympathetic activity and reduces vagal tone. This leads to an increase in heart rate (tachycardia), strengthened ventricular contraction, and vasoconstriction, which collectively raise blood pressure. The baroreceptor reflex is a classic example of a negative feedback loop that maintains haemodynamic stability during postural changes, such as standing up quickly, where a sudden drop in pressure is rapidly corrected.
相反，当血压下降时，压力感受器放电频率降低会解除对交感活动的抑制并减弱迷走张力。这导致心率加快（心动过速）、心室收缩力增强以及血管收缩，这些共同作用使血压升高。压力感受器反射是一个典型的负反馈回路，可在体位改变期间（如快速站立）维持血流动力学稳定，此时压力骤降会被迅速纠正。
Baroreceptors adapt to prolonged changes in pressure, so they are more effective at short-term regulation than long-term control. In chronic hypertension, the set point of the baroreflex resets to a higher pressure, allowing the high pressure to be maintained without sustained reflex bradycardia. This resetting is partly due to changes in ion channel expression and receptor sensitivity, underscoring the plasticity of the nervous control of the heart.
压力感受器对持久的压力变化会产生适应，因此它们在短期调节中比长期控制更有效。在慢性高血压中，压力反射的设定点会重置到较高压力，从而在无持续反射性心动过缓的情况下维持高压。这种重置部分源于离子通道表达和受体敏感性的变化，突显了心脏神经控制的可塑性。
11. Chemoreceptor Modulation and Oxygen Delivery | 化学感受器的调节与氧输送
Peripheral chemoreceptors in the carotid and aortic bodies primarily detect changes in arterial O₂, CO₂, and pH. When arterial PO₂ falls (hypoxia), or PCO₂ rises (hypercapnia) causing a fall in pH (acidosis), these receptors increase their firing rate. The afferent signals also travel to the medulla, where they stimulate increased ventilation and also influence the cardiovascular centre. The primary cardiac response to peripheral chemoreceptor stimulation is a sympathetically mediated tachycardia and an increase in blood pressure, thereby improving oxygen delivery.
颈动脉体和主动脉体中的外周化学感受器主要负责检测动脉血O₂、CO₂和pH的变化。当动脉PO₂下降（低氧），或PCO₂上升（高碳酸血症）导致pH下降（酸中毒）时，这些感受器放电频率增加。传入信号同样传至延髓，刺激通气增强，同时影响心血管中枢。外周化学感受器刺激引发的主要心脏反应是交感神经介导的心动过速和血压升高，从而改善氧输送。
In contrast, the direct effect of severe hypoxia on the SAN and cardiac conducting cells is to slow the heart rate, but the peripheral chemoreflex overrides this local effect through the autonomic nervous system. This is why during asphyxiation, the heart rate may initially rise before falling drastically when the direct depressant effects become overwhelming. The chemoreceptor mechanism illustrates how chemical homeostasis is intimately tied to cardiovascular control.
相比之下，严重缺氧对SAN和心脏传导细胞的直接作用是减慢心率，但外周化学反射通过自主神经系统覆盖了这一局部效应。这就是为什么在窒息时，心率最初可能上升，随后当直接抑制作用变得不可抗拒时才会急剧下降。化学感受器机制说明了化学稳态与心血管控制息息相连。
Central chemoreceptors in the medulla are sensitive to changes in cerebrospinal fluid pH, primarily influenced by PCO₂. An increase in brain CO₂ activates central chemoreceptors, which stimulate the sympathetic nervous system, raising heart rate and peripheral resistance. Together with the baroreflex, these chemoreflexes form a sophisticated network that adjusts cardiac output to match tissue metabolism.
延髓中的中枢化学感受器对脑脊液pH变化敏感，主要受PCO₂影响。脑CO₂升高会激活中枢化学感受器，刺激交感神经系统，提高心率和外周阻力。这些化学反射与压力反射一起，组成一个复杂的网络，可根据组织代谢调整心输出量。
12. Integrating the Controls: A Homeostatic Overview | 综合控制：稳态概述
The control of the heartbeat can be summarised as a hierarchy: intrinsic myogenic rhythm generated by the SAN provides the baseline; the autonomic nervous system superimposes rapid, beat-by-beat adjustments; and circulating hormones like adrenaline provide a more sustained modulation. The medullary cardiovascular centre acts as the central processor, continuously integrating afferent inputs from baroreceptors, chemoreceptors, and higher brain centres to set the appropriate sympathetic and parasympathetic tone.
心跳的控制可概括为一种层次结构：SAN产生的内在肌源性节律提供基线；自主神经系统叠加了快速、逐拍的调整；而肾上腺素等循环激素则提供了更持久的调节。延髓心血管中枢起着中央处理器的作用，持续整合来自压力感受器、化学感受器和高级脑中枢的传入信息，以设定适宜的交感和副交感张力。
During exercise, chemoreceptors and muscle mechanoreceptors signal a rise in CO₂ and metabolic demand, causing vagal withdrawal and sympathetic activation. Heart rate and stroke volume both increase, raising cardiac output from about 5 L min⁻¹ at rest to as high as 20–35 L min⁻¹ in elite athletes. Simultaneously, baroreflex sensitivity is dampened to allow the
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