📚 The Control of the Heartbeat | 心跳的调控机制
The heartbeat is not merely a mechanical pump driven by nerves; it originates from within the heart muscle itself. In A-Level biology, understanding the control of the heartbeat requires knowledge of the cardiac conduction system, the roles of the autonomic nervous system, and the feedback mechanisms that adjust heart rate according to the body’s demands.
心跳并非仅仅由神经驱动的机械泵活动，它起源于心肌自身。在A-Level生物学中，理解心跳的调控需要掌握心脏传导系统、自主神经系统的作用，以及根据机体需求调节心率的反馈机制。
1. The Cardiac Cycle and Pressure Changes | 心动周期与压力变化
The cardiac cycle is the sequence of events in one heartbeat, consisting of atrial systole, ventricular systole and complete diastole. During atrial systole, blood is forced from the atria into the relaxed ventricles. Ventricular systole then raises the pressure inside the ventricles, closing the atrioventricular (AV) valves and opening the semilunar valves, so that blood is ejected into the aorta and pulmonary artery. When ventricular pressure falls below the arterial pressure during diastole, the semilunar valves close to prevent backflow.
心动周期是指一次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。心房收缩期间，血液被心房压入松弛的心室；随后心室收缩使心室内压升高，关闭房室瓣并推开半月瓣，使血液射入主动脉和肺动脉。当心室压力在舒张期降至动脉压以下时，半月瓣关闭，防止血液逆流。
At a resting rate of about 75 beats per minute, one cycle lasts about 0.8 s: atrial systole 0.1 s, ventricular systole 0.3 s, and complete diastole 0.4 s. The long diastolic period allows the heart muscle to receive oxygen via the coronary circulation and allows the atria to refill with blood.
当静息心率约为75次/分钟时，一个心动周期约0.8秒：心房收缩0.1秒、心室收缩0.3秒、全心舒张0.4秒。较长的舒张期使心肌通过冠状动脉获得氧气供应，并让心房得以重新充盈血液。
2. Cardiac Muscle Is Myogenic | 心肌的肌源性
Cardiac muscle is described as myogenic because its contraction is initiated by the muscle itself, not by nervous impulses. This distinguishes it from skeletal muscle, which is neurogenic. Even when a piece of cardiac muscle is isolated in a warm oxygenated solution, it can continue to beat rhythmically for some time without any nerve supply.
心肌被称为“肌源性”组织，是因为其收缩由肌肉自身引发，而非神经冲动。这与骨骼肌不同，骨骼肌是“神经源性”的。即使将一小块心肌离体置于温暖、含氧的溶液中，在没有神经支配的情况下，它也能持续有节律地搏动一段时间。
The molecular basis of this automaticity is an unstable resting membrane potential in specialised pacemaker cells. These cells slowly depolarise during diastole until they reach threshold and fire an action potential, a property known as the pacemaker potential.
这种自动性的分子基础在于特化起搏细胞的不稳定静息膜电位。这些细胞在舒张期缓慢去极化，直到达到阈值并发放动作电位，这一特性称为起搏电位。
3. The Sinoatrial Node (SAN) | 窦房结——起搏点
The SAN is a small patch of specialised muscle cells located in the wall of the right atrium, close to the opening of the vena cava. It initiates electrical impulses spontaneously at about 100–110 times per minute at rest. From the SAN, the wave of excitation spreads across both atria through gap junctions in the atrial muscle fibres, causing atrial systole.
窦房结是位于右心房壁、靠近腔静脉开口处的一小组特化肌细胞。静息时它以每分钟约100–110次的频率自发产生电冲动。兴奋波从窦房结出发，通过心房肌纤维间的缝隙连接传播到两个心房，引起心房收缩。
Because the atria and ventricles are separated by non-conducting fibrous tissue, the impulse can only reach the ventricles via the atrioventricular node (AVN), located on the floor of the right atrium. The delay at the AVN of about 0.1 s ensures that atrial contraction is completed before ventricular contraction begins.
由于心房与心室之间由不导电的纤维组织隔开，冲动只能通过位于右心房底部附近的房室结到达心室。房室结处约0.1秒的延迟确保了心房收缩在心室收缩开始之前完成。
4. The Atrioventricular Node and the Purkyne Fibres | 房室结与浦肯野纤维
The AVN relays the impulse more slowly than the atrial wall, producing the essential delay mentioned above. The impulse then travels along the bundle of His, which runs down the interventricular septum and divides into right and left bundle branches.
房室结的传导速度比心房壁更慢，从而产生上述关键的延迟。随后冲动沿希氏束向下穿行于室间隔，并分为左、右束支。
The Purkyne fibres fan out from the bundle branches and penetrate the ventricular muscle. Because their conduction velocity is very high (up to 4 m s⁻¹), both ventricles depolarise almost simultaneously, producing a powerful, coordinated ventricular systole from the apex upwards.
浦肯野纤维从束支呈扇形展开进入心室肌。由于其传导速度极快（可达4 m s⁻¹），左右心室几乎同时去极化，产生自心尖向上的强有力且协调的心室收缩。
5. Cardiac Action Potentials | 心肌动作电位
Cardiac action potentials differ from those of neurones. After rapid depolarisation caused by the opening of voltage-gated Na⁺ channels, there is a prolonged plateau phase maintained by a slow inward current of Ca²⁺ balanced by an outward current of K⁺. This plateau lasts about 0.25 s.
心肌动作电位与神经元的动作电位不同。在电压门控Na⁺通道开放引起快速去极化之后，存在一个由Ca²⁺缓慢内流和K⁺外流共同维持的较长平台期。该平台期持续约0.25秒。
The plateau and the long refractory period that follows prevent the heart from undergoing tetanus (sustained contraction). Because cardiac muscle cannot be restimulated during contraction, the heart always relaxes between beats, allowing the chambers to fill with blood before the next contraction.
平台期及其后较长的不应期可防止心脏发生强直收缩（持续性收缩）。由于心肌在收缩期间不能被再次刺激，心脏总能在两次搏动之间舒张，使心腔在下次收缩前得以充盈血液。
6. The Electrocardiogram (ECG) | 心电图
The electrocardiogram (ECG) is a recording of the electrical activity of the heart detected by electrodes placed on the skin. Each wave or complex corresponds to a specific electrical event in the cardiac cycle.
心电图（ECG）是通过贴附在皮肤上的电极记录到的心脏电活动。每个波或波群对应心动周期中一个特定的电事件。
| ECG feature | Electrical event | Meaning |
|---|---|---|
| P wave | Atrial depolarisation | Spreads from SAN through both atria |
| QRS complex | Ventricular depolarisation | Follows conduction through AVN, bundle of His and Purkyne fibres |
| T wave | Ventricular repolarisation | Relaxation of ventricles before next cycle |
Atrial repolarisation is hidden within the QRS complex because its electrical signal is small compared with that of ventricular depolarisation. The PR interval represents the time taken for the impulse to pass from the atria through the AVN to the ventricles.
心房复极被隐藏在QRS波群中，因为其电信号比心室去极化小得多。PR间期代表冲动从心房经房室结传至心室所需的时间。
7. Nervous Control from the Medulla Oblongata | 延髓的神经调控
Although the SAN sets the basic rhythm, its rate is continually adjusted by the autonomic nervous system. The medulla oblongata contains two opposing centres: the cardioacceleratory centre, which sends impulses through sympathetic fibres (the accelerator nerve), and the cardioinhibitory centre, which sends impulses through parasympathetic fibres in the vagus nerve.
虽然窦房结决定基本节律，其速率仍由自主神经系统不断调整。延髓内有两个功能相反的中枢：心加速中枢，通过交感纤维（加速神经）发放冲动；以及心抑制中枢，通过迷走神经内的副交感纤维发放冲动。
At rest, the cardioinhibitory centre is dominant, so the resting heart rate is about 70 beats per minute rather than the SAN’s intrinsic rate of about 110 beats per minute. Sympathetic stimulation releases noradrenaline, which increases the rate of depolarisation of SAN cells and therefore raises heart rate. Parasympathetic stimulation releases acetylcholine, which slows SAN depolarisation and lowers heart rate.
静息时，心抑制中枢占优势，因此静息心率约为70次/分钟，而不是窦房结内在的约110次/分钟。交感刺激释放去甲肾上腺素，加快窦房结细胞去极化速率，从而使心率升高。副交感刺激释放乙酰胆碱，减慢窦房结去极化，使心率降低。
Published by TutorHao | A-Level Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导