📚 Control of Heart Beat | 心跳的控制
The heart must beat in a coordinated way to pump blood effectively around the body. In A-level Biology, understanding how the heartbeat is initiated and controlled is essential because it links muscle cell physiology, electrical conduction, nervous coordination and homeostasis. The human heart is myogenic, meaning its rhythm originates inside the cardiac muscle itself, but the rate and force of contraction are constantly adjusted by nerves, hormones and chemical changes in the blood.
心脏必须有节律地收缩，才能有效地将血液泵送到全身。在A-level生物课程中，理解心跳如何被启动和控制非常重要，因为它将肌细胞生理、电传导、神经协调和稳态联系在一起。人的心脏具有自律性，即节律起源于心肌自身，但收缩的频率和力度会不断受到神经、激素和血液化学变化的调节。
1. Myogenic nature of cardiac muscle | 心肌的自律性
Cardiac muscle is described as myogenic because it can initiate its own electrical impulses and contract without any nerve impulse from the central nervous system. If the heart is removed from the body and kept in an oxygenated saline solution at a suitable temperature, it continues to beat for a period of time. This property depends on specialised cardiac muscle cells called pacemaker cells, which are able to depolarise spontaneously. The intrinsic rhythm of the heart is therefore generated by the heart itself, not by the brain.
心肌被描述为具有自律性，因为它能够自行产生电冲动并收缩，而不需要来自中枢神经系统的神经冲动。如果将心脏从体内取出，并保存在适宜温度的充氧生理盐水中，心脏仍可继续搏动一段时间。这一特性依赖于称为起搏细胞的特化心肌细胞，它们能够自动去极化。因此，心脏的基本节律是由心脏自身产生的，而不是由大脑产生的。
The basic resting rate produced by the heart’s own pacemaker is around 100 beats per minute. However, in a resting human, the actual heart rate is usually about 70 beats per minute because the parasympathetic nervous system suppresses the intrinsic rate. Nerves and hormones do not create the heartbeat, but they modify the rate, force and response to the body’s needs.
心脏自身起搏点产生的基础静息频率约为每分钟100次。然而，静息时人的实际心率通常约为每分钟70次，因为副交感神经系统抑制了固有频率。神经和激素并不产生心跳，但它们会调节心率、收缩力以及心脏对身体需求的反应。
2. The sinoatrial node as pacemaker | 窦房结作为起搏点
The sinoatrial node, usually abbreviated as SAN, is a small mass of specialised cardiac muscle cells located in the wall of the right atrium, close to where the superior vena cava enters the heart. The SAN has the fastest rate of spontaneous depolarisation among all cardiac muscle tissues, so it is the primary pacemaker of the heart. It sets the rhythm for the whole heart because the electrical impulse it generates spreads to all other regions before those regions can depolarise on their own.
窦房结通常缩写为SAN，是位于右心房壁、靠近上腔静脉进入心脏处的一小团特化心肌细胞。SAN在所有心肌组织中具有最快的自动去极化速率，因此它是心脏的初级起搏点。它为整个心脏设定节律，因为它产生的电冲动会在其他区域自行去极化之前传导到所有其他区域。
The cells of the SAN do not have a stable resting membrane potential. Instead, during the relaxation phase of the cardiac cycle, their membrane potential gradually drifts towards threshold because of slow inward movement of sodium ions and calcium ions. When threshold is reached, an action potential is produced. This gradual depolarisation before the action potential is called the pacemaker potential or prepotential. The steeper the pacemaker potential, the faster the SAN fires and therefore the faster the heart rate.
SAN细胞没有稳定的静息膜电位。相反，在心舒张期，由于钠离子和钙离子缓慢内流，它们的膜电位逐渐向阈电位漂移。当达到阈电位时，就产生一个动作电位。动作电位之前的这种逐渐去极化称为起搏电位或前电位。起搏电位的斜率越大，SAN发放冲动的频率越快，因此心率越快。
3. Spread of excitation through the atria | 兴奋在心房的传导
From the sinoatrial node, the wave of electrical excitation spreads rapidly across the walls of both atria. This spread occurs through gap junctions and intercalated discs between adjacent cardiac muscle cells, allowing ions to pass directly from one cell to the next. The result is that both atria contract almost simultaneously, pushing blood into the ventricles. This phase is called atrial systole.
电兴奋波从窦房结出发，迅速传播到两个心房的壁上。这种传播通过相邻心肌细胞之间的缝隙连接和闰盘进行，使离子直接从一个细胞传到下一个细胞。其结果是两个心房几乎同时收缩，将血液推入心室。这一阶段称为心房收缩。
There is a layer of connective tissue called the atrioventricular ring that separates the atria from the ventricles. This ring is electrically non-conducting, so the wave of excitation cannot pass directly from the atria to the ventricles. The only normal electrical connection between the upper and lower chambers is through a specialised region called the atrioventricular node. This arrangement ensures that ventricular contraction is delayed and occurs only after atrial contraction has finished.
心房的壁和心室的壁之间有一层称为房室环的结缔组织。这层组织不导电，因此兴奋波不能直接从心房传到心室。上下心腔之间唯一的正常电联系是通过一个特化区域，即房室结。这种结构保证了心室收缩被延搁，并且只在心房收缩完成之后才发生。
4. The atrioventricular node and the delay | 房室结与延搁
The atrioventricular node, abbreviated as AVN, is located in the septum near the base of the atria, just above the ventricles. When the wave of excitation reaches the AVN from the atria, it is held there for about 0.1 seconds before being transmitted onwards. This is called the AVN delay or atrioventricular delay. The delay is caused by the small diameter of the conducting cells and their relatively slow conduction velocity.
房室结缩写为AVN，位于心房底部附近、心室上方的室间隔中。当兴奋波从心房到达AVN时，它会在那里被延迟约0.1秒，然后继续向下传导。这称为房室结延搁。延搁是由传导细胞直径较小以及传导速度相对较慢造成的。
The functional importance of the AVN delay is that it allows the atria to finish contracting and emptying blood into the ventricles before the ventricles begin to contract. If the ventricles contracted at the same time as the atria, the heart would not fill properly and pumping would be inefficient. If conduction through the AVN is too slow, a condition known as heart block may develop, where some or all atrial impulses fail to reach the ventricles.
房室结延搁的功能意义在于，它使心房能够先完成收缩并将血液排入心室，心室才开始收缩。如果心房和心室同时收缩，心脏就不能正常充盈，泵血效率会降低。如果AVN传导过慢，就可能出现传导阻滞，即部分或全部心房冲动不能到达心室。
5. Bundle of His and Purkyne fibres | 希氏束与浦肯野纤维
After the AVN delay, the electrical impulse travels rapidly down the Bundle of His, which lies in the interventricular septum. The Bundle of His splits into left and right bundle branches, which run down the two sides of the septum. These branches then divide into smaller conducting fibres called Purkyne fibres, also spelled Purkinje fibres, which spread throughout the thick muscular walls of the ventricles.
经过房室结延搁后，电冲动沿希氏束快速向下传导，希氏束位于室间隔中。希氏束分为左束支和右束支，沿室间隔两侧下行。这些束支再分成更小的传导纤维，称为浦肯野纤维，遍布心室厚实的肌壁。
Purkyne fibres conduct action potentials very rapidly. This rapid conduction ensures that the mass of ventricular muscle receives the impulse at almost the same time, so the ventricles contract together rather than in a slow wave. The contraction begins at the apex of the heart and spreads upwards, which pushes blood efficiently towards the pulmonary artery and the aorta. This coordinated ventricular contraction is essential for producing enough pressure to eject blood into the circulation.
浦肯野纤维传导动作电位的速度非常快。这种快速传导确保心室肌几乎同时接收到冲动，因此心室是同步收缩的，而不是缓慢地波状收缩。收缩从心尖开始并向上传播，将血液有效地推向肺动脉和主动脉。这种协调的心室收缩对于产生足够的压力将血液射入循环系统至关重要。
6. Ventricular contraction and the cardiac cycle | 心室收缩与心动周期
When the wave of excitation spreads through the ventricular walls, the ventricles enter systole. The coordinated contraction increases the pressure inside the ventricles. When ventricular pressure rises above the pressure in the aorta and pulmonary artery, the semilunar valves open and blood is ejected. After the action potentials end and the cells repolarise, the ventricles relax and enter diastole. Ventricular pressure falls below atrial pressure, so the atrioventricular valves open and the ventricles begin to fill with blood again.
当兴奋波传播到心室壁时，心室进入收缩期。协调的收缩使心室内的压力升高。当心室压力升高到超过主动脉和肺动脉内的压力时，半月瓣打开，血液被射出。动作电位结束且细胞复极后，心室舒张并进入舒张期。心室压力降到低于心房压力时，房室瓣打开，心室再次开始充盈血液。
The orderly sequence of atrial systole, ventricular systole and diastole depends directly on the conduction pathway. The SAN initiates the impulse, the atria contract, the AVN delays the impulse, the Bundle of His and Purkyne fibres conduct it rapidly to the ventricles, and the ventricles contract. Valves prevent backflow and ensure one-way blood flow through the heart.
心房收缩、心室收缩和舒张的有序顺序直接依赖于传导通路。SAN发起冲动，心房收缩，AVN延搁冲动，希氏束和浦肯野纤维将冲动快速传导到心室，然后心室收缩。瓣膜防止血液倒流，并确保血液在心脏内单向流动。
7. Autonomic nervous system control | 自主神经系统控制
Although the heartbeat is initiated by the SAN, the rate and force of contraction are constantly modified by the autonomic nervous system. The cardiovascular centre in the medulla oblongata of the brain sends impulses along two sets of nerves: the sympathetic cardiac nerve and the parasympathetic vagus nerve. These two pathways have opposite effects on the heart.
尽管心跳由SAN启动，但心率和收缩力会不断受到自主神经系统的调节。大脑延髓中的心血管中枢通过两组神经发送冲动：心交感神经和副交感迷走神经。这两条通路对心脏的作用相反。
The sympathetic cardiac nerve releases noradrenaline at the SAN and throughout the cardiac muscle. Noradrenaline increases the slope of the pacemaker potential, so the SAN reaches threshold more quickly and heart rate increases. It also increases the force of ventricular contraction, so stroke volume increases. The vagus nerve releases acetylcholine at the SAN. Acetylcholine makes the pacemaker potential less steep and hyperpolarises the cells slightly, so heart rate decreases. At rest, parasympathetic tone from the vagus nerve keeps the heart rate below the intrinsic SAN rate of about 100 beats per minute.
心交感神经在SAN以及整个心肌中释放去甲肾上腺素。去甲肾上腺素增加起搏电位的斜率，使SAN更快达到阈电位，从而加快心率。它还增强心室收缩力，因此每搏输出量增加。迷走神经在SAN处释放乙酰胆碱。乙酰胆碱使起搏电位的斜率降低，并使细胞轻微超极化，因此心率减慢。静息时，来自迷走神经的副交感紧张使心率保持在SAN固有频率约每分钟100次以下。
8. Chemical and stretch receptors in control | 化学感受器与压力感受器
The medulla oblongata does not control heart rate randomly; it responds to information from receptors in the circulatory system. Baroreceptors are pressure receptors located in the carotid sinus and the aortic arch. They detect changes in arterial blood pressure by responding to the stretch of the artery wall. When blood pressure falls, the frequency of impulses from the baroreceptors decreases. The cardiovascular centre responds by increasing sympathetic activity and decreasing parasympathetic activity, so heart rate and stroke volume rise, helping to restore blood pressure.
延髓并不是随意控制心率的，它会对循环系统中的感受器传来的信息作出反应。压力感受器是位于颈动脉窦和主动脉弓的压力受体。它们通过感受动脉壁的牵拉程度来检测动脉血压的变化。当血压下降时，压力感受器发出的冲动频率降低。心血管中枢通过增加交感活动并降低副交感活动来作出反应，使心率和每搏输出量升高，从而帮助恢复血压。
When blood pressure rises, the opposite happens: baroreceptor firing increases, the medulla reduces sympathetic activity and increases parasympathetic activity, and heart rate falls. Chemoreceptors in the carotid bodies and aortic bodies detect changes in blood chemistry, especially the partial pressures of carbon dioxide and oxygen, and the pH of the blood. During exercise, increased CO₂ and decreased pH stimulate the chemoreceptors. These send impulses to the medulla, which then increases heart rate and ventilation to deliver more oxygen to the muscles and remove excess CO₂.
当血压升高时，则发生相反的变化：压力感受器放电增加，延髓降低交感活动并增加副交感活动，心率下降。颈动脉体和主动脉体中的化学感受器检测血液化学成分的变化，尤其是二氧化碳分压、氧分压和血液pH。运动时，CO₂升高和pH降低会刺激化学感受器。它们向延髓发送冲动，延髓随后加快心率和通气，从而向肌肉输送更多氧气并排出多余的CO₂。
9. Role of adrenaline and hormones | 肾上腺素与激素的作用
During exercise, stress or excitement, the adrenal medulla releases adrenaline, also called epinephrine, into the bloodstream. Adrenaline is a hormone, but it also acts as a chemical messenger that binds to beta-adrenergic receptors on the cells of the SAN and on cardiac muscle cells. Binding of adrenaline increases the rate of spontaneous depolarisation at the SAN, so heart rate rises. It also increases the force of ventricular contraction, so stroke volume rises. Because adrenaline travels in the blood, its effect may last longer than the local effect of noradrenaline released from sympathetic nerve endings.
在运动、应激或兴奋时，肾上腺髓质向血液中释放肾上腺素。肾上腺素是一种激素，但也作为一种化学信使，与SAN细胞和心肌细胞上的β-肾上腺素能受体结合。肾上腺素与受体结合后，加快SAN的自动去极化速率，使心率上升。它还增强心室收缩力，使每搏输出量增加。由于肾上腺素通过血液运输，其作用可能比交感神经末梢释放的去甲肾上腺素的局部作用持续时间更长。
Thyroxine, a hormone produced by the thyroid gland, also influences heart rate. Thyroxine increases the overall metabolic rate of cells and makes the heart more sensitive to adrenaline and noradrenaline. As a result, excessive thyroxine can lead to a rapid heart rate, while too little thyroxine can cause a slow heart rate. This shows that heart control is not only electrical and nervous but also chemical and hormonal.
甲状腺分泌的甲状腺素也会影响心率。甲状腺素提高细胞的整体代谢率，并使心脏对肾上腺素和去甲肾上腺素更加敏感。因此，甲状腺素过多会导致心率过快，而甲状腺素过少则会导致心率过慢。这表明心脏的控制不仅依赖于电活动和神经，还依赖于化学和激素。
10. Electrocardiogram interpretation | 心电图的解读
The electrical activity of the heart can be recorded using electrodes placed on the skin. This recording is called an electrocardiogram, or ECG. A normal ECG shows a repeating pattern with several distinct waves. The P wave represents atrial depolarisation, which leads to atrial contraction. The QRS complex represents ventricular depolarisation, which leads to ventricular contraction, and it masks the smaller signal of atrial repolarisation. The T wave represents ventricular repolarisation, which leads to ventricular relaxation.
心脏的电活动可以通过放置在皮肤上的电极进行记录。这种记录称为心电图，简称ECG。正常心电图显示出具有几个明显波形的重复模式。P波代表心房去极化，随后发生心房收缩。QRS波群代表心室去极化，随后发生心室收缩，并且它掩盖了较弱的心房复极信号。T波代表心室复极，随后心室舒张。
| ECG feature | What it represents |
|---|---|
| P wave | Atrial depolarisation, followed by atrial systole |
| PR interval | Time from atrial depolarisation to ventricular depolarisation, including the AVN delay |
| QRS complex | Ventricular depolarisation, followed by ventricular systole |
| T wave | Ventricular repolarisation, followed by ventricular diastole |
The PR interval is particularly useful in assessing the AVN delay. A normal PR interval shows that the impulse has travelled from the SAN to the ventricles through the proper pathway. If the PR interval is abnormally long, this suggests slower conduction through the AVN. If QRS complexes are missing, some impulses may be blocked. Irregular rhythms can indicate problems with the pacemaker or conduction system. Therefore the ECG provides a simple but powerful way to observe the electrical events that control the heartbeat.
PR间期特别有助于评估房室结延搁。正常的PR间期表明冲动已经通过正确的通路从SAN传到心室。如果PR间期异常延长，则提示房室结传导变慢。如果QRS波群缺失，说明部分冲动可能被阻断。节律不规则可提示起搏点或传导系统出现问题。因此，心电图提供了一种简单而有效的方法来观察控制心跳的电活动。
11. Summary
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Cancel reply