📚 The Cardiac Cycle | 心动周期
The cardiac cycle describes the sequence of mechanical and electrical events that occur during a single heartbeat. It includes the coordinated contraction (systole) and relaxation (diastole) of the atria and ventricles, ensuring that blood is pumped efficiently through the pulmonary and systemic circuits. For A‑Level students, understanding pressure changes, valve movements and how these relate to the electrocardiogram (ECG) is essential.
心动周期描述了一次心跳过程中发生的一系列机械与电活动。它包括心房和心室协调的收缩（收缩期）与舒张（舒张期），确保血液有效泵入肺循环和体循环。对 A‑Level 学生而言，理解压力变化、瓣膜活动及其与心电图（ECG）的关系至关重要。
1. What is the Cardiac Cycle? | 什么是心动周期？
The cardiac cycle is all the events that take place from the start of one heartbeat to the beginning of the next. In a resting adult with a heart rate of 75 beats per minute, one cycle lasts approximately 0.8 seconds. This cycle is divided into three major phases: atrial systole, ventricular systole and diastole. During each phase, pressure gradients drive the opening and closing of the atrioventricular (AV) valves and semilunar valves, thereby directing blood flow in a unidirectional manner.
心动周期是指从一次心跳开始到下一次心跳开始之间发生的所有事件。在静息心率约 75 次/分的成人中，一个周期持续约 0.8 秒。该周期可分为三个主要时相：心房收缩期、心室收缩期和舒张期。每一时相中，压力梯度驱动房室瓣与半月瓣的开放和关闭，从而保证血液单向流动。
2. Phases of the Cardiac Cycle | 心动周期的时相
The cardiac cycle can be broken down into three chronologically ordered phases. Firstly, atrial systole contracts the atria, pushing the final volume of blood into the ventricles. This is followed by ventricular systole, where the ventricles contract and eject blood into the arteries. Finally, diastole is the relaxation period during which the chambers refill with blood. The sequence is repeated with every heartbeat, with the duration of each phase being adjusted by the autonomic nervous system.
心动周期可按时间顺序分为三个时相。首先是心房收缩期，心房收缩将最后一部分血液推入心室。随后是心室收缩期，心室收缩并将血液射入动脉。最后是舒张期，即心脏各腔室舒张并重新充盈血液。这一序列在每一次心跳中重复，自主神经系统会调节各时相的持续时间。
The table below summarises the three main phases and the key events occurring in each.
下表概括了三个主要时相以及每一时相中的关键事件。
| Phase | Key Events |
|---|---|
| Atrial Systole (~0.1 s) | Atria contract, AV valves open, semilunar valves closed, ventricles fill actively. |
| Ventricular Systole (~0.3 s) | Ventricles contract, AV valves close, semilunar valves open, blood ejected. |
| Diastole (~0.4 s) | All chambers relax, semilunar valves close, ventricles fill passively, then atrial systole begins again. |
中文翻译：心房收缩期（约 0.1 秒）：心房收缩，房室瓣开放，半月瓣关闭，心室主动充盈。心室收缩期（约 0.3 秒）：心室收缩，房室瓣关闭，半月瓣开放，血液射出。舒张期（约 0.4 秒）：所有腔室舒张，半月瓣关闭，心室被动充盈，然后心房收缩重新开始。
3. Atrial Systole | 心房收缩期
Atrial systole is the first phase of the cardiac cycle, initiated by the depolarisation of the sinoatrial (SA) node. The atria contract, increasing the pressure inside the atrial chambers. This pressure rise forces the remaining blood – approximately 20‑30% of ventricular filling – through the open AV valves (tricuspid and bicuspid/mitral) into the relaxed ventricles. Throughout this phase, the semilunar valves remain shut because the arterial pressure is still higher than the pressure inside the ventricles. The contraction of the atria lasts about 0.1 s and corresponds to the P wave on the ECG.
心房收缩期是心动周期的第一个时相，由窦房结的去极化启动。心房收缩，使心房内压力升高。这一压力升高迫使剩余血液（约占心室充盈量的 20‑30%）通过开放的房室瓣（三尖瓣和二尖瓣）进入舒张的心室。整个过程中，由于动脉压力仍高于心室内压，半月瓣保持关闭。心房收缩持续约 0.1 秒，对应心电图上的 P 波。
Pressure changes: Atrial pressure rises slightly from about 2 mmHg to 5 mmHg. Ventricular pressure remains low, and ventricular volume reaches its end‑diastolic value.
压力变化：心房压力从约 2 mmHg 略微上升至 5 mmHg。心室压力仍低，心室容积达到舒张末期容积。
4. Ventricular Systole | 心室收缩期
Ventricular systole begins immediately after the QRS complex of the ECG. It can be subdivided into isovolumetric contraction and the ejection period. As the ventricles contract, intraventricular pressure rises sharply. Once this pressure exceeds atrial pressure, the AV valves snap shut, producing the first heart sound ('lub'). For a brief moment, both AV and semilunar valves are closed, so ventricular volume stays constant – this is the isovolumetric contraction phase. When intraventricular pressure surpasses the pressure in the aorta (about 80 mmHg) and pulmonary artery, the semilunar valves are forced open, and blood is ejected. During ejection, ventricular pressure peaks around 120 mmHg in the left ventricle and 25 mmHg in the right ventricle. At the end of ejection, the ventricles start to relax, and the semilunar valves close, generating the second heart sound ('dub').
心室收缩期紧随心电图的 QRS 波群开始，可细分为等容收缩期和射血期。心室收缩时，室内压急剧上升。一旦超过心房内压力，房室瓣猛然关闭，产生第一心音（“lub”）。在短暂时刻，房室瓣和半月瓣均处于关闭状态，心室容积保持不变——此即等容收缩期。当室内压超过主动脉（约 80 mmHg）和肺动脉内的压力时，半月瓣被推开，血液射出。射血期间，左心室压力峰值约 120 mmHg，右心室约 25 mmHg。射血末期，心室开始舒张，半月瓣关闭，产生第二心音（“dub”）。
The total duration of ventricular systole is approximately 0.3 seconds. The ejection fraction – the percentage of end‑diastolic volume ejected – is normally around 60‑70%.
心室收缩期总持续时间约 0.3 秒。射血分数（射出的血液占舒张末期容积的百分比）正常约为 60‑70%。
5. Diastole | 舒张期
Diastole is the relaxation and refilling phase, lasting about 0.4 seconds at rest. It starts with isovolumetric relaxation: as the ventricles relax, intraventricular pressure drops below the pressure in the aorta and pulmonary artery, causing the semilunar valves to close. At this point all four valves are closed, so ventricular volume remains constant for a short period. Once ventricular pressure falls below atrial pressure, the AV valves open, and blood flows passively from the atria into the ventricles. This is the rapid filling phase. Later, diastasis – a slower filling phase – occurs, and finally the cycle is completed by the next atrial systole. The majority of ventricular filling (about 70‑80%) occurs passively during early diastole, highlighting the importance of venous return.
舒张期是舒张和充盈的阶段，在静息状态下持续时间约为 0.4 秒。舒张期始于等容舒张：心室舒张时，室内压降至低于主动脉和肺动脉压，导致半月瓣关闭。此时四个瓣膜均关闭，心室容积在短时间内保持不变。当室内压低于心房压时，房室瓣打开，血液被动地从心房流入心室。这是快速充盈期。随后出现减慢充盈期（diastasis），最终由下一次心房收缩完成整个周期。心室充盈的大部分（约 70‑80%）在舒张早期被动发生，这凸显了静脉回流的重要性。
At the end of diastole, the ventricles are filled with their end‑diastolic volume (around 120‑130 mL in the left ventricle).
舒张末期，心室充满舒张末期容积（左心室约 120‑130 mL）。
6. Heart Sounds and Valve Actions | 心音与瓣膜活动
Heart sounds are produced by the closure of heart valves. The first heart sound (S1), often described as 'lub', occurs at the beginning of ventricular systole when the AV valves close. The second heart sound (S2), 'dub', occurs at the beginning of diastole when the semilunar valves close. Abnormal heart sounds, or murmurs, can indicate valve dysfunction such as stenosis or incompetence. In a healthy heart, valve opening is normally silent because the pressure difference opens the cusps smoothly.
心音由心脏瓣膜的关闭产生。第一心音（S1），通常描述为“lub”，出现在心室收缩初期房室瓣关闭时。第二心音（S2），“dub”，出现在舒张初期半月瓣关闭时。异常心音或杂音可能提示瓣膜狭窄或关闭不全等功能障碍。在健康心脏中，瓣膜开放通常无声，因为压力差使瓣叶平稳推开。
The precise timing of S1 and S2 relative to the cardiac cycle is used clinically to assess heart health. For example, a split S2 may be normal during inspiration.
S1 和 S2 相对于心动周期的精确时间被临床用于评估心脏健康。例如，吸气时出现的分裂 S2 可能是正常现象。
7. Pressure Changes during the Cardiac Cycle | 心动周期中的压力变化
Pressure profiles in the left atrium, left ventricle and aorta provide a classic graph that illustrates the cardiac cycle. At the start of atrial systole, left atrial pressure increases slightly, causing the 'a' wave. During ventricular systole, left ventricular pressure rises steeply, crosses aortic pressure and then falls as ejection finishes. Aortic pressure rises to a systolic peak (~120 mmHg) and then falls to a diastolic level (~80 mmHg); the dicrotic notch (incisura) marks the closure of the aortic semilunar valve. Left atrial pressure exhibits a small 'c' wave (due to AV valve bulging) and a 'v' wave (due to venous return during ventricular systole) before the valve opens.
左心房、左心室和主动脉的压力曲线形成了展示心动周期的经典图形。心房收缩初期，左心房压力轻微升高，形成“a”波。心室收缩期，左心室压力急剧上升，超过主动脉压力，然后随着射血结束而下降。主动脉压力升至收缩期峰值（约 120 mmHg），然后降至舒张期水平（约 80 mmHg）；重搏切迹（降中峡）标志着主动脉半月瓣的关闭。左心房压力在房室瓣打开前表现出一小“c”波（由房室瓣膨出引起）和“v”波（因心室收缩期间的静脉回流所致）。
A typical summary of left‑sided pressures (in mmHg) is:
左侧压力的典型值（单位 mmHg）如下：
| Chamber/Vessel | Systolic | Diastolic |
|---|---|---|
| Left atrium | ~5 | ~2 |
| Left ventricle | ~120 | ~5 |
| Aorta | ~120 | ~80 |
Right‑sided pressures are lower (right ventricle ~25 mmHg systolic; pulmonary artery ~25/8 mmHg).
右侧压力较低（右心室收缩压约 25 mmHg；肺动脉约 25/8 mmHg）。
8. The ECG and the Cardiac Cycle | 心电图与心动周期
The electrocardiogram (ECG) records the electrical activity that triggers mechanical events. The P wave represents atrial depolarisation and therefore precedes atrial systole. The QRS complex corresponds to ventricular depolarisation, initiating ventricular systole. The T wave indicates ventricular repolarisation, which occurs just before ventricular diastole. Thus, the ECG is a valuable tool for linking the electrical and mechanical phases. There is a short delay between the electrical signal and the resulting contraction because of the slow conduction through the AV node, which allows complete ventricular filling.
心电图记录引发机械活动的电信号。P 波代表心房去极化，因此先于心房收缩出现。QRS 波群对应心室去极化，启动心室收缩。T 波表示心室复极化，发生在心室舒张之前。因此，心电图是将电活动与机械时相联系起来的宝贵工具。由于房室结传导缓慢，在电信号和随之而来的收缩之间存在短暂延迟，这使得心室能够完全充盈。
Clinically, a prolonged Q‑T interval, for example, can signal a risk of arrhythmia, while an abnormal P wave may indicate atrial enlargement.
临床上，例如 Q‑T 间期延长可提示心律失常风险，而异常 P 波可能表明心房扩大。
9. Duration and Regulation of the Cardiac Cycle | 心动周期的时长与调节
At a heart rate of 75 bpm, one cardiac cycle lasts 0.8 s: atrial systole 0.1 s, ventricular systole 0.3 s, diastole 0.4 s. When heart rate increases, the duration of all phases shortens, but diastole is disproportionately reduced. This can reduce ventricular filling time and potentially affect stroke volume. The autonomic nervous system modulates heart rate: sympathetic stimulation (via noradrenaline) increases heart rate and the force of contraction, while parasympathetic stimulation (via the vagus nerve, releasing acetylcholine) decreases heart rate. Circulating hormones such as adrenaline also accelerate the cycle.
心率 75 次/分时，一个心动周期持续 0.8 秒：心房收缩期 0.1 秒，心室收缩期 0.3 秒，舒张期 0.4 秒。心率增加时，各时相持续时间均缩短，但舒张期缩短得不成比例。这会减少心室充盈时间，并可能影响每搏输出量。自主神经系统调节心率：交感神经刺激（通过去甲肾上腺素）提高心率和收缩力，而副交感神经刺激（通过迷走神经释放乙酰胆碱）降低心率。肾上腺素等循环激素也能加快心动周期。
een heart rate and cycle duration is given by: Cycle duration (s) = 60 / heart rate (bpm).
心率与周期时长的关系为：周期时长（秒）= 60 / 心率（次/分）。
10. Calculating Cardiac Output | 心输出量的计算
Cardiac output (CO) is the volume of blood pumped by each ventricle per minute. It is a key indicator of heart performance and can be calculated using the formula:
心输出量（CO）是每个心室每分钟泵出的血液量，是衡量心脏功能的关键指标，计算公式如下：
Cardiac Output = Heart Rate × Stroke Volume
stroke volume (SV) is the volume of blood ejected by a ventricle in one beat, typically 70 L/min. During exercise, both HR and SV can increase, causing cardiac output to rise to 20‑25 L/min in trained athletes.
心率（HR）为每分钟心跳次数，每搏输出量（SV）为每个心室一次搏动射出的血液量，静息时通常约为 70 mL。因此，典型静息值为：CO = 75 次/分 × 0.07 L = 5.25 L/分。运动时，心率和每搏输出量均可增加，使训练有素的运动员的心输出量升至 20‑25 L/分。
Stroke volume itself is influenced by preload (the degree of ventricular stretch), contractility (force of contraction) and afterload (the resistance against which the ventricle ejects blood). These factors link directly to the cardiac cycle phases.
每搏输出量本身受前负荷（心室拉伸程度）、收缩力（收缩力量）和后负荷（心室射血所遇阻力）的影响。这些因素直接与心动周期各时相联系。
11. Key Terminology Summary | 关键术语总结
The following table lists essential terms for mastering the cardiac cycle topic.
下表列出了掌握心动周期所需的关键术语。
| Term | Definition | 中文 |
|---|---|---|
| Atrial systole | Contraction of the atria | 心房收缩 |
| Ventricular systole | Contraction of the ventricles | 心室收缩 |
| Diastole | Relaxation phase of the heart | 舒张期 |
| Stroke volume (SV) | Volume of blood pumped per beat (~70 mL) | 每搏输出量 |
| Cardiac output (CO) | SV × HR; typically ~5 L/min | 心输出量 |
| AV valves | Valves between atria and ventricles (tricuspid, bicuspid) | 房室瓣 |
| Semilunar valves | Aortic and pulmonary valves | 半月瓣 |
| Isovolumetric contraction | Brief period when ventricles contract but all valves are closed | 等容收缩 |
| Isovolumetric relaxation | Brief period when ventricles relax but all valves are closed | 等容舒张 |
| End‑diastolic volume (EDV) | Ventricular volume just before contraction (~120 mL) | 舒张末期容积 |
| End‑systolic volume (ESV) | Ventricular volume after contraction (~50 mL) | 收缩末期容积 |
12. The Cardiac Cycle during Exercise | 运动时的心动周期
During exercise, sympathetic stimulation and circulating adrenaline increase heart rate and myocardial contractility. The duration of a cardiac cycle shortens dramatically; at 180 bpm, the cycle may last only 0.33 s. Diastole is shortened the most, which reduces passive filling time. To maintain stroke volume, the enhanced contractility empties the ventricles more completely (lower ESV) and venous return is boosted by the skeletal muscle and respiratory pumps. Atrial systole becomes proportionally more important, contributing up to 40% of ventricular filling at very high heart rates. Consequently, cardiac output can rise three‑ to five‑fold, meeting the increased oxygen demand of active muscles.
运动时，交感神经刺激和循环中的肾上腺素增快心率并增强心肌收缩力。心动周期的时长显著缩短；心率 180 次/分时，周期可能仅持续 0.33 秒。舒张期缩短最为显著，从而缩短了被动充盈时间。为维持每
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