📚 A-Level Biology: Detailed Explanation of the Stages of the Cardiac Cycle | A-Level 生物：心动周期各阶段详解
The cardiac cycle describes the sequence of events that occurs when the heart beats. In A-Level Biology, you need to understand the pressure changes, valve movements, and electrical coordination that drive blood through the heart. This article breaks down every stage step by step.
心动周期描述的是心脏每次搏动时发生的一系列事件。在 A-Level 生物中，你需要理解推动血液流过心脏的压力变化、瓣膜运动以及电协调机制。本文将逐步拆解每一个阶段。
1. Key Structures and the Conduction System | 关键结构及传导系统
The heart’s rhythmic contraction is initiated by the sinoatrial node (SAN), a group of specialised muscle cells in the wall of the right atrium. The SAN acts as the heart’s natural pacemaker, generating electrical impulses roughly 70 times per minute at rest.
心脏的节律性收缩由窦房结（SAN）启动，窦房结位于右心房壁内，是一群特化的肌细胞。窦房结充当心脏的天然起搏器，静息时每分钟产生约 70 次电冲动。
The electrical impulse spreads across both atria, causing them to contract. It then reaches the atrioventricular node (AVN), which delays the impulse by about 0.1 seconds. This delay ensures that atrial contraction finishes before ventricular contraction begins. The impulse then travels down the bundle of His, through the Purkinje fibres, and up the ventricular walls, triggering ventricular contraction from the apex upwards.
电冲动扩散到两个心房，使其收缩。随后冲动到达房室结（AVN），房室结将冲动延迟约 0.1 秒。这一延迟确保心房收缩结束之后心室收缩才开始。之后冲动沿希氏束下行，经浦肯野纤维传至心室壁，自心尖向上触发心室收缩。
- SAN: pacemaker, initiates atrial systole | 窦房结：起搏点，启动心房收缩
- AVN: delays impulse, allows atrial filling | 房室结：延迟冲动，允许心房充盈
- Bundle of His & Purkinje fibres: rapid conduction to ventricles | 希氏束和浦肯野纤维：快速传导至心室
2. Atrial Systole | 心房收缩期
Atrial systole is the first active phase of the cardiac cycle. The SAN fires, and both atria contract simultaneously. This contraction forces the remaining blood (about 20–25% of ventricular filling) through the atrioventricular valves into the ventricles.
心房收缩期是心动周期的第一个主动阶段。窦房结发出冲动后，左右心房同时收缩。此收缩将剩余血液（约占心室充盈量的 20–25%）经房室瓣推入心室。
During this phase, the atrioventricular valves (tricuspid and bicuspid/mitral) are open, while the semilunar valves (pulmonary and aortic) are closed. Atrial pressure rises slightly above ventricular pressure, which helps push blood passively and actively into the ventricles. At the end of atrial systole, the ventricles contain their maximum volume, known as the end-diastolic volume (EDV).
在此阶段，房室瓣（三尖瓣和二尖瓣）开放，而半月瓣（肺动脉瓣和主动脉瓣）关闭。心房压力略微升高至高于心室压力，帮助血液被动和主动地进入心室。心房收缩期结束时，心室容纳最大血量，即舒张末期容积（EDV）。
EDV ≈ 120–130 cm³ (resting adult) | 舒张末期容积 ≈ 120–130 cm³（静息成人）
3. Ventricular Systole: Isovolumetric Contraction | 心室收缩期：等容收缩
Ventricular systole begins when the ventricles receive the electrical impulse via the Purkinje fibres. The ventricular walls contract inward, causing the pressure inside the ventricles to rise sharply. As ventricular pressure exceeds atrial pressure, the atrioventricular valves snap shut, producing the first heart sound (“lub”).
心室收缩期始于心室通过浦肯野纤维接收到电冲动。心室壁向内收缩，使心室内压力急剧升高。当心室压力超过心房压力时，房室瓣迅速关闭，产生第一心音（”lub”）。
At this early stage, both sets of valves are closed because ventricular pressure is still lower than the pressure in the aorta and pulmonary artery. The volume of blood in the ventricles remains constant, hence the term isovolumetric contraction. The muscle fibres are generating tension but not yet shortening.
在此早期阶段，两组瓣膜均关闭，因为心室压力仍低于主动脉和肺动脉内的压力。心室内的血容量保持不变，因此称为等容收缩。肌纤维正在产生张力，但尚未缩短。
4. Ventricular Systole: Ejection | 心室收缩期：射血
As ventricular contraction continues, the pressure in the left ventricle rises above the aortic pressure (about 80 mmHg at rest), and the pressure in the right ventricle rises above the pulmonary arterial pressure (about 15 mmHg). The semilunar valves are forced open, and blood is ejected rapidly into the aorta and pulmonary trunk.
随着心室收缩继续进行，左心室压力升至高于主动脉压（静息时约 80 mmHg），右心室压力升至高于肺动脉压（约 15 mmHg）。半月瓣被迫打开，血液被迅速射入主动脉和肺动脉干。
Peak ventricular pressure during ejection reaches about 120 mmHg in the left ventricle. The volume of blood ejected per beat is called the stroke volume (SV), typically about 70 cm³ at rest. The volume remaining after ejection is the end-systolic volume (ESV), about 50–60 cm³.
射血期间左心室峰值压力可达约 120 mmHg。每搏射出的血量称为每搏输出量（SV），静息时约为 70 cm³。射血后剩余的血量为收缩末期容积（ESV），约 50–60 cm³。
Stroke Volume = EDV − ESV | 每搏输出量 = 舒张末期容积 − 收缩末期容积
5. Ventricular Diastole: Isovolumetric Relaxation | 心室舒张期：等容舒张
After ejection, the ventricles begin to relax. Ventricular pressure falls rapidly. When the pressure in the ventricles drops below the pressure in the aorta and pulmonary artery, the semilunar valves close. This closure produces the second heart sound (“dub”).
射血之后，心室开始舒张。心室压力迅速下降。当心室内压力降至低于主动脉和肺动脉中的压力时，半月瓣关闭。此关闭产生第二心音（”dub”）。
For a brief moment, all four heart valves are again closed: the semilunar valves have just closed, and the atrioventricular valves have not yet opened because ventricular pressure is still higher than atrial pressure. Ventricular volume remains constant, so this phase is called isovolumetric relaxation.
在短暂瞬间，四个心瓣膜再次全部关闭：半月瓣刚刚关闭，而房室瓣尚未打开，因为心室压力仍高于心房压力。心室容积保持不变，因此该阶段称为等容舒张。
6. Ventricular Diastole: Filling | 心室舒张期：充盈
As the ventricles continue to relax, their pressure falls below that of the atria. The atrioventricular valves open, and blood flows from the atria into the ventricles. About 70–80% of ventricular filling occurs passively during this phase, without any atrial contraction.
随着心室继续舒张，其压力降至低于心房压力。房室瓣打开，血液从心房流入心室。约 70–80% 的心室充盈在此阶段被动完成，无需心房收缩。
During this phase, the atria are also receiving blood from the venae cavae and pulmonary veins. Near the end of ventricular filling, the atria contract again (the next atrial systole), adding the final volume. This completes the cycle, and the process repeats.
在此阶段，心房也在从腔静脉和肺静脉接收血液。在心室充盈接近结束时，心房再次收缩（下一次心房收缩期），加入最终血量。至此周期完成，过程重复发生。
7. Pressure–Time Graph of the Cardiac Cycle | 心动周期压力-时间曲线
You must be able to interpret the classic pressure–time graph of the left side of the heart. The graph shows simultaneous pressure traces for the aorta, left ventricle, and left atrium over one cycle.
你必须能够解读心脏左侧经典的 压力-时间曲线。该曲线显示一个周期内主动脉、左心室和左心房的同步压力变化。
| Phases | AV valves | Semilunar valves |
| Atrial systole | Open | Closed |
| Isovolumetric contraction | Closed | Closed |
| Ventricular ejection | Closed | Open |
| Isovolumetric relaxation | Closed | Closed |
| Ventricular filling | Open | Closed |
Key features to identify: the aortic pressure trace rises when the semilunar valves open, and shows a small dip (“dicrotic notch”) when the semilunar valves close. The ventricular pressure trace is lower than the atrial trace during filling but becomes the highest during systole.
需要识别的关键特征：主动脉压力曲线在半月瓣打开时上升，并在半月瓣关闭时出现一个小凹陷（”重搏波切迹”）。心室压力曲线在充盈期低于心房曲线，但在收缩期达到最高。
8. Heart Sounds and Valve Function | 心音与瓣膜功能
The two main heart sounds, often described as “lub-dub,” are caused by the closure of heart valves. The first sound (“lub”) occurs at the start of ventricular systole when the atrioventricular valves close. The second sound (“dub”) occurs at the start of ventricular diastole when the semilunar valves close.
两个主要心音常被描述为”lub-dub”，由心脏瓣膜关闭引起。第一心音（”lub”）出现在心室收缩开始时，由房室瓣关闭产生。第二心音（”dub”）出现在心室舒张开始时，由半月瓣关闭产生。
Valve defects can cause abnormal heart murmurs. For example, a leaking atrioventricular valve allows backflow of blood during ventricular systole, producing a distinct hissing sound between “lub” and “dub.”
瓣膜缺陷可导致异常的心脏杂音。例如，房室瓣关闭不全时，心室收缩期血液会返流，在”lub”和”dub”之间产生明显的嘶嘶声。
9. Electrical Activity and the ECG | 电活动与心电图
An electrocardiogram (ECG) records the electrical activity of the heart. The P wave corresponds to atrial depolarisation (atrial systole). The QRS complex corresponds to ventricular depolarisation, which triggers ventricular systole. The T wave corresponds to ventricular repolarisation, which precedes ventricular diastole.
心电图（ECG）记录心脏的电活动。P 波对应心房去极化（心房收缩）。QRS 波群对应心室去极化，触发心室收缩。T 波对应心室复极化，发生在心室舒张之前。
Understanding the ECG helps you link electrical events to mechanical events in the cardiac cycle. Remember: the electrical event always precedes the mechanical contraction it triggers. The AVN delay appears on the ECG as the PR segment, giving the atria time to contract before the ventricles.
理解心电图有助于你将心动周期中的电事件与机械事件联系起来。记住：电事件总是先于其触发的机械收缩。房室结延迟在心电图上表现为 PR 段，使心房有时间在心室收缩之前完成收缩。
10. Cardiac Output and Its Regulation | 心输出量及其调节
Cardiac output (CO) is the volume of blood pumped by one ventricle per minute. It is calculated by multiplying heart rate (HR) by stroke volume (SV):
心输出量（CO）是每分钟由一侧心室泵出的血液量。计算公式为心率（HR）乘以每搏输出量（SV）：
Cardiac Output = Heart Rate × Stroke Volume | 心输出量 = 心率 × 每搏输出量
At rest, with HR = 70 beats min⁻¹ and SV = 70 cm³, CO ≈ 4900 cm³ min⁻¹, roughly 5 dm³. During exercise, both HR and SV increase, raising CO to meet higher oxygen demand.
静息时，HR = 70 次 min⁻¹，SV = 70 cm³，CO ≈ 4900 cm³ min⁻¹，约为 5 dm³。运动时，心率和每搏输出量均增加，使心输出量升高以满足更高的氧气需求。
The cardiac centre in the medulla oblongata regulates heart rate via the autonomic nervous system. Sympathetic nerves release noradrenaline, increasing HR and stroke volume. Parasympathetic (vagus) nerves release acetylcholine, decreasing HR. Adrenaline from the adrenal medulla also increases both HR and contractility.
延髓中的心加速中枢和心抑制中枢通过自主神经系统调节心率。交感神经释放去甲肾上腺素，增加心率和每搏输出量。副交感神经（迷走神经）释放乙酰胆碱，降低心率。肾上腺髓质释放的肾上腺素也会增加心率和心肌收缩力。
11. Common Exam Misconceptions | 常见考试误区
One common mistake is assuming that atrial contraction is necessary for ventricular filling. In reality, most ventricular filling is passive. Another error is thinking that the atrioventricular valves open during ventricular systole; they are actually forced closed by the high ventricular pressure.
一个常见误区是认为心房收缩是心室充盈所必需的。实际上，大部分心室充盈是被动的。另一个错误是认为房室瓣在心室收缩期开放；实际上，心室高压迫使它们关闭。
- Do not call the isovolumetric phases “constant pressure” — it is constant volume with rapidly changing pressure | 不要把等容阶段说成”恒压”——它是容积不变而压力快速变化
- Remember the left ventricle generates higher pressure than the right ventricle | 记住左心室产生的压力高于右心室
- Both atria contract simultaneously; both ventricles contract simultaneously | 左右心房同时收缩；左右心室同时收缩
12. Summary and Key Equations | 总结与关键公式
The cardiac cycle is a continuous sequence of atrial systole, ventricular systole, and diastole. Pressure differences drive blood flow, and valve closures produce heart sounds. The conduction system ensures coordinated timing, while the autonomic nervous system adjusts cardiac output according to demand.
心动周期是心房收缩、心室收缩和舒张的连续序列。压力差驱动血液流动，瓣膜关闭产生心音。传导系统确保协调的时间顺序，自主神经系统则根据需求调节心输出量。
Master the order of events, the pressure traces, and the valve positions in each phase. These form the basis of many A-Level exam questions on this topic.
掌握各阶段的事件顺序、压力曲线和瓣膜位置，这是许多 A-Level 考试题目中关于本主题的基础。
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