A-Level Biology: The Cardiac Cycle and Cardiac Output

The Cardiac Cycle: An Overview 心脏周期概述

The cardiac cycle describes the complete sequence of mechanical events that occur during one heartbeat. It is a precisely coordinated process that alternates between contraction (systole) and relaxation (diastole), ensuring unidirectional blood flow through the four chambers of the heart. The cycle lasts approximately 0.8 seconds at a resting heart rate of 75 beats per minute, and each phase is governed by pressure gradients that open and close the atrioventricular (AV) and semilunar valves. Understanding the cardiac cycle is fundamental to A-Level Biology because it integrates anatomy, physiology, and the principles of pressure-driven flow, all of which appear regularly in exam questions on circulation and transport in mammals.

心脏周期描述了在一次心跳中发生的完整机械事件序列。这是一个精确协调的过程，交替进行收缩（收缩期）和舒张（舒张期），确保血液通过心脏四个腔室的单向流动。在静息心率75次/分钟时，周期大约持续0.8秒，每个阶段都由压力梯度控制，压力梯度使房室瓣和半月瓣打开和关闭。理解心脏周期对A-Level生物学至关重要，因为它整合了解剖学、生理学和压力驱动流动的原理，这些内容经常出现在有关哺乳动物循环和运输的考试题目中。

Structure of the Heart: Chambers and Valves 心脏的结构：腔室和瓣膜

The mammalian heart is a double pump composed of four chambers: two thin-walled atria that receive blood from the veins, and two thick-walled ventricles that pump blood out into the arteries. The right atrium receives deoxygenated blood from the vena cavae and passes it through the tricuspid valve into the right ventricle, which pumps it to the lungs via the pulmonary artery. The left atrium receives oxygenated blood from the pulmonary veins and passes it through the bicuspid (mitral) valve into the left ventricle, whose muscular wall is approximately three times thicker than the right ventricle’s because it must generate sufficient pressure to pump blood throughout the entire systemic circulation. Four valves ensure one-way flow: the tricuspid and bicuspid AV valves prevent backflow into the atria during ventricular contraction, while the pulmonary and aortic semilunar valves prevent backflow from the arteries into the ventricles during relaxation.

哺乳动物的心脏是一个双泵，由四个腔室组成：两个薄壁的心房接收来自静脉的血液，两个厚壁的心室将血液泵入动脉。右心房从腔静脉接收去氧血液，通过三尖瓣将其传递到右心室，右心室将血液通过肺动脉泵送到肺部。左心房从肺静脉接收含氧血液，通过二尖瓣将其传递到左心室，左心室的肌肉壁大约比右心室厚三倍，因为它必须产生足够的压力将血液泵送到整个体循环。四个瓣膜确保单向流动：三尖瓣和二尖瓣房室瓣防止心室收缩时血液回流到心房，而肺动脉瓣和主动脉瓣半月瓣防止舒张时血液从动脉回流到心室。

Atrial Systole: Active Filling of the Ventricles 心房收缩期：心室的主动充盈

Atrial systole marks the beginning of the cardiac cycle and lasts approximately 0.1 seconds. During this phase, the sinoatrial node (SAN) initiates a wave of electrical excitation that spreads across both atria, causing them to contract simultaneously. The contraction raises atrial pressure above ventricular pressure, forcing the remaining 20-30% of blood ： which had been passively flowing into the ventricles during the preceding diastole ： through the open AV valves. This “atrial kick” is particularly important during exercise, when the shortened diastolic interval means passive ventricular filling is reduced and the atria’s active contribution becomes proportionally larger. Throughout atrial systole, the semilunar valves remain closed because ventricular pressure is still lower than arterial pressure.

心房收缩标志着心脏周期的开始，持续约0.1秒。在此阶段，窦房结（SAN）启动一波电兴奋，传播到两个心房，使它们同时收缩。收缩使心房压力升高到心室压力之上，迫使剩余的20-30%血液通过开放的房室瓣进入心室：这些血液在前一个舒张期中已经被动地流入心室。这种”心房冲击”在运动期间尤为重要，因为缩短的舒张间隔意味着被动心室充盈减少，心房主动贡献的比例相应增大。在整个心房收缩期间，半月瓣保持关闭，因为心室压力仍然低于动脉压力。

Ventricular Systole: Ejecting Blood into the Arteries 心室收缩期：将血液射入动脉

Ventricular systole lasts approximately 0.3 seconds and can be divided into two sub-phases: isovolumetric contraction and ventricular ejection. At the onset of ventricular contraction, the rising pressure inside the ventricles immediately exceeds atrial pressure, snapping the AV valves shut ： this closure produces the first heart sound (“lub”). For a brief moment, both the AV and semilunar valves are closed while ventricular pressure continues to rise; because the blood volume inside the ventricles does not change, this is called isovolumetric contraction. Once ventricular pressure surpasses the pressure in the aorta (left side, ~80 mmHg) and pulmonary artery (right side, ~10 mmHg), the semilunar valves are forced open and blood is ejected into the arteries. The left ventricle ejects approximately 70 mL of blood per beat at rest ： the stroke volume.

心室收缩期持续约0.3秒，可分为两个亚阶段：等容收缩和射血。在心室收缩开始时，心室内升高的压力立即超过心房压力，使房室瓣关闭：这种关闭产生第一心音（”lub”）。在短暂的时刻内，房室瓣和半月瓣都关闭，而心室压力继续上升；由于心室内血量不变，这被称为等容收缩。一旦心室压力超过主动脉（左侧，约80 mmHg）和肺动脉（右侧，约10 mmHg）的压力，半月瓣被迫打开，血液被射入动脉。左心室在静息状态下每次搏动射出约70 mL血液：即每搏输出量。

Diastole: Relaxation and Passive Filling 舒张期：松弛和被动充盈

Diastole occupies approximately 0.4 seconds ： the longest phase of the cardiac cycle at rest. Ventricular diastole begins with isovolumetric relaxation: as the ventricles relax, their internal pressure drops sharply, falling below arterial pressure. This causes the semilunar valves to close ： producing the second heart sound (“dub”) ： while the AV valves remain shut because ventricular pressure is still above atrial pressure. Once ventricular pressure drops below atrial pressure, the AV valves open and blood that has been accumulating in the atria during ventricular systole rushes passively into the relaxed ventricles. Approximately 70% of ventricular filling occurs passively during early diastole, before the next atrial contraction adds the final 30% in the subsequent atrial systole.

舒张期占据约0.4秒：是静息时心脏周期中最长的阶段。心室舒张始于等容舒张：随着心室松弛，其内部压力急剧下降，降至动脉压力以下。这导致半月瓣关闭：产生第二心音（”dub”）：而房室瓣保持关闭，因为心室压力仍高于心房压力。一旦心室压力降至心房压力以下，房室瓣打开，在心室收缩期间积聚在心房的血液被动地涌入松弛的心室。约70%的心室充盈在舒张早期被动发生，然后下一次心房收缩在随后的心房收缩期中加入最后30%。

Pressure Changes During One Cardiac Cycle 一个心脏周期中的压力变化

Exam questions frequently ask students to interpret graphs showing pressure changes in the left atrium, left ventricle, and aorta throughout a single cardiac cycle. The left ventricular pressure curve is the most dramatic: it rises steeply from near-zero during isovolumetric contraction, peaks at approximately 120 mmHg during ejection, and then falls rapidly during isovolumetric relaxation before settling back to near-zero in diastole. Aortic pressure follows a similar but slightly delayed trajectory: it rises to a peak (systolic pressure, ~120 mmHg) when the semilunar valve opens, then gradually declines as blood flows into the systemic circulation, with a distinctive dicrotic notch caused by the brief backflow that closes the aortic valve. Atrial pressure remains low throughout (~0-5 mmHg), with a small rise during atrial systole and a small increase during ventricular systole as the AV valves bulge back into the atria.

考试题目经常要求学生解释显示单个心脏周期中左心房、左心室和主动脉压力变化的图表。左心室压力曲线最为剧烈：在等容收缩期间从接近零急剧上升，在射血期间达到约120 mmHg的峰值，然后在等容舒张期间迅速下降，最后在舒张期回到接近零。主动脉压力遵循相似但略微延迟的轨迹：当半月瓣打开时升高到峰值（收缩压，约120 mmHg），然后随着血液流入体循环逐渐下降，并有一个由关闭主动脉瓣的短暂回流引起的特征性重搏切迹。心房压力始终较低（约0-5 mmHg），在心房收缩期间有小幅上升，在心室收缩期间当房室瓣向心房膨胀时也有小幅增加。

Myogenic Stimulation: The Sinoatrial and Atrioventricular Nodes 肌源性刺激：窦房结和房室结

Unlike skeletal muscle, which requires nervous stimulation to contract, cardiac muscle is myogenic ： it can initiate its own electrical impulses without external input. The heart’s natural pacemaker is the sinoatrial node (SAN), a specialised cluster of cells located in the wall of the right atrium. The SAN spontaneously depolarises at a rate of approximately 60-100 times per minute, generating an electrical wave that spreads across both atria and triggers atrial systole. The wave of excitation cannot pass directly from the atria to the ventricles because a non-conducting layer of fibrous tissue separates them. Instead, the electrical signal is delayed for approximately 0.1 seconds at the atrioventricular node (AVN), the only conducting pathway between the atria and ventricles. This delay is physiologically crucial: it allows the atria to complete their contraction and fully empty into the ventricles before ventricular systole begins.

与需要神经刺激才能收缩的骨骼肌不同，心肌是肌源性的：它可以在没有外部输入的情况下自行产生电脉冲。心脏的天然起搏器是窦房结（SAN），它是位于右心房壁中的一个特化细胞簇。SAN以大约每分钟60-100次的速率自发去极化，产生一个传播到两个心房并触发心房收缩的电波。兴奋波不能直接从心房传递到心室，因为一层不导电的纤维组织将它们隔开。相反，电信号在房室结（AVN）：心房和心室之间唯一的传导通路：处延迟约0.1秒。这种延迟在生理上至关重要：它允许心房完成收缩并在心室收缩开始之前完全排空到心室中。

The Electrocardiogram: Interpreting the ECG Trace 心电图：解读ECG波形

The electrical activity of the heart can be recorded non-invasively as an electrocardiogram (ECG). A typical ECG trace displays three distinctive waves that correspond to the electrical events of the cardiac cycle. The P wave represents atrial depolarisation and occurs just before atrial systole; its small amplitude reflects the relatively small muscle mass of the atria. The QRS complex represents ventricular depolarisation ： a much larger deflection because the ventricles have far greater muscle mass ： and it coincides with the onset of ventricular systole; the electrical activity of atrial repolarisation is hidden within this complex. The T wave represents ventricular repolarisation and occurs just before ventricular diastole begins. In exam questions, students may be asked to calculate heart rate from an ECG by measuring the R-R interval or to identify abnormalities such as tachycardia (heart rate above 100 bpm) and bradycardia (below 60 bpm).

心脏的电活动可以通过心电图（ECG）无创地记录下来。典型的ECG波形显示三个与心脏周期电事件相对应的特征波。P波代表心房去极化，发生在心房收缩之前；其幅度较小反映了心房相对较小的肌肉质量。QRS波群代表心室去极化：其偏移幅度更大，因为心室的肌肉质量大得多：并且与心室收缩的开始同时发生；心房复极化的电活动隐藏在这个波群中。T波代表心室复极化，发生在心室舒张开始之前。在考试中，学生可能被要求通过测量R-R间期从ECG计算心率，或识别异常如心动过速（心率超过100 bpm）和心动过缓（低于60 bpm）。

Cardiac Output: Stroke Volume × Heart Rate 心输出量：每搏输出量 × 心率

Cardiac output (CO) is the volume of blood pumped by each ventricle per minute, calculated as the product of stroke volume (SV) and heart rate (HR): CO = SV × HR. At rest, a typical adult has a stroke volume of approximately 70 mL and a heart rate of 72 bpm, yielding a cardiac output of roughly 5 litres per minute ： approximately equal to the body’s total blood volume. During strenuous exercise, heart rate can increase to 180-200 bpm and stroke volume to 120-140 mL, raising cardiac output to 20-25 L/min in trained athletes. Stroke volume itself is determined by three factors: preload (the degree of ventricular stretch before contraction, governed by venous return ： Starling’s law of the heart), contractility (the intrinsic strength of ventricular contraction, enhanced by sympathetic stimulation and adrenaline), and afterload (the arterial pressure against which the ventricle must pump; higher afterload reduces stroke volume).

心输出量（CO）是每个心室每分钟泵出的血液体积，计算为每搏输出量（SV）和心率（HR）的乘积：CO = SV × HR。在静息状态下，一个典型成年人的每搏输出量约为70 mL，心率约为72 bpm，产生约每分钟5升的心输出量：大约等于身体的总血量。在剧烈运动期间，心率可增加到180-200 bpm，每搏输出量可增加到120-140 mL，在训练有素的运动员中心输出量可提升到20-25 L/min。每搏输出量本身由三个因素决定：前负荷（收缩前心室拉伸的程度，由静脉回流量决定：斯塔林心脏定律）、收缩性（心室收缩的内在强度，由交感神经刺激和肾上腺素增强）、和后负荷（心室必须克服的动脉压力；较高的后负荷会降低每搏输出量）。

Key Bilingual Terms and Exam Tips 关键双语术语和考试技巧

Cardiac cycle · 心脏周期 | Systole · 收缩期 | Diastole · 舒张期 | Sinoatrial node (SAN) · 窦房结 | Atrioventricular node (AVN) · 房室结 | Stroke volume · 每搏输出量 | Cardiac output · 心输出量 | Electrocardiogram (ECG) · 心电图 | Isovolumetric contraction · 等容收缩 | Dicrotic notch · 重搏切迹 | Myogenic · 肌源性的 | Preload · 前负荷 | Afterload · 后负荷 | Contractility · 收缩性 | Semilunar valve · 半月瓣 | Atrioventricular valve · 房室瓣 | Starling’s law · 斯塔林定律 | Venous return · 静脉回流量

Exam Tips 考试建议: When answering cardiac cycle questions, always describe pressure changes first before mentioning valve movements ： valves open and close passively in response to pressure gradients, not by their own action. For ECG interpretation, remember the mnemonic “PQRST”: P wave precedes atrial contraction, QRS precedes ventricular contraction, and T wave precedes ventricular relaxation. Calculation questions on cardiac output are common ： always show the formula CO = SV × HR and express your answer in L/min. If the question provides values in mL, convert to litres by dividing by 1000. Finally, when explaining the myogenic nature of the heart, emphasise that the SAN sets the intrinsic rhythm and that nervous and hormonal inputs (sympathetic and parasympathetic nerves, adrenaline) only modify the rate rather than initiate contraction.

考试建议: 回答心脏周期问题时，始终先描述压力变化再提及瓣膜运动：瓣膜是根据压力梯度被动打开和关闭的，而不是自行运动的。对于ECG解读，记住助记符”PQRST”：P波在心房收缩之前，QRS在心室收缩之前，T波在心室舒张之前。关于心输出量的计算题很常见：始终展示公式CO = SV × HR，并以L/min表示答案。如果题目给出的值是mL，除以1000转换为升。最后，在解释心脏的肌源性特性时，强调SAN设定固有节律，而神经和激素输入（交感和副交感神经、肾上腺素）只改变速率而不是启动收缩。