The Cardiac Cycle, Animation · Alila Medical Media
本视频通过动画详细讲解心脏周期的各个阶段，包括收缩期与舒张期，适合练习医学英语和心血管系统知识。
The cardiac cycle refers to the sequence of events that occur and repeat with every heartbeat.
心脏周期是指每次心跳时发生并重复的一系列事件。
It can be divided into 2 major phases: systole and diastole, each of which subdivides into several smaller phases.
心脏搏动过程可以分为两个主要阶段：收缩期（systole）和舒张期（diastole），每个阶段又可进一步细分为若干更小的子阶段。
Systole and diastole, when not specified otherwise, refer to ventricular contraction and relaxation, respectively.
除非另有说明，否则“收缩期（systole）”和“舒张期（diastole）”分别指心室的收缩和舒张过程。
Reminders: - Blood flows from higher to lower pressure.
提醒： - 血液是从压力较高的地方流向压力较低的地方的。
- Contraction increases the pressure within a chamber, while relaxation lowers the pressure.
- 收缩会增加腔室内的压力，而松弛则会降低压力。
- AV valves open when atrial pressures are higher than ventricular pressures and close when the pressure gradient is reversed.
- 房室瓣在心房压力高于心室压力时打开，在压力梯度方向逆转时关闭。
Similarly, semilunar valves open when ventricular pressures are higher than aortic/pulmonary pressures, and close when the reverse is true.
同样地，半月瓣在心室压力高于主动脉/肺动脉压力时打开；当情况相反时（即心室压力低于主动脉/肺动脉压力时），半月瓣会关闭。
The cycle is initiated with the firing of the SA node that stimulates the atria to depolarize.
这个循环过程始于窦房结（SA node）的放电，该放电会刺激心房发生去极化（即细胞内的电荷分布发生变化）。
This is represented by the P-wave on the ECG.
这在心电图（ECG）上表现为 P 波。
Atrial contraction starts shortly after the P-wave begins, and causes the pressure within the atria to increase, forcing blood into the ventricles.
心房收缩在 P 波开始后不久便随即发生，这会导致心房内的压力升高，从而迫使血液流入心室。
Atrial contraction, however, only accounts for a
然而，心房收缩仅占……（具体比例未在原文中给出）。
fraction of ventricular filling, because at this
心室充盈的比例；因为在这个阶段……
point, the ventricles are already almost full due to passive blood flow down the ventricles through the open AV valves.
实际上，由于血液通过开放的房室瓣被动地流入心室，心室已经几乎充满了血液。
As atrial contraction completes, atrial pressure begins to fall, reversing the pressure gradient across the AV valves, causing them to close.
当心房收缩完成时，心房压力开始下降，这会改变房室瓣两侧的压力差，从而导致房室瓣关闭。
The closing of the AV valves produces the first heart sound, S1, and marks the beginning of systole.
二尖瓣和三尖瓣的关闭产生了第一心音（S1），标志着心缩期的开始。
At this point, ventricular depolarization, represented by the QRS complex, is half way through, and the ventricles start to contract,
此时，心室去极化过程（由QRS波群表示）已经进行到一半，心室开始收缩。
rapidly building up pressures inside the ventricles.
脑室内部的压力正在迅速上升。
For a moment, however, the semilunar valves remain closed, and the ventricles contract within a closed space.
然而，在某一瞬间，半月瓣仍然处于关闭状态，心室在封闭的空间内继续收缩。
This phase is referred to as isovolumetric contraction, because no blood is ejected and ventricular volume is unchanged.
这个阶段被称为等容收缩（isovolumetric contraction），因为在收缩过程中没有血液被排出，心室容积也没有发生变化。
Ventricular ejection starts when ventricular pressures exceed
心室射血过程始于心室压力超过某个临界值时。
the pressures within the aorta and pulmonary artery; the aortic and pulmonic valves open and blood is ejected out of the ventricles.
主动脉和肺动脉内的压力；主动脉瓣与肺动脉瓣打开，血液从心室中被挤出。
This is the rapid ejection phase.
这就是快速喷射阶段。
As ventricular repolarization, reflected by the T-wave, begins, ventricular pressure starts to fall and the force of ejection is reduced.
当心室复极化开始（这一过程通过T波表现出来）时，心室压力开始下降，同时射血力量也会减弱。
When ventricular pressures drop below aortic and pulmonary pressures, the semilunar valves close, marking the end of systole and beginning of diastole.
当心室压力降至主动脉压力和肺动脉压力以下时，半月瓣会关闭，这标志着收缩期的结束和舒张期的开始。
Closure of semilunar valves produces the second heart sound, S2.
半月瓣的关闭产生了第二心音（S2）。
The first part of diastole is, again, isovolumetric, as the ventricles relax with all valves closed.
舒张期的第一个阶段仍然是等容期：此时心室处于松弛状态，所有瓣膜均处于关闭状态。
Ventricular pressure drops rapidly but their volumes remain unchanged.
心室的压力会迅速下降，但其容积保持不变。
Meanwhile, the atria are being filled with blood and atrial pressures rise slowly.
与此同时，心房开始充血，心房压力逐渐升高。
Ventricular filling starts when ventricular pressures drop below atrial pressures, causing the AV valve to open, allowing blood to flow down the ventricles passively.
心室充盈过程始于心室压力降至 atrial pressures（心房压力）以下时：此时房室瓣（AV valve）打开，血液便被动地流入心室。
The atria contract to finish the filling phase and the cycle repeats itself.
心房收缩，从而完成血液充盈的过程；随后这个循环会再次重复。