The Cardiac Cycle | 心动周期

📚 The Cardiac Cycle | 心动周期

The cardiac cycle describes the sequence of mechanical and electrical events that occur during one complete heartbeat. It includes contraction (systole) and relaxation (diastole) of the atria and ventricles, coordinated by the conduction system of the heart. Understanding pressure changes, valve movements and volume changes is essential for A-Level Biology.

心动周期描述一次完整心跳中发生的机械与电活动事件序列。它包括心房和心室的收缩（收缩期）与舒张（舒张期），由心脏传导系统协调。理解压力变化、瓣膜开闭和容积变化是 A-Level 生物学的核心内容。

1. Overview of the cardiac cycle | 心动周期概述

A single cardiac cycle lasts about 0.8 seconds at a resting heart rate of 75 beats per minute. The cycle can be divided into atrial systole, ventricular systole and diastole. Although both sides of the heart contract together, the left side generates much higher pressures because it pumps blood through the systemic circulation.

在静息心率 75 次/分钟时，一个心动周期约持续 0.8 秒。周期可分为心房收缩期、心室收缩期和舒张期。虽然左右两侧心脏同时收缩，但左侧产生的压力高得多，因为它将血液泵入体循环。

The terms systole and diastole refer to contraction and relaxation respectively. When describing the cardiac cycle, always state whether the event occurs during atrial systole, ventricular systole or diastole, and link it to valve opening or closing.

收缩期和舒张期分别指收缩和舒张。在描述心动周期时，应始终说明事件发生在心房收缩期、心室收缩期还是舒张期，并将其与瓣膜的开闭联系起来。

2. The heart as a pump: chambers and valves | 心脏作为泵：腔室与瓣膜

The mammalian heart has four chambers: two thin-walled atria and two thick-walled ventricles. The left ventricle has the thickest muscular wall because it must generate enough pressure to drive blood around the entire body.

哺乳动物心脏有四个腔室：两个壁薄的心房和两个壁厚的心室。左心室的肌壁最厚，因为它必须产生足够的压力将血液推向全身。

Four main valves prevent backflow. The atrioventricular (AV) valves, namely the tricuspid valve on the right and the bicuspid or mitral valve on the left, lie between the atria and ventricles. The semilunar valves, namely the aortic and pulmonary valves, lie at the base of the major arteries leaving the heart.

四个主要瓣膜防止血液倒流。房室瓣（右侧三尖瓣，左侧二尖瓣/双尖瓣）位于心房与心室之间。半月瓣（主动脉瓣和肺动脉瓣）位于离开心脏的大动脉基部。

Valves are opened and closed by pressure differences, not by muscle contraction of the valve cusps themselves. The chordae tendineae and papillary muscles prevent the AV valves from turning inside out during ventricular contraction.

瓣膜的开闭由压力差驱动，而不是依靠瓣膜本身肌肉收缩。腱索和乳头肌可防止房室瓣在心室收缩时翻转。

3. Initiation and conduction: SAN to Purkyne fibres | 起始与传导：从窦房结到浦肯野纤维

The heartbeat is myogenic, meaning it is initiated within the cardiac muscle itself. The sinoatrial node (SAN) in the wall of the right atrium acts as the pacemaker and sends out electrical impulses.

心跳是肌源性的，即由心肌自身发起。位于右心房壁的窦房结（SAN）充当起搏器并发出电冲动。

The impulse spreads across both atria, causing atrial contraction. It is then delayed at the atrioventricular node (AVN) for about 0.1 seconds. This delay allows the ventricles to fill before they contract.

冲动传遍两侧心房，引起心房

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