Contains the full lesson along with a supporting toolkit, including teachers’ notes.
Nothing is more central to our concept of survival than our dependence on the heart. Nothing signals our distress faster than a quickened heartbeat. The heart pumps blood through thecardiovascular system around the body, delivering oxygen and nutrients. The rate at which blood needs to be delivered depends on the energy being used by the body; it must work precisely or serious injury or death can occur. In this lesson we look at the mechanism of the heart and the importance of its electrical system. We discuss some problems that can arise and how two modern medical devices – pacemakers and defibrillators – can come to the rescue.
The Cardiac Cycle:
The cardiac cycle is the sequence of events that occurs when the heart beats. There are two phases of the cardiac cycle. In the diastole phase, the heart ventricles are relaxed and the heart fills with blood. In the systole phase, the ventricles contract and pump blood to the arteries.
How Does the Heart Know When to Beat?
Each heartbeat starts with an electrical signal. The signals are sent and received by special electrical cells within the heart called nodes. As the signals move through the heart, they tell the chambers that pump blood (called atria and ventricles) when to contract and squeeze blood out of the heart. When you're active, the signal rate from the nodes increases causing the heart to pump blood at a higher rate. When resting, the signals return to a normal frequency. The SA (sinoatrial) node (positioned in the right atrium), is the heart's natural pacemaker. It starts each heartbeat by sending an electrical signal that tells the atria to contract. The AV (atrioventricular) node (positioned on the wall between the rightatrium and the right ventricle) receives the signal and directs it to the ventricles.