Introduction to the heart’s electrical conduction system

The heart lies in the space between the lungs, which is called the mediastinum. It is approximately the size of a person’s fist and consists of four muscular chambers, two atria and two ventricles. The role of the heart is to pump blood around the body, enabling oxygen, nutrients and waste products to be transported to and from the different cells and organs. Blood travels through the heart in the following way– oxygen depleted blood returning from the body cells enters the right atrium, and at the same time, blood which has been oxygenated in the lungs returns to the left atrium. The increase in volume of blood in the atria then starts to flow into both ventricles.

This is facilitated by contraction of the atrial walls. Once the ventricles are full of blood, they in turn contract and eject their contents. Blood from the right ventricle travels to the lungs to pick up oxygen, and blood which has already been oxygenated in the left ventricle travels to the rest of the body’s cells and organs. In order for blood to be pumped around the body as efficiently as possible, the atria and ventricles need to contract in sequence. And it is the role of the heart’s electrical conduction system to control this. The conduction system is comprised of specialised cells embedded within the heart’s muscular walls.

These cells are able to generate and conduct electrical impulses, which in turn stimulate the heart muscle to contract. The conduction system works as follows. Firstly, the sinus node generates an electrical impulse which sweeps across both atria in a wave-like fashion. This spread of the impulse is called a wave of depolarisation. The atria respond by contracting and this ejects blood into the ventricles. Meanwhile, the impulse reaches the atrioventricular node, which lies between the atria and the ventricles, before travelling on through the bundle of His and right and left bundle branches. The ventricles, which have now had sufficient time to fill with blood, contract in response to the impulse. The process starts again with another impulse being generated.

As each cycle originates at the sinus node, it is referred to as a sinus beat. And where this cycle occurs repeatedly, it is called sinus rhythm. This represents the normal sequence of events leading to controlled electrical activation and contraction of the heart. We will revisit the concept of sinus rhythm in week two of the course.