The normal heart rhythm is controlled by the heart’s natural pacemaker, the sinus node, which is why it is termed sinus rhythm. On average, most people’s heart rate is between 60 and 80 beats per minute at rest although it may be slower if the person is fit, relaxed or just through natural individual variation.
The heart is a muscle pump made up of many thousands of muscle fibres. The two atriums make up the top half. They are smaller chambers that receive blood as it returns from the body or the lungs. When they squeeze they push blood through the tricuspid and mitral valves into the ventricles. The ventricles are bigger chambers and the left ventricle in particular is muscular and strong. When the ventricles squeeze, blood is elected out of the heart and pushed into the lungs by the right ventricle or around the rest of the body by the left ventricle. It is this push of blood by the left ventricule that cause the pulse that can be felt at the wrist or any awareness of the heart beat in the chest or neck.
Heart muscle fibres are able to conduct electricity from one fibre to another, allowing an electrical impulse to spread through the heart chambers. As electricity passes through a muscle fibre it makes it twitch. Once the electrical impulse has passed through the fibre relaxes and waits for the next impulse to come.
In a normal heart beat the electrical impulse is generated by the sinus node which sits in the top of the right atrium. The electricity spreads very rapidly and smoothly through the atriums so all the muscle fibres twitch together and two atria contract (squeeze), pushing blood into the ventricles. The fibres then relax, the atriums expand and refill and wait for the next electrical impulse.
As the tricuspid and mitral valves that separate the atriums from the ventricles act like a layer of electrical insulation, there is a special conduction system to let the electrical impulse travel from the atriums to the ventricles. The atrioventricular node (AV node) acts like a junction box. It slows the electrical impulse down so that the ventricles have time to complete their filling and it sends the electrical impulse from the atriums into the His bundle that passes through the insulating valve rings and enters into the septum (muscle wall) between the two ventricles.
After the electricity has travelled through the AV node and His bundle it spreads quickly through the ventricles using special fibres called the bundle branches. This rapid spread of electricity causes all the ventricular muscle fibres to twitch at the same time and the ventricles contract and vigorously pump blood out of the heart. The electrical impulse then fades away, the muscle fibres relax and the ventricles expand, filling up with blood and waiting for the next impulse.
In a normal rhythm there is a one-way flow of electricity from top to bottom with a coordinated squeezing of atriums first, followed by the ventricles. When the atriums contract the ventricles fill and when the ventricles contact the atriums fill. The speed at which the heart pumps (heart rate), is dictated by the sinus node. The sinus node is influenced by many things including the nervous system, adrenaline levels, activity, illness and medications. Sinus tachycardia occurs when the sinus node increases the heart rate to more than 100 beats per minute. This is a natural occurrence during exercise as well as with strong emotions such as excitement or anxiety.