Normal Heart Rhythm
Normally, the heart beats in a regular, organized way, at a rate of 60-100 beats per minute.
The heart during normal rhythm (“sinus rhythm”)
The heart is a muscular pump about the size of a clenched fist, which delivers blood containing oxygen to the body. It is divided into two upper chambers, or “atria”, which collect blood returning via the great veins, and two lower chambers or “ventricles”, which pump blood out through the aorta (main artery) and the lungs.
This is because it is driven by the “sinus node”, a clump of specialized cells situated in the atria, which emits electrical impulses. The sinus node is sometimes referred to as the heart’s natural pacemaker. These electrical impulses spread through the atria and then into the ventricles via a connecting cable (the “AV node”). The sinus node controls the timing of the heart, according to the needs of the body. An example of this is during exercise, when the heart rate speeds up. When the heart is beating normally like this, we refer to it as “sinus rhythm” or “normal sinus rhythm”.
The heart is an amazing machine!
The heart really is an amazing machine. It is actually a big muscle that pumps at a rate of about 72 beats per minute and pumps out 5 litres of blood every minute. The heart will keep doing its job for every minute in every hour, for 24 hours in every day, for 365 days a year, for every year of your life. When you exercise or are frightened or run a fever your heart rate can increase up to 140 beats a minute and pump out 15 litres of blood a minute.
This means that in a lifetime of 70 years, the heart will have beaten at least 2,650,000,000 times. If it was used to pump water into a medium sized swimming pool, it would fill it in one week. In your lifetime, your heart will have generated enough power to pull 64 adult elephants up Table Mountain. Yet, it can never be sent in for a service and if it stops beating for longer than three minutes you will die.
The heart performs its task through the collaborative efforts of three components:
• The heart structure (the muscle's chambers and valves)
• The electrical system (the signals that tell the heart to beat)
• The circulatory system (the blood pathways)
The Heart Structure
The heart muscle is divided into four chambers — the left and right atria (the upper chambers) and the left and right ventricles (the lower chambers). With each heartbeat, the atria draw blood into the heart and send it on to the ventricles, which push the blood out of the heart. Valves located between the chambers and at the ends of the ventricles, open and close like doorways to make sure the blood continues to move forward in the right direction.
The Electrical System
What keeps the heart pumping is a built-in electrical system. An electrical impulse generator, called the “sinus node,” sends signals from the right atrium to trigger the heartbeat, like a natural pacemaker. The electrical current follows a web of pathways through the heart, causing the chambers to squeeze and release in a steady, rhythmic sequence that draws blood into the heart and pushes it out.
The Circulatory System
Arteries are the vessels that carry “fresh” blood, rich in oxygen, from the left side of the heart to smaller vessels, called capillaries, throughout the body. Veins return “used” blood from the capillaries to the right side of the heart, which routes it to the lungs to pick up fresh oxygen. The fresh blood then returns to the left side of the heart, and the cycle repeats.
When Things Don't Work Together
Trouble in any part of the heart’s components can disrupt the entire system and lead to problems elsewhere. For instance, a clogged artery can cause a heart attack, which can damage a part of the heart muscle, which can disrupt the electrical pathway in the damaged area, which can cause a disturbance in the heart rhythm (an arrhythmia).
Keeping Things Running Smoothly
Keeping this interrelated system healthy is largely under your control. Following the commonly prescribed advice to eat well, exercise, avoid excess alcohol and refrain from smoking can go a long way toward maintaining good heart health. However, some heart problems can occur despite the best efforts to stay healthy. Ongoing research continues to lead to new medicines, procedures and interventions to help minimize heart problems and keep the whole system running smoothly.