The Conduction System
The intricate timing system that controls the rhythmic beating of your heart is handled by the heart's electrical, or conduction system. This system is the circuitry that conducts electrical impulses throughout the muscle of the heart. These electrical impulses stimulate the heart muscle to contract and squeeze blood out of the heart and into the arteries.
The sinus node, the heart's natural pacemaker, consists of a group of cells in the upper part of the right atrium. Also called the sinoatrial node, it's the normal point of origin of the electrical impulses.
Once the electrical signal is generated by the sinus node, it moves from cell to cell down through the heart until it reaches the atrioventricular (AV) node, a cluster of cells in the center of the heart between the atria and ventricles. The AV node acts as a gate that slows the electrical current before it's allowed to pass through to the ventricles. This delay ensures that the atria have had a chance to fully contract before the ventricles are stimulated.
After clearing the AV node, the electrical current is channeled to the ventricles by special fibers embedded in the walls of the lower parts of the heart. It's this transfer of electrical impulses that doctors measure when they take an electrocardiogram to measure how well a heart is beating.
The autonomic nervous system, the same system that automatically controls blood pressure, breathing and excretion, for example, controls the firing of the sinus node, which triggers the start of the cardiac cycle. This system can make snap decisions, causing the sinus node to increase the heart rate to twice normal within only 3 to 5 seconds. This flexibility is important during exercise, when the heart must rapidly increase its beating speed to keep up with the body's increased demand for oxygen.
As you age, your heart rate slows. The decrease is not noticeable during everyday activities, but your maximal heart rate during exercise tends to decrease as you grow older. For example, your maximal heart rate at age 25 is about 200 beats per minute; by age 65, it has dropped to about 155 beats per minute.
Heart Rates for Different Ages
Normal Heart Rates at Rest
Average Maximal Heart Rates