A heartbeat begins with a contraction of the heart's two upper chambers (the atria). The left atrium gets oxygen-rich blood from the lungs; the right atrium gets oxygen-poor blood from the rest of the body. When the atria contract, they push blood to the two lower chambers of the heart (the ventricles). The left ventricle pumps oxygen-rich blood to all parts of the body, while the right ventricle pumps oxygen-poor blood to the lungs. Normally, the heart beats 60 to 100 times a minute in a regular and orderly fashion.
What kicks off each heartbeat is an electrical impulse that begins in a small group of specialized muscle cells called the sinoatrial (SA) node. The SA node, also known as the natural pacemaker, is located in the wall of the right atrium.
The signal from the SA node spreads quickly to the atria, causing them to contract. The signal then travels to another specialized bundle of cells, the atrioventricular (AV) node, and on to the ventricles through special pathways called bundle branches. The signal reaches the ventricles a fraction of a second after instructing the atria to contract. This lets the ventricles fill with blood before contracting.
It is normal for the heart to speed up during periods of exercise or emotional anxiety, answering the body's need for more oxygen and to get rid of more waste products. Fever, anemia, an overactive thyroid gland, and some drugs (such as decongestants, caffeine, amphetamines, and cocaine) can also cause the heart to pump faster than normal.
Abnormal heart rhythms develop when there is a problem in the heart's electrical system. These can make the heart beat faster than it should, or with an irregular or disordered rhythm. Rhythm abnormalities, which range from mild to life threatening, can change the regular contraction of either the atria or the ventricles.