Throughout history, the human heart has been thought to have special powers. Writers and poets have called it the seat of emotion, the source of love, and the wellspring of bravery.
In fact, the human heart is only a pump. But what a pump! It starts as a microscopic part of the human embryo. It begins beating during the 4th week of pregnancy and must continue beating without any long pause for as long as that person lives. It has four thin, flexible valves that must open and close with each heartbeat 35 million times a year for an average lifetime. The only rest period for the heart muscle is the short time between heartbeats. No mechanical device yet invented can duplicate the performance of the human heart.
The heart has four compartments, or chambers. The upper chambers are called atria, and the lower chambers are called ventricles. Blood returning from the rest of the body enters the right atrium through two big veins called the superior and inferior vena cavae. Blood flows through the tricuspid valve, located between the right atrium and right ventricle, and the right ventricle pumps it through the pulmonic valve to the lungs.
As it passes through the lungs, the blood picks up a fresh supply of oxygen, gets rid of carbon dioxide, and flows into the left atrium. From there, it flows through the mitral valve into the left ventricle. The left ventricle has a thick muscle that pumps the blood through the aortic valve to the largest blood vessel in the body, the aorta. The aorta distributes the blood back to the rest of the body, with a small amount going to the heart muscle through the coronary arteries.
The four heart valves work in a pattern to keep blood flowing in 1 direction. They are made of thin tissue and open and close easily. The tricuspid, pulmonic, and aortic valves all have three cusps (leaflets). The mitral valve has only two leaflets.
The mitral and tricuspid valves are larger than the others, and their margins, or free edges, are supported by strands of heart tissue called chordae tendineae. These small cords help keep the valves closed during contraction (squeezing) of the heart.
Every normal heartbeat starts in a group of specialized cells in the right atrium, called the sinus node. These pacemaker cells (P cells) discharge an electrical impulse to start each contraction of the heart. The sinus node is connected to a circuit of tissue pathways of special cells that distribute the electrical impulse to the rest of the heart. The impulse causes the atria to contract and push blood from the atria into the ventricles. The impulse travels down the tissue pathways to a junction called the atrioventricular (AV) node. Then the impulse proceeds down the special cell pathways to the ventricles. The ventricles contract, pushing blood out to the lungs and the rest of the body.
The usual rate of heartbeats when you are resting is between 50 and 100 beats per minute. The rate of beating can be made faster or slower by the body's activity, nervous system, hormones, or drugs. The heart will continue to beat even if all the nerves leading to it are cut.
The heart is much like other body muscles. However, it is much more sensitive to oxygen supply than other muscles. A lack of enough oxygen-carrying blood to the heart muscle quickly leads to a decrease in its ability to squeeze.
