Location and Structures
The heart is a muscular four chambered organ. It is normally about the size of your fist. It sits just behind and to the left of the sternal bone in the pleural cavity called the mediastium. The two upper chambers of the heart are called the atrium and the two lower pumping chambers are called the ventricles.
The wall that seperates the atria and the ventricles into right and left sides is called the septum. Blood is pumped through the heart with the help of four heart valves that prevent back flow and keep the blood moving in the appropriate direction. These four valves are: the tricuspid valve, located between the right atrium and the right ventricle; the pulmonary valve, located between the right ventricle and the pulmonary artery; the mitral valve, located between the left atrium and left ventricle, and; the aortic valve, located between the left ventricle and the aorta. The mitral valve normally has two flaps (also referred to as cusps or leaflets). The remaining valves normally have three flaps.
Blood Supply and Circulation
There are five major blood vessels that communicate to and from the heart. The inferior vena cava and the superior vena cava collect blood from the venous circulation (commonly referred to as "blue" blood) and return it to the right side of the heart. The blood is emptied into the right atrium, through the tricuspid valve to the right ventricle, then to the lungs via the pulmonary artery. In the lungs, the blood collects oxygen (commonly referred to as "red" blood), releases carbon dioxide, and then is returned to the left atrium via four pulmonary veins. From there, blood goes through the mitral valve to the left ventricle and is pumped under high pressure by the left ventricle out to the body via the aorta.
The Heart Beat and Electrical System
The heart has an automatic conduction system that signals each beat. This inherent pacemaker system is called the sinus node located within the right atrial wall. The sinoatrial node produces an electrical impulse that travels to a central conduction site called the AV node and is then sent down to the ventricular muscle. This in turn causes a contraction of the ventricular muscle ejecting blood to both the lungs and the aorta. The number of contractions each minute is what is referred to as the heart rate or pulse. In newborns and young children, this heart rate is much faster (100-170 beats/minute) than in older children and/or adults (70 - 100 beats/minute).