Blood flows through the heart in one direction. This unidirectional flow is made possible by the action of four valves inside the heart: tricuspid, mitral, aortic, and pulmonic. The tricuspid valve is located between the right atrium and right ventricle and opens during diastole, allowing blood to pass from right atrium to right ventricle. When the right ventricle contracts in systole, the higher pressure in the right ventricle forces the tricuspid valve to close and the pulmonic valve to open. Since blood cannot pass backwards through the closed tricuspid valve, it is forced out of the right ventricle through the open pulmonic valve into the pulmonary artery and on to the lungs for oxygenation. The oxygenated blood returns to the heart from the lungs via the pulmonary vein, which empties into the left atrium. Separating the left atrium from the left ventricle is the mitral valve. During diastole, the mitral valve opens and blood from the left atrium fills the left ventricle. When the left ventricle contracts during systole, the mitral valve closes, the aortic valve opens, and blood is vigorously ejected through the aortic valve and into the aorta, which carries oxygenated blood to the body. The "lub-dub" sound that one hears when listening through a stethoscope is actually the sound of moving blood striking against the valves that "slam" shut as the heart contracts and relaxes. A heart murmur is an extra sound that may be heard if there is turbulence of blood flow across a valve.

 

 

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