Publisher's Synopsis
Human heart valves are remarkable structures. These tissue-paper thin membranes attached to the heart wall constantly open and close to regulate blood flow (causing the sound of a heartbeat). This flexing of the tissue occurs day after day, year after year. In fact, the tissue withstands about 80 million beats a year, or 5 to 6 billion beats in an average lifetime. Each beat is an amazing display of strength and flexibility.Human heart valves lie at the exit of each of the four heart chambers and maintain one-way blood flow through the heart chamber. The four heart valves make sure that blood always flows freely in a forward direction and that there is no backward leakage.Blood flows from the right and left atria into the ventricles through the open tricuspid and mitral valves.When the ventricles are full, the tricuspid and mitral valves shut. This prevents blood from flowing backward into the atria while the ventricles contract.As the ventricles begin to contract, the pulmonic and aortic valves are forced open and blood is pumped out of the ventricles. Blood from the right ventricle passes through the open pulmonic valve into the pulmonary artery, and blood from the left ventricle passes through the open aortic valve into the aorta and the rest of the body.
