The oxygen rich blood returns to the left atrium and is delivered to the body when the left ventricle contracts. (smallanimalchanel.com)
Because of the pressure created when the left ventricle contracts, the mitral valve wears out in many dogs. (thedrakecenter.com)
However, in MMVD the thickening of the mitral valve results in an imperfect seal and allows blood to «leak» backward into the atrium as the ventricle contracts. (epictrial.com)