Myocardium
The muscular wall of the heart. It contracts to pump blood out of the heart and then relaxes as the heart refills with returning blood.
Anatomy
Mitral Valve
The valve at the junction of the left atrium with the left ventricle, which controls flow of blood into the ventricle, before each beat, and prevents backflow.
Lung
One of a pair of organs of respiration, situated in the chest cavity on either side of the heart and enclosed by a serous membrance