Light micrograph of a section through human heart muscle showing parenchyma myocarditis. The parenchyma of the heart is muscle tissue (cardiac muscle cells). The nerves, intrinsic blood vessels and connective tissue of the heart comprise the stroma. Myocarditis is a disease marked by the inflammation of the heart muscle known as the myocardium, the muscle layer of the heart wall. This muscle is responsible for the contracting and relaxing to pump blood in and out of the heart and the rest of the body.