Answer
The greater the workload of a chamber, the thicker myocardium it develops.
Work Step by Step
The atri hace a much easier job than ventricles. The former simply pump blood localy to the ventricles below . Therfore they don't nneed very thic walls; the average ticness of the myocardium of the ventricles is aout 2,5mm. On the cotrary the ventricles have much harder tasks t pump blood out of the heart to near or distant locations. The right ventricle pomps blood to the lunns . this is a harder job than that done by the atroa . Cosequently the myocardium of the right ventricle is about 5 mm thick, almost twice the thickness of the atrial myocardium. But the left ventricle powers the systemic circuation and has the greatest workload of all the cardiac chambers. As expected the left ventricle has the thickest myocardium of all, about 12.5 mm.
In certain cases of cardiovascular pathology , the arteries can become narrowed by the deposition of atherosclerotic, fatty plaques in arteries. This requires harder effort and stronger contraction of the left ventricle to get the blood flowing through these narrowed arteries. In response to the increased workload, the left ventricular myocardium gets thicker than 15-20mm in diameter. This condition of abnormal myocardial thickness goes by the clinical name of cardiomegaly.
