Final answer:
The myocardium, a layer of cardiac muscle cells, represents the tissue level of organization, essential for the heart's pumping function.
Step-by-step explanation:
The layers of muscle tissue that form the bulk of the heart's wall, specifically the myocardium, represent the tissue level of organization in biological systems. The myocardium is the thickest layer of the heart, comprised largely of cardiac muscle cells.
These cells work cohesively in a complex arrangement to enable the heart to pump blood efficiently through the body. The myocardium, along with the epicardium and endocardium, creates a multilayered structure that houses the chambers and valves of the heart, effectively maintaining one-way blood flow during the cardiac cycle.