Final answer:
The percentage of cardiac muscle cells that are contractile is 4) 99 percent. These cells are responsible for the contractions that pump blood through the body and constitute the majority of cells in the heart's atria and ventricles.
Step-by-step explanation:
The percentage of cardiac muscle cells that are contractile is 99 percent. Cardiac muscle cells can be categorized into myocardial contractile cells and myocardial conducting cells. The contractile cells are responsible for the contractions that pump blood throughout the body and constitute most of the cells in the heart's atria and ventricles. These critical cardiomyocytes are well-equipped with myofibrils, which enable them to contract and push blood through the circulatory system. Conversely, myocardial conducting cells, which only make up about 1 percent of the cardiac muscle cells, form the conduction system of the heart and are less involved in the contraction process.