Cardiac muscle cells and red blood cells play different functional roles in an organism. Cardiac muscle cells work to keep the heart pumping through involuntary movements. Cardiac muscle cells form a highly branched cellular network in the heart. They are connected end to end by intercalated disks and are organized into layers of myocardial tissue that are wrapped around the chambers of the heart.
Red blood cells carry oxygen from the lungs to the rest of the body. Then they make the return trip, taking carbon dioxide back to the lungs to be exhaled. In humans, mature red blood cells are flexible and oval biconcave disks. They lack a cell nucleus and most organelles, in order to accommodate maximum space for hemoglobin.
So, different structure of cardiac muscle cells and red blood cells defines their functional specialization.