In the lungs specifically (not counting the nose, mouth, or trachea), various cells serve individual functions. Alveolar macrophages (dust cells), for example, play a critical role in clearing the alveoli of pathogens and in maintaining homeostasis. Individual epithelial cells of the alveoli are incredibly thin—as low as one cell layer—so that oxygen can easily diffuse into the blood stream at the alveolar-capillary barrier. The next level of organization would be the tissues of the lungs. These would include structures such as the alveoli, the bronchioles, the arteries that supply blood to the bronchioles, and so on. The next level of organization, the organ, would be a combination of all of these individual tissues to make the lung itself. Finally, the last level of organization would be a combination of organs. The lungs, trachea, and mouth and nose, for example, form the respiratory system.
The logic of cell organization is the same for the heart. Individual cells include the cardiac muscle cells, which are involved in self-regulating heart systole; red blood cells that travel through the heart; and nerve cells that supply signals stimulating contraction. The next level of organization, cardiac tissue, includes the three main layers of the heart—the endocardium, myocardium, and epicardium—which serve various protective and supply functions. Other tissues also include valves, arteries, and Purkinje fibers (tissues which carry nerve impulses to the myocardium). All of these tissues taken collectively give you the heart itself, the next highest level of organization. Finally, the heart and the body's arteries, veins, and blood comprise the entire organ system: the circulatory system.