In a multicellular organism the levels of organization are: cells, tissues, organ, organ system, and organism (from smallest to largest). All parts are specialized (hold a particular structure to complete a particular function). However, all organ systems work together for the good of the whole.
The respiratory system is composed of the lungs. Its main function is the intaking of oxygen and exhaling of carbon dioxide.
The oxygen that is inhaled by the respiratory system is delivered to the rest of the body via the circulatory system. The hemoglobin in the red blood cells attaches to the oxygen. As the blood travels through the veins and arteries, it drops off the oxygen from the lungs to the cells and picks up the carbon dioxide from the cells and delivers it to the lungs to be exhaled.
Some of the oxygen that is inhaled by the respiratory system and delivered by the circulatory system is used during the digestive processes of cellular respiration in the digestive system. In this process, food (glucose) combines with oxygen to produce carbon dioxide, water, and energy. Hence, some of the carbon dioxide that the blood carries to the lungs is produced during the breakdown of organic materials in the digestive system.