Major organs of this system are grouped into two broad categories which are respiratory tract and lungs. However, it cannot perform its functions without the assistance of pulmonary blood circulation and oxygen binding protein, hemoglobin. The air passes through a long tube, trachea, and reaches lungs which play the role of a pivotal organ. After the respiratory gases are exchanged at the alveolar level, the waste and unabsorbed gages are exhaled back into air via the same passage.

Respiratory Tract

This tubular air passage runs from oral and nasal cavities through pharynx, larynx and trachea, ultimately, opening into lungs. Pharyngeal region, at the top, contains three smaller cavities including nasopharynx, oropharynx and laryngopharynx which are close to nose, mouth and larynx, respectively. In the next step, air enters larynx, also called voice-box that is composed of true and false vocal cards. It is interesting to note that this is the region where you produce all types of mild, harsh and melodious sounds for instructing, rebuking or delighting others. Vocal cords are like the strings of violin and, when air is forced through them, they are set to vibration and produce voices of varying pitch and volume.

Lungs

These are two lobe-like structures situated in the chest region of body and can be easily inflated or deflated like a balloon. Two sets of ribs, each protecting a lobe, not only protect them but also help in their expansion and contraction. In this respect, the role of diaphragm is also quite significant as it rises when air is to be expelled out, and lowers when air is inhaled to allow maximum place for the accommodation of incoming gases.