I'm assuming that you are referring to the passage of inhaled air in the human respiratory tract.Inhaled gas - 'air' - passes through either the mouth or nose to reach the back of the 'throat;' the area called the pharynx. From here, the inhaled air passes into the trachea, which is the structure at the front of the neck (in front of the oesophagus, which the passage down which food travels.) The trachea bifurcates, or 'divides into two,' at the level just below the 1st rib. One division of the trachea passes into each of the right and left lungs.From here, air passes into the 'bronchus' of each lung. These 'bronchi' go on to supply the tiny, microscopic sacs of air called 'alveolar sacs' in the lungs. Blood vessels wrap themselves around these alveolar sacs to allow for Oxygen to diffuse or 'move into' the blood, and for Carbon Dioxide to leave the blood.The diaphragm (a large located between the thorax or 'chest' and the abdomen) is the major muscle involved in breathing. It contracts and lowers during inspiration, which pulls air into the lungs due to changes in pressure.I hope that helps!Oscar