Air passes through the structures of the vertebrate respiratory system in the following order:
Mouth or Nostrils → Nasopharynx/ Oral Pharynx → Glottis → Trachea → (Lungs: Right and Left Bronchi → Bronchioles → Alveoli)
Gas exchange occurs between the alveolar sacs and the small blood vessels that are located on the alveolar sacs. It is here that oxygen is delivered from the lungs to the bloodstream. The oxygen attaches to the hemoglobin that is found within red blood cells. The red blood cells then transport the oxygen to cells. The oxygen is used by the mitochondria of the vertebrate eukaryotic cells in the process that is known as cellular respiration. During cellular respiration, oxygen gas and the sugar glucose are converted into water, carbon dioxide gas, and an energy source called adenosine triphosphate (ATP). The blood that circulates within the bloodstream then picks up the carbon dioxide that is produced during cellular respiration. The blood transports the carbon dioxide delivered back to the alveoli of the respiratory system in order to be eliminated from the body.