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The pathway for respiration in the human respiratory system is as follows: First air enters through the external nares and passes through the nasal cavity. From this point air then travels to the nasopharynx. After this it then travels through the oropharynx, followed by the laryngopharynx. Those three together make up the pharynx. Air then travels through the larynx and moves next through the trachea to the primary bronchus. Next, air passes through the secondary bronchus and the tertiary bronchus (collectively the bronchi) to reach the bronchioles. At this point air travels through the terminal bronchiole and then the respiratory bronchiole. Air then travels through the alveolar duct to the alveolar sac and finally the alveolus itself. It is at this point when air is in the alveoli that gas exchange actually takes place and oxygen enters the blood stream. Interestingly, there are over 300 million alveoli in two adult lungs, covering a surface area almost equal to a single side of a tennis court! Hope this helps!
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