When humans breathe in air, most of the time it is through the nose, though we can also breathe in through our mouths. Assuming air comes in through the nose, the correct order is:
- Nasal cavity. This is a hollow portion of the skull containing hairs and mucus, and both warms and adds moisture to the incoming air, and removes foreign matter such as dust. This cleaning by mucus continues throughout the respiratory system.
- Pharynx (throat). Both air to the lungs, and food to the gut, pass through the pharynx; the epiglottis, a flap of tissue, helps keep food and water from entering the lungs.
- Larynx (voice box). Air passes over vocal folds in this area, enabling speech.
- Trachea (wind pipe). This tube has supporting cartilage; it stays open so that air may easily pass through.
- Bronchi. These airways split from the trachea and enter each lung; they become increasing small, and are termed secondary and tertiary (3rd) bronchi in the lobes of the lungs.
- Alveolar ducts. These connect the bronchioles with the alveoli.
- Alveoli. These are very small sacs in the lungs that are in such close contact with the capillaries of the bloodstream that oxygen and carbon dioxide can diffuse across the membranes; higher oxygen content of the inhaled air allows oxygen to enter the blood, and higher carbon dioxide level in the blood allows it to diffuse back to the alveoli so that it can be exhaled.