The upper half of the respiratory system consists of the nasal cavity, larynx and pharynx, while the lower half consists of the trachea, bronchi and lungs. The upper part of the system conducts gases, while the lower part is responsible for gas exchange.
Air enters the respiratory system via the nose and mouth and passes through the sinuses, which regulate moisture and temperature, before moving to the trachea, which filters the air. The trachea branches to form two tubes called bronchi. Small hairs, or cilia, line the bronchi and function to transport mucus away from the lungs. Mucus is essential in trapping foreign material from the air for later removal by coughing or sneezing.
The lungs are the next stop in the respiratory system. The left lung consists of two lobes and, because of the position of the heart, is smaller than the three-lobed right lung. Small air sacs, or alveoli, fill both lungs and function in gas exchange. Oxygenated blood from the lungs moves through tiny capillaries in the alveoli to the pulmonary vein and on to the heart for transport to the rest of the body. Meanwhile, blood laden with carbon dioxide moves from the pulmonary vein and into the alveoli for exhalation from the body.