The nose is one of the key external parts to the respiratory system. Nostrils open into the nasal cavity which has a cilia-covered membrane. Mucus is produced in the nasal cavity, moistening air and enclosing any foreign particles. Mucus is then either spat out or swallowed. In this way, the air is moistened. The air is also heated by capillaries in the nasal cavity. These two together help the sense of smell.

From either the nasal cavity or the oral cavity (mouth), air enters the pharynx (throat), which leads to the trachea and esophagus. The esophagus leads to the digestive system. The trachea (wind pipe) is thin and covered with cilia and mucus. Particles from the

air are trapped here as well and propelled by the cilia back to the pharynx. When a person swallows, their larynx (voice box) acts as a barrier between their esophagus and trachea. The upper-most piece of cartilage, the epiglottis, acts as a cover over the trachea to prevent food from being inhaled.

​​Behind the breastbone, the trachea divides to form two primary bronchi. The right primary bronchus passes onto the right lung and the left primary bronchus passes onto the left lung. The lungs are large cone-shaped structures inside the chest cavity. They rest on the diaphragm and are each surrounded by a pleural membrane. It is between the lung and the outside of the membrane there is fluid which allows for smooth movement during breathing. Intercostal muscles control that breathing movement by closing and expanding the ribs. The primary bronchi continue into the lungs, where they branch off into a network called a bronchial tree. Off of each bronchial tree there are clusters of aveoli, the average person having roughly 480 million. Each aveolus has a thin, delicate membrane and is surrounded by capillaries, which is how the exchange of gases takes place (oxygen, carbon dioxide, water vapor).