Inhalation is the biological process of drawing air into the lungs, driven primarily by the contraction of the diaphragm and external intercostal muscles. This action expands the chest cavity, lowering the pressure inside the lungs compared to the atmosphere, which causes air to rush inward. The air follows a specific anatomical pathway to deliver oxygen to the body’s tissues for cellular energy production, concluding with the exchange of gases deep within the chest.
The Upper Airway
The air’s journey begins as it is pulled through the external openings of the nose into the nasal cavity, though it can also enter via the oral cavity. The nasal cavity serves as the initial air processing center, where a rich blood supply and mucosal lining begin to condition the inhaled air. This environment works to warm the incoming air, bringing its temperature close to that of the body, and to humidify it, preventing desiccation of the lower airways.
Particulate matter, such as dust and pollen, is immediately filtered out by the coarse hairs lining the nostrils, known as vibrissae. Deeper within the cavity, a sticky blanket of mucus traps smaller particles, which are then swept toward the pharynx by tiny, hair-like projections called cilia. The pharynx is a muscular tube that acts as a common passageway for both air and swallowed food, connecting the nasal cavity and the mouth to the structures below.
The Conducting Zone
From the pharynx, the air moves into the larynx, often called the voice box, which is a cartilaginous structure positioned superior to the trachea. The larynx’s main protective function is carried out by the epiglottis, a flap of cartilage that automatically covers the opening to the airway during swallowing, preventing food from entering the lower respiratory tract. Air then enters the trachea, a tube about 10 to 12 centimeters long, which descends into the chest.
The trachea, or windpipe, is a rigid yet flexible tube maintained by a series of 16 to 20 C-shaped rings of hyaline cartilage. These rings prevent the airway from collapsing as air pressure changes during breathing. At its lower end, the trachea bifurcates into the right and left primary bronchi, which direct the airflow into the respective lungs. These structures constitute the conducting zone, a region designed purely for air transport where no gas exchange takes place.
The Respiratory Zone
Once inside the lungs, the primary bronchi divide extensively, forming the bronchial tree, which includes secondary and tertiary bronchi. As these tubes branch further, they become progressively smaller, eventually losing their supportive cartilage plates. They transition into the tiny, muscular tubes known as bronchioles, which lack cartilage entirely and instead rely on smooth muscle to regulate their diameter and airflow.
The terminal bronchioles represent the final segment of the conducting zone before giving way to the respiratory zone. This zone begins with the respiratory bronchioles, which lead into delicate passageways called alveolar ducts. The airflow terminates in clusters of microscopic air sacs known as alveoli, which are thin-walled, hollow cavities.
Each alveolus is enveloped by a dense network of pulmonary capillaries, creating a remarkably thin blood-air barrier. This immense surface area, estimated to be around 75 square meters across both lungs, facilitates the rapid movement of gases. Oxygen from the inhaled air passively diffuses across the alveolar and capillary walls into the bloodstream. Simultaneously, carbon dioxide, a waste product carried by the blood, diffuses out of the capillaries and into the alveoli, ready to be exhaled.