The respiratory tract is a continuous system of passages that moves air from the atmosphere into the lungs for gas exchange. For clarity, this pathway is divided into two major regions: the upper airway and the lower airway. This division marks a significant transition in the structures’ function and protection mechanisms. Identifying the precise anatomical structure that separates these two regions provides a clearer understanding of how the body manages the air we breathe.
Understanding the Upper Airway Structures
The upper respiratory tract serves as the initial entry point for air, beginning with the nose and the nasal cavity. These structures contain specialized linings and turbinates that create turbulence. This preparation involves filtering out large particulate matter, warming the air to body temperature, and humidifying it with moisture.
Air then moves into the pharynx, a muscular tube divided into three sections. The nasopharynx sits behind the nasal cavity, the oropharynx is behind the mouth, and the laryngopharynx is the lowest part, leading toward both the esophagus and the voice box. The pharynx acts as a shared pathway for both the respiratory and digestive systems.
The upper airway also includes the superior portion of the larynx, or voice box. This segment contains the epiglottis, a flexible flap of cartilage that guards the airway opening. These upper components condition the air and provide an initial defense before the air travels deeper into the body.
The Critical Dividing Point
The larynx is the structure that contains the precise boundary between the upper and lower airways. The most commonly accepted and functionally significant landmark is the vocal folds, also known as the glottis. The portion of the larynx that sits above the vocal folds is grouped with the upper airway, while everything below them marks the start of the lower airway.
This division at the vocal folds is functionally significant because it separates the shared respiratory and digestive tract from the purely respiratory tract. The vocal folds are capable of tightly closing the airway entrance, preventing food or liquid from entering the lungs, a protective action known as guarding against aspiration. The second convention places the division slightly lower, at the inferior border of the cricoid cartilage, the ring-shaped cartilage forming the base of the larynx.
The cricoid cartilage is the only complete ring of cartilage, providing rigid structural support at the transition point. Regardless of whether the vocal folds or the cricoid cartilage is specified, this region represents the gatekeeper of the lower respiratory system. This area is also where the air begins its journey into the conducting zone, a system of tubes designed solely for the movement of air.
Exploring the Lower Airway Structures
Immediately following the dividing point within the larynx, the lower airway begins its descent into the chest cavity. The first major structure is the trachea, a tube reinforced by C-shaped rings of hyaline cartilage that prevent its collapse. The trachea extends downward until it splits into the two primary bronchi leading into the lungs.
These primary bronchi then undergo branching, forming the bronchial tree, which consists of secondary and tertiary bronchi. This branching system becomes progressively narrower, leading into the smaller tubes called bronchioles. The walls of these lower airway structures are lined with ciliated cells and mucus-producing goblet cells, forming the mucociliary escalator, which continuously cleans the air passages by trapping and sweeping debris upward.
The terminal bronchioles eventually lead to the respiratory bronchioles, alveolar ducts, and finally the alveoli, which are thin-walled air sacs. This is the respiratory zone, where oxygen diffuses into the bloodstream and carbon dioxide moves out, completing the respiratory cycle.