The respiratory system is a network of organs and passages that brings oxygen into the body and expels carbon dioxide. It is divided into the upper and lower tracts. The lower respiratory tract (LRT) conducts air deep into the chest and performs gas exchange.
Defining the Lower Respiratory Tract Boundary
The division between the upper and lower respiratory tracts is anatomically precise. The larynx, or voice box, acts as the transition zone between the two sections. The boundary is specifically established at the level of the vocal folds (vocal cords) within the larynx.
Everything located below this point is considered part of the LRT and is situated within the thoracic cavity. This includes the trachea, the branching system of the bronchi and bronchioles, and the lungs. The LRT focuses on air conduction and oxygen transfer into the bloodstream.
The Trachea: The Main Airway
The first major structure of the LRT is the trachea, or windpipe, which extends downward from the larynx into the chest cavity. This tube is approximately 10 to 12 centimeters long in an adult. It is held open by C-shaped rings of hyaline cartilage, which prevents the airway from collapsing during breathing.
There are typically 15 to 20 incomplete cartilaginous rings along the trachea’s length. The C-shape is open at the posterior side, adjacent to the esophagus. This design allows the esophagus to expand when a person swallows food. The trachea descends into the mediastinum until it bifurcates into the right and left main bronchi at the carina.
The Bronchial Tree and Bronchioles
The trachea’s division marks the beginning of the bronchial tree, a network of progressively smaller air passages. The two main bronchi (primary bronchi) enter their respective lungs and begin to branch. The right main bronchus is shorter, wider, and descends more vertically than the left, which can affect the path of aspirated foreign objects.
Each primary bronchus divides into secondary (lobar) bronchi, with three on the right and two on the left, each supplying one lobe of the lung. These tubes continue to subdivide into tertiary (segmental) bronchi, which deliver air to specific segments within the lung lobes. As the airways repeatedly branch, the structural composition changes. The supporting cartilage rings found in the larger bronchi gradually decrease and are replaced by small, irregular plates.
The branching continues until the airways are less than one millimeter in diameter, becoming the bronchioles, which entirely lack cartilage. These smaller tubes are primarily composed of smooth muscle. This muscle allows them to regulate airflow by constricting or dilating. The final passages are the terminal bronchioles, which mark the end of the air-conducting zone and lead into the respiratory zone where gas exchange occurs.
The Lungs: Site of Gas Exchange
The lungs house the entire bronchial tree and are the destination for inhaled air. Within the lungs, the respiratory zone begins with the respiratory bronchioles and ends with the alveoli. Alveoli are microscopic, balloon-shaped structures clustered at the ends of the air passages, resembling a bunch of grapes.
The human lungs contain an estimated average of 480 million alveoli, collectively providing a massive surface area for gas exchange, ranging from 70 to 130 square meters. This immense area maximizes the transfer efficiency of oxygen and carbon dioxide. The walls of the alveoli are thin, often only a single cell thick, minimizing the distance gases must travel.
A dense mesh of pulmonary capillaries surrounds each alveolus, forming the alveolar-capillary membrane. Oxygen inhaled into the alveoli diffuses across this thin membrane into the capillary blood, while carbon dioxide diffuses out to be exhaled. This arrangement ensures an efficient supply of oxygen and the removal of metabolic waste.