The lungs are not simply two large, uniform organs; their architecture is highly organized into distinct, tiered compartments. The largest divisions are the lobes, with three in the right lung and two in the left. Within these lobes, the next level of organization is the bronchopulmonary segment, which functions as the smallest anatomically and physiologically independent unit of the lung. This concept of segmentation is fundamental to understanding how the lungs manage airflow, blood supply, and disease containment.
Defining the Segment’s Structure
The lung structure begins with the trachea, branching into main bronchi, then lobar (secondary) bronchi for each lobe. These airways continue to divide, with each lobar bronchus giving rise to several segmental (tertiary) bronchi. Each segmental bronchus supplies air exclusively to one bronchopulmonary segment, establishing its physical and functional boundary.
Following the course of the airway, a branch of the pulmonary artery also travels directly into the center of the segment, alongside the bronchus. This arrangement, with the air tube and the arterial blood supply running centrally, ensures that each unit has its own dedicated system for gas exchange. The segments themselves are generally pyramidal in shape, with the point, or apex, directed toward the lung root (hilum) and the wide base forming part of the lung’s outer surface.
The boundaries separating these segments are defined by layers of connective tissue, known as intersegmental planes. These planes are traversed by the pulmonary veins, which drain the oxygenated blood from the segment’s capillaries. The unique positioning of the veins between segments, rather than running centrally with the bronchus and artery, provides a natural, identifiable boundary.
Because the left lung is slightly smaller and contains the space for the heart, it typically has a different number of segments than the right. The right lung consistently divides into ten segments across its three lobes (three in the upper, two in the middle, and five in the lower lobe). The left lung commonly features either eight or nine segments, as certain adjacent segments, like the apical and posterior segments of the upper lobe, often fuse into a single unit.
Functional Independence and Airflow Dynamics
The specialized vascular and bronchial arrangement allows each segment to operate with a high degree of autonomy. The dedicated segmental bronchus ensures airflow, while the accompanying pulmonary artery branch provides deoxygenated blood for oxygen absorption. This parallel system means that ventilation (air supply) and perfusion (blood supply) are localized and self-contained within each segment.
This structural independence provides a mechanism for internal damage control. If a blockage occurs in a segmental bronchus, the loss of airflow is restricted to that single segment. The connective tissue planes (fibrous septa) separating the segments also help contain localized infection or inflammation.
This localized containment significantly limits the spread of fluid or disease to neighboring segments. Unlike lobes, which are often less completely separated by fissures, the segments are distinct subunits. The ability to localize an issue helps preserve the function of the surrounding healthy lung tissue.
Clinical Relevance: Localizing Disease and Surgery
Knowledge of bronchopulmonary segments is foundational to modern respiratory medicine, guiding both diagnosis and treatment. When a patient presents with a localized lung issue, such as pneumonia or a tumor, physicians use advanced imaging like Computed Tomography (CT) scans to pinpoint the exact segment affected. Identifying the segment accurately helps interpret the disease pattern and guide treatment choices.
For instance, a physician can note that a consolidation is restricted to the right middle lobe’s medial segment, which offers important information about the likely cause and potential trajectory of the illness. This precision allows for highly targeted interventions, such as focused antibiotic delivery or localized drainage procedures.
The segmentation is particularly consequential in thoracic surgery, where it enables a procedure called a segmentectomy. Historically, removing a diseased portion of the lung often required removing an entire lobe, a procedure known as a lobectomy. Segmentectomy, however, involves the targeted removal of only the affected segment.
This lung-sparing approach is possible because the intersegmental veins provide a clear, easily identifiable surgical plane. Surgeons can isolate and sever the central bronchus and artery supplying the diseased segment, then dissect along the plane marked by the veins, preserving the surrounding healthy segments. Segmentectomy is increasingly favored for treating small, peripheral lung cancers and benign lesions, as it maximizes the preservation of overall lung function for the patient.