The lungs are paired organs responsible for the continuous exchange of oxygen and carbon dioxide, a process known as respiration. These organs are organized into distinct, independent architectural units known as bronchopulmonary segments. This segmentation is a fundamental feature of lung anatomy, allowing for localized function and remarkable surgical precision. The total number of these segments varies slightly between the two lungs, typically falling within a range of 18 to 20 segments combined.
Defining Bronchopulmonary Segments
Bronchopulmonary segments represent the smallest functional and structural units of the lung, each acting as a self-contained respiratory district. The anatomical basis for this segmentation lies in the branching pattern of the airways and blood vessels. Each segment is supplied by its own tertiary, or segmental, bronchus, which delivers air to the alveoli within that unit.
A dedicated tertiary branch of the pulmonary artery also runs alongside the bronchus, carrying deoxygenated blood into the segment for gas exchange. This specific and independent supply of air and blood is the defining characteristic of a segment. The segments are separated from their neighbors by thin layers of connective tissue, which contain the pulmonary veins and lymphatic vessels.
This arrangement means that the veins and lymphatics drain along the periphery of the segment, not through its center. The connective tissue septa create discrete units, allowing a diseased segment to be removed without compromising the function of the adjacent lung tissue.
Each segment is roughly pyramidal in shape, with its apex pointing toward the hilum, where the bronchi and major vessels enter the lung. The base of the pyramid faces the outer surface of the lung, or the pleura.
The Numerical Breakdown of Lung Segments
The total count of bronchopulmonary segments is 10 in the right lung and typically 8 to 10 in the left lung, resulting in 18 to 20 segments combined. The difference is due to the smaller size of the left lung, which accommodates the heart, and the resulting fusion of some segments. The right lung is consistently divided into 10 segments across its three lobes.
The right lung’s upper lobe contains three segments, the middle lobe has two segments, and the lower lobe is composed of five segments. These segments are identified using a standard numerical nomenclature (S1, S2, S3, etc.), which corresponds to the tertiary bronchus supplying that area. This consistent naming convention allows medical professionals worldwide to precisely identify a specific region of the lung.
The left lung has only two lobes (upper and lower) and typically has fewer distinct segments due to merging. For instance, the apical and posterior segments often fuse to form a single apicoposterior segment in the upper lobe. Similarly, the medial and anterior basal segments may combine into one anteromedial basal segment in the lower lobe.
The left lung is frequently described as having eight segments, though variations exist. The left upper lobe is composed of the apicoposterior segment, the anterior segment, and the two segments of the lingula. The lower lobe typically contains four segments, reflecting the fusion of the two basal segments.
Clinical Relevance of Lung Segmentation
The precise mapping of bronchopulmonary segments is a foundational requirement in thoracic medicine, particularly in surgical planning. Knowing the exact boundaries allows surgeons to perform a segmentectomy, the targeted removal of a single diseased segment. This procedure is often favored over a lobectomy (removal of an entire lobe) because it preserves a greater volume of healthy lung tissue.
Accurate segmentation is also paramount for localizing disease, as certain infections, such as pneumonias or lung abscesses, often remain contained within a single segment. Early-stage lung cancer or localized fungal infections can be pinpointed using high-resolution imaging. The segmental nomenclature provides a precise language for diagnosis and communication among medical professionals tracking pulmonary pathology.
The segmental organization is also utilized in respiratory therapy through a technique called postural drainage. This technique involves positioning a patient in specific ways to use gravity to help drain mucus and secretions from a particular segment toward the larger airways. The positions are tailored to align the segmental bronchus with the direction of gravity, making clearance more effective for patients with conditions like cystic fibrosis or bronchiectasis.