Human lungs are divided into discrete, self-contained units called bronchopulmonary segments. The right lung has ten and the left typically has eight to ten, depending on how you count a couple of segments that frequently merge. Each segment receives its own dedicated bronchus and its own branch of the pulmonary artery, and thin walls of connective tissue mark the borders between neighboring segments. This architecture is far from a textbook curiosity: it underpins how surgeons remove tumors, how inhaled particles settle in the airways, and how congenital lung problems are managed in children.
How the Segments Are Arranged
The right lung has three lobes, and those lobes are subdivided into ten segments. The upper lobe holds the apical, posterior, and anterior segments. The middle lobe has the lateral and medial segments. The lower lobe is the largest, containing five segments: superior, medial basal, anterior basal, lateral basal, and posterior basal. On the left side, things are a bit different because the heart takes up space. The left lung has only two lobes, and several segments that are separate on the right tend to fuse on the left. The apicoposterior segment, for instance, is a single unit combining what would be two distinct segments on the right. The lingula, which is the left lung’s rough equivalent of the right middle lobe, has its own superior and inferior segments. Despite the asymmetry, each segment on both sides functions as an independent respiratory unit with its own airway and blood supply.
The naming system most clinicians use today traces back to an international agreement reached at a 1949 congress of specialists from multiple countries and disciplines. Before that, different surgical teams used different labels for the same structures, which created confusion as lung surgery became more sophisticated.
How Segments Form Before Birth
The blueprint for every segment is laid down remarkably early in development. Around day 26 after conception, two tiny buds push outward from the front wall of the primitive foregut, one for each lung. These buds then undergo a repetitive cycle of growth and branching, a process called branching morphogenesis, that builds the future airway tree.
Most of this branching happens during the pseudoglandular stage of lung development, when roughly the first 20 generations of future airways are formed. By the end of that stage, even the earliest generations of the smallest air ducts that will eventually participate in gas exchange are already sketched out.
1PubMed Central. Development of the lungThe segmental pattern, then, is not something the lung figures out gradually through childhood. It is essentially hardwired by mid-gestation, which is why congenital malformations that affect a specific segment are confined to that segment’s territory rather than spilling across the entire lobe.
The Boundaries Between Segments
What keeps one segment functionally separate from its neighbor is a thin partition of connective tissue called the intersegmental septum. These septa are visible on CT scans as faint lines of slightly denser tissue. Histologically, each septum consists of three layers: two outer layers made of alveolar walls and a deeper middle layer of collagen fibers and interstitial fluid.
2PubMed Central. Kohn’s pores are not responsible for collateral ventilation between inflated and deflated segmentsA CT scan finding that sometimes puzzles radiologists is a faint line of increased density in the lower lobe. This corresponds to a septum of thin, loose connective tissue bounded on one side by the base of the pulmonary ligament and on the other by a vertically oriented vein.
3PubMed. Intersegmental (intersublobar) septum of the lower lobe in relation to the pulmonary ligamentThe veins are a subtle but important part of the picture. While each segment has its own artery running alongside its bronchus, the veins that drain the segments do not follow the same rule. Instead, the pulmonary veins run between segments, sitting in the connective tissue septa and collecting blood from the segments on either side. Surgeons rely on this arrangement as a landmark when identifying the boundary between adjacent segments during an operation.
4European Society of Radiology. Shortest guide to anatomy of pulmonary arteries evaluated in multislice computed tomography angiographyCollateral Ventilation and What It Means
A question that matters a great deal during certain lung procedures is whether air can leak from one segment into another. This phenomenon, collateral ventilation, determines how well a deflated segment stays deflated once its bronchus is blocked. It was long assumed that tiny pores in the alveolar walls (Kohn’s pores) allowed air to cross between neighboring segments. Research has shown otherwise: Kohn’s pores are not found within the intersegmental septum itself, so they are not the main route for air moving between inflated and deflated segments during surgery.
5PubMed Central. Kohn’s pores are not responsible for collateral ventilation between inflated and deflated segmentsCollateral ventilation matters especially in the treatment of severe emphysema. Doctors can place one-way valves inside a bronchus to block airflow to a diseased lobe, causing it to collapse and giving healthier parts of the lung more room to expand. But if air freely crosses from adjacent segments into the treated one, the lobe will not deflate properly, and the procedure fails. Confirming that collateral ventilation is absent before placing those valves is a critical step.
6PubMed Central. Endobronchial Valves for the Treatment of Advanced EmphysemaAnatomical Variations Are Surprisingly Common
Textbook diagrams depict a tidy, symmetrical branching pattern, but the reality is messier. A clinical-anatomic study that examined pairs of lungs found that only about 37% had no anatomical variation at all in either lung. The right lower lobe showed the highest rate of variation at roughly 25%, while the middle lobe and left lower lobe had the lowest at about 12%. There were no significant differences in variation rates between men and women.
7PubMed. The bronchial segmentation and its anatomical variationsThe upper lobes are another hotspot for variation, particularly in the pulmonary arteries. Accessory arteries from adjacent segments crop up frequently, and the number of arterial branches supplying a given segment can differ from person to person.
8PubMed Central. Pulmonary vascular anatomy & anatomical variantsEven within a single segment, the internal arrangement can vary substantially. A study of nearly 1,900 patients looked at branching patterns in the medial basal segment (segment 7) of the right lower lobe and identified five distinct patterns for how the bronchus and artery relate to the inferior pulmonary vein. The most common pattern accounted for about 73% of patients, but the remaining four patterns collectively accounted for more than a quarter, with some involving the bronchus splitting across both sides of the vein.
9PubMed. Branching patterns of segmental bronchi and arteries in the medial basal segmentFor a radiologist interpreting a scan, these variations can mimic disease if you do not expect them. For a surgeon, encountering an unexpected arterial branch mid-operation is the kind of surprise that can cause bleeding. Both specialties need a working awareness that the “standard” segmental anatomy is really just the most common version among many.
Segmentectomy and Cancer Surgery
The reason lung segments matter most in modern medicine is surgery. For decades, removing an entire lobe (lobectomy) was the default operation for early-stage lung cancer. That approach works well oncologically, but it sacrifices a lot of healthy tissue. Because each segment is a self-contained unit with identifiable borders, surgeons can remove just the affected segment and leave the rest of the lobe intact.
A landmark randomized trial published in The Lancet compared segmentectomy to lobectomy in patients with small peripheral non-small-cell lung cancers. At a median follow-up of about seven years, five-year overall survival was roughly 94% for segmentectomy and 91% for lobectomy. The trial confirmed that segmentectomy was not just as good as lobectomy for survival but was statistically superior, with the benefit holding across all predefined patient subgroups. The researchers concluded that segmentectomy should be the standard surgical procedure for that population.
10The Lancet. Segmentectomy versus lobectomy in small-sized peripheral non-small-cell lung cancer (JCOG0802/WJOG4607L)Segmentectomy is also favored for patients whose lung function is already compromised, such as people with severe COPD or other conditions that would make surviving the larger operation risky.
11PubMed Central. Segmentectomy versus lobectomy. Which factors are decisive for an optimal oncological outcome?How Much Lung Function Segmentectomy Preserves
The practical payoff of removing a segment instead of a lobe is measurable in breathing tests. Within the first couple of months after surgery, patients who undergo lobectomy lose an average of about 25% of their baseline forced expiratory volume (a standard measure of how forcefully you can exhale), while segmentectomy patients lose an average of about 18%. By a year out, the gap narrows but persists: lobectomy patients settle at a loss of roughly 11% and segmentectomy patients at about 5%.
12European Respiratory Review. Lung segmentectomy: does it offer a real functional benefit over lobectomy?A meta-analysis pooling data from multiple studies confirmed that these differences are real and consistent. Across measures of lung capacity and gas exchange, segmentectomy patients preserved significantly more function than lobectomy patients.
13PubMed Central. The impact of segmentectomy versus lobectomy on pulmonary function in patients with non-small-cell lung cancerA 4-to-7 percentage-point difference might sound modest, but for someone who already has limited lung reserve, that margin can mean the difference between climbing stairs comfortably and becoming short of breath during routine activities.
3D Modeling and Surgical Planning
One of the trickiest parts of segmentectomy is identifying exactly where the target segment’s borders, arteries, and veins are in a specific patient, especially given the high rate of anatomical variation. Three-dimensional reconstruction from CT scans has become a valuable tool for this. In a trial of robotic segmentectomy using 3D lung models, about 81% of planned segmental resections were completed successfully. The surgical plan was modified after reviewing the 3D model in about 36% of cases, and surgeons reported a significant increase in confidence when the models were available.
14JTCVS Techniques. Three-dimensional virtual lung reconstruction in robotic segmentectomyArtificial intelligence is accelerating this process further. AI-driven systems can generate patient-specific 3D models in five to ten minutes, improving the identification of anatomical variations, tumor localization, and margin planning. Clinical studies report that integrating these models into routine workflows reduces operative complexity and increases surgeon satisfaction.
15PubMed. Artificial Intelligence-Driven Three-Dimensional Reconstruction in Lung Cancer SurgeryThe direction of travel here is clear: as imaging and computing power improve, the old challenge of “which branches belong to which segment in this particular patient” is becoming less of a guessing game.
Where Inhaled Particles Settle
Segmental anatomy is not just a surgical concern. It also determines where harmful particles end up when you breathe them in. Three main factors govern particle deposition: the physics of the particles themselves, the anatomy of the airways, and the airflow patterns within them. Among the anatomical parameters, the diameters, lengths, and branching angles of airway segments are the key variables that determine where particles land.
16PubMed Central. Factors influencing the deposition of inhaled particlesThe branching points where one airway splits into two are especially vulnerable. Computational modeling has shown that a small fraction of cells sitting right at the ridge where an airway divides can receive doses of toxic particles hundreds of times higher than the average dose across the whole airway. This finding supports the hypothesis that the tendency for lung cancer to develop preferentially at certain sites may be driven by concentrated deposition of carcinogenic particles at these branching ridges.
17PubMed. Local particle deposition patterns may play a key role in the development of lung cancerIn other words, segmental anatomy does not just organize the lung into surgical units. It creates a landscape of variable exposure risk, with certain cells receiving far more punishment from pollutants or tobacco smoke than their neighbors.
Congenital Malformations and Pediatric Surgery
Babies can be born with abnormal tissue masses in the lung, the most common being congenital pulmonary airway malformations (CPAMs). These are clusters of abnormal cystic or solid tissue that typically occupy one segment or part of a lobe. Because CPAMs are segmental in nature, the same anatomical principle that allows cancer surgeons to remove a single segment applies in pediatric surgery too.
Traditionally, lobectomy was the go-to procedure for CPAMs. More recently, segmental resection has emerged as a viable lung-sparing alternative. In one pediatric series, 56% of patients with a specific type of malformation underwent segmental resection, and about 79% of those procedures were completed thoracoscopically (through small incisions, without opening the chest). None of the patients who had segmental resection experienced recurrent disease. The segmentectomy group had a median hospital stay of two days compared to seven days for the lobectomy group, with no difference in complication rates.
18Journal of Pediatric Surgery. Segmental resection for the treatment of congenital pulmonary malformationsThis matters especially in young children, whose remaining lung tissue has the potential to grow and compensate over time. Preserving even one or two extra segments in a two-year-old can translate into meaningfully better lung capacity decades later.
19PubMed Central. Surgical Management of Congenital Pulmonary Airway Malformations (CPAM) in an Infant and a ToddlerLung Segments Across Species
Humans are not the only mammals with segmented lungs, but the details differ considerably from one species to the next. Dissection studies comparing the lungs of sheep, goats, cats, rabbits, and monkeys have found that branching patterns differ not only between species but even between different lobes within the same animal. The types of cells lining the airways also vary from lobe to lobe and from species to species.
20American Review of Respiratory Disease. Structure as Revealed by Airway Dissection: A Comparison of Mammalian LungsThis variability has practical implications for medical research. Scientists studying occupational lung diseases, for instance, need animal models whose lung architecture resembles the human version closely enough for results to translate. A recent comparative review concluded that artiodactyls, particularly small pig breeds and goats, are the best models because their respiratory bronchioles and lobular structures most closely match human anatomy.
21PubMed Central. Comparative anatomy of respiratory bronchioles and lobular structures in mammalsRodents, by contrast, have much simpler airway branching and lack true respiratory bronchioles altogether, which is one reason findings in mouse lung models do not always hold up in humans. For anyone reading about a lung study done in mice, the species gap in segmental anatomy is worth keeping in mind.