Lung fissures are anatomical structures that play a fundamental role in the architecture of the respiratory system. They are formed by a deep folding of the visceral pleura, the delicate membrane covering the surface of the lungs. These folds divide the lung tissue into distinct, functional compartments known as lobes, allowing for localized movement and expansion during breathing.
The Standard Anatomy of Lung Fissures
The human lungs possess two primary types of fissures, which differ between the right and left sides due to the heart’s position. The right lung is divided into three lobes—upper, middle, and lower—by two fissures. The left lung, which is slightly smaller, has only two lobes—upper and lower—separated by a single fissure.
The oblique fissure, also called the major fissure, is present in both lungs. It runs diagonally from the posteriosuperior aspect (near the fourth or fifth thoracic vertebrae) down toward the anteroinferior surface, ending near the diaphragm. In the right lung, this diagonal cut separates the lower lobe from both the middle and upper lobes. On the left side, the oblique fissure divides the upper lobe from the lower lobe.
The horizontal fissure, sometimes called the minor fissure, is found exclusively in the right lung. This fissure begins at the oblique fissure near the midaxillary line and runs forward horizontally, separating the upper lobe from the middle lobe. The fissures extend completely from the lung’s surface inward to the hilum, allowing the lobes to move somewhat independently during respiration.
Fissures in Medical Imaging and Disease Spread
Fissures are important landmarks for medical professionals interpreting chest imaging, such as X-rays and Computed Tomography (CT) scans. On these images, a fissure appears as a thin, sharp line representing the double layer of visceral pleura. The horizontal fissure is often visible on a frontal chest X-ray, while the oblique fissures are usually best visualized on a lateral view.
These structures act as natural boundaries, which can affect the progression of respiratory diseases. A well-defined, complete fissure can contain an infection, such as pneumonia, limiting the inflammatory process to a single lobe. This containment is known as lobar consolidation, and it is a pattern that helps clinicians localize the disease.
The fissures also guide the movement of fluid within the chest cavity, particularly in cases of pleural effusion. Fluid tends to collect within the space created by the fissure, and the appearance of this fluid on an X-ray can help determine its exact location. If a fissure is incomplete, however, the fluid or infection may be able to spread directly into an adjacent lobe without following the expected anatomical path.
Common Anatomical Variations
The standard anatomical description of lung fissures is not universally present, and variations are common. A frequent finding is an incomplete fissure, where the visceral pleura does not fully separate the adjacent lobes. For example, the horizontal fissure in the right lung is incomplete in approximately two-thirds of people and may be entirely absent in about ten percent of the population.
This incomplete separation allows for a connection between the lung segments, which permits air to move between lobes, a phenomenon called collateral ventilation. For patients undergoing lung surgery, such as a lobectomy, an incomplete fissure is significant because it is associated with a higher risk of post-operative air leakage. The absence of a complete barrier also means that a disease process may cross from one lobe to the next more easily.
Accessory fissures are another common variation, representing additional folds that create smaller, sometimes non-functional, subdivisions. The Azygos fissure is the most recognized accessory type, occurring when the Azygos vein indents the right upper lobe. This variation forms a distinct segment known as the Azygos lobe, and while generally harmless, it is important for radiologists to recognize these variations to avoid misinterpreting them as signs of disease.