Lung fissures are a normal aspect of human anatomy, representing the folds that separate the different sections of the lungs. These structures are not a health problem but are standard features formed by infoldings of the visceral pleura, the thin membrane covering the lungs’ surface. Their presence creates distinct boundaries within the lung tissue.
These anatomical partitions are integral to how the lungs are organized. Each lung is divided into large subunits, and the fissures are the dividing lines between them. This organization allows for a more efficient pulmonary structure. The fissures themselves are composed of two layers of this pleura, creating a defined separation.
Anatomy and Function of Lung Fissures
The primary role of lung fissures is to divide the lungs into lobes, allowing for their independent movement and expansion during breathing. The right lung is larger and is separated into three lobes: the superior, middle, and inferior. This division is accomplished by two main fissures: the oblique fissure separates the inferior lobe from the middle and superior lobes, while the horizontal fissure divides the superior and middle lobes.
The left lung is smaller to accommodate the heart and is divided into two lobes: the superior and inferior. A single, prominent oblique fissure separates these two lobes. The fissures are not superficial lines; they are deep invaginations of the visceral pleura that extend into the lung tissue, facilitating the sliding motion of the lobes against one another during inhalation and exhalation.
This segmental organization is advantageous, allowing each lobe to function as a distinct unit. This separation can help prevent the spread of localized infections from one lobe to another. It also provides surgeons with clear anatomical planes, enabling the removal of a diseased lobe (a lobectomy) while preserving the function of the remaining healthy lung tissue.
Anatomical Variations in Fissures
The classic anatomical description of lung fissures is not universal, as variations are common in the general population. These deviations from the standard structure are typically benign and do not signify disease. The most frequent variations include incomplete fissures, where the separation between lobes is not fully formed, and accessory fissures, which are extra fissures that further subdivide a lobe. Incomplete fissures occur when lung tissue connects the adjacent lobes at the base of the fissure.
Studies have found that incomplete fissures are more common than fully complete ones. For instance, one study observed that the horizontal fissure in the right lung was incomplete in 79% of cases, and the oblique fissure was incomplete in 65% of right lungs and 58% of left lungs. The absence of a fissure is another possible variation, with the right horizontal fissure being the most commonly absent. These variations arise during embryonic development when the spaces between developing lung buds fail to separate completely.
Accessory fissures are also a common finding, occurring at the boundaries of bronchopulmonary segments. Some recognized accessory fissures include the superior accessory fissure, the inferior accessory fissure, and the azygos fissure. The azygos fissure is notable because it is a developmental anomaly where the vein is displaced, carrying layers of pleura with it. The prevalence of accessory fissures is estimated to be around 7-10%.
Clinical Significance in Medical Imaging
In clinical practice, lung fissures are important landmarks on medical imaging like chest X-rays and computed tomography (CT) scans. Radiologists and physicians use the fissures to precisely locate abnormalities within the lungs. Identifying whether a nodule, an area of pneumonia, or a mass is in a specific lobe is fundamental for diagnosis and treatment planning. The fissures provide a clear map, allowing for accurate localization of disease processes.
Changes in the appearance of the fissures can also provide diagnostic clues. A thickened fissure, for example, may indicate the presence of fluid, inflammation, or even neoplastic conditions. When fluid accumulates in the pleural space, a condition known as pleural effusion, it can track along the fissures. This creates specific radiological signs that help clinicians identify the presence and extent of the effusion. An incomplete fissure can sometimes alter the typical appearance of fluid collection on an X-ray.
On CT scans, fissures are seen as thin, sharp lines or as less dense bands. Advanced imaging like high-resolution CT provides a clear depiction of the fissures, aiding in the identification of subtle abnormalities. Their appearance and relationship to other structures on imaging are of great interest to clinicians, as they can reflect underlying health issues that require further investigation.