A pleural fissure is an extension of the pleural space between lobes of the lungs, formed by a double layer of visceral pleura. These fissures are crucial anatomical divisions that allow the lung lobes to move independently during respiration.
Understanding Pleural Fissures
Pleural fissures are natural clefts or invaginations of the visceral pleura, the delicate membrane that directly covers the surface of the lungs. They project inward from the lung surface, creating distinct boundaries between the different lung lobes. This unique arrangement ensures that each lobe can expand and contract somewhat independently, facilitating efficient gas exchange.
Anatomy and Formation
The lungs are enveloped by a double-layered serous membrane called the pleura. The inner layer, the visceral pleura, adheres tightly to the lung surface, including deep into the fissures. The outer layer, the parietal pleura, lines the chest wall, diaphragm, and mediastinum. Between these two layers lies the pleural space, which contains a thin film of pleural fluid, reducing friction during breathing.
Pleural fissures are created when the visceral pleura folds inward, essentially creating a double layer that extends deep into the lung tissue, dividing it into lobes. This means that the pleural space itself extends into these fissures.
Types of Pleural Fissures
Humans typically have two primary fissures, with an additional one often present in the right lung.
| Fissure Type | Location | Divides | Common in |
|---|---|---|---|
| Major (Oblique) Fissure | Runs obliquely from the posterior chest wall downwards and forwards. | Divides the superior lobe from the inferior lobe in both lungs. | Both lungs |
| Minor (Horizontal) Fissure | Runs horizontally across the right lung at the level of the 4th rib. | Divides the superior lobe from the middle lobe in the right lung. | Right lung only |
| Accessory Fissures | Less common, variable in location (e.g., azygos fissure). | Can create additional "lobules" or segments within a lobe. | Variable |
Function and Significance
The primary function of pleural fissures is to facilitate the independent movement of lung lobes. During inhalation and exhalation, different lobes may expand and contract at varying rates, and the fissures allow this flexibility, preventing restrictive motion.
Key characteristics include:
- Double Layer of Visceral Pleura: Each fissure is lined by two layers of the visceral pleura.
- Extension of Pleural Space: The potential space between these two visceral pleura layers within the fissure is an extension of the main pleural cavity.
- Division of Lobes: They create distinct anatomical and functional compartments within the lungs.
- Visible on Imaging: Fissures are routinely observed on chest X-rays and computed tomography (CT) scans.
Clinical Relevance and Imaging
Understanding pleural fissures is crucial in medical imaging and diagnosis:
- Radiological Landmark: Fissures serve as important anatomical landmarks on chest X-rays and CT scans, helping to localize pathology to specific lung lobes or segments.
- Fluid Accumulation: In conditions like pleural effusion, fluid can collect within the fissures, appearing as a thick, linear opacity or a lens-shaped collection on imaging.
- Consolidation and Collapse: The position of a fissure can shift if a lung lobe undergoes collapse (atelectasis) or becomes consolidated due to pneumonia, providing diagnostic clues.
- Tumor Spread: The fissures can act as barriers or pathways for the spread of certain lung diseases, including infections and malignancies.
- Incomplete Fissures: Not all fissures are complete, meaning the visceral pleura might not fully extend through the lung parenchyma. This can sometimes lead to collateral air movement between lobes.
For instance, an X-ray showing a thickened minor fissure could indicate inflammation or fluid in the right middle lobe, guiding further investigation. Similarly, a CT scan can precisely map the extent of a tumor relative to the fissures, aiding in surgical planning.
