# Pleural Cavities: Anatomy, Boundaries, and Clinical Notes The pleural cavities are the two potential spaces within the thorax, each lined by a continuous serous membrane called the pleura, that enclose the right and left lungs and allow them to slide against the chest wall during breathing. Each cavity is bounded internally by the mediastinum, externally by the ribs and intercostal muscles, cranially by the thoracic inlet, and caudally by the diaphragm. Understanding the pleural cavities matters because they sit at the center of some of the most common and most urgent thoracic emergencies in veterinary practice. Air that leaks into the pleural space (pneumothorax) or fluid that accumulates there (pleural effusion) can compress the lungs and rapidly cause life-threatening dyspnea. Knowing the boundaries of each cavity, the location of the recesses, and the completeness of the mediastinum in different species directly guides where a clinician places a needle for thoracocentesis and how fluid or air is expected to distribute across the thorax. ## The Two Pleural Sacs and Their Serous Membranes Each lung sits inside its own closed serous sac. The two sacs are the right pleural cavity and the left pleural cavity. In most domestic mammals these two cavities are functionally separate because the mediastinum forms a partition between them, but the degree of separation varies by species. The serous membrane of each sac has two layers that are continuous with each other at the root of the lung: - **Parietal pleura.** This layer lines the inner surface of the thoracic wall, the thoracic surface of the diaphragm, and the lateral surfaces of the mediastinum. It is named by the structure it covers: costal pleura (over the ribs), diaphragmatic pleura (over the diaphragm), mediastinal pleura (over the mediastinum), and the pleural cupula or cervical pleura that extends slightly through the thoracic inlet. - **Visceral pleura.** This layer is tightly adherent to the surface of the lung and dips into the interlobar fissures. Because it is fused with the lung, it cannot be surgically separated from the lung surface without damaging the underlying tissue. The two layers are continuous where the parietal pleura reflects onto the lung at the hilus, the point where the bronchi, pulmonary vessels, and nerves enter and leave the lung. The thin film of serous fluid between the layers lubricates their movement so the lung can slide against the chest wall with each breath. Normal mesothelial cells that line the pleura have bush-like elongated microvilli that enmesh hyaluronic acid-rich lubricants, and this arrangement minimizes the work of breathing [1]. The mesothelial cells are joined by narrow but labile intercellular junctions, which lets fluid and electrolytes pass freely between them [1]. The pleural cavity itself is only a potential space. In health it contains just a few milliliters of fluid, not a visible gap. The lung is held against the chest wall by the mechanical coupling between the two, and this coupling keeps the lung inflated [1]. When air or fluid enters the space, the coupling is disrupted and the lung recoils toward the hilus. ## Boundaries of the Pleural Cavity Each pleural cavity is a three-dimensional space with distinct walls. Knowing these boundaries is what lets a clinician predict where fluid will pool and where a needle can safely be placed. ### The Chest Wall: Ribs and Intercostal Muscles The lateral and ventral boundaries are formed by the ribs, the intercostal muscles that fill the spaces between them, and the associated fascia. The costal pleura lines the inner surface of this wall. The intercostal muscles are arranged in layers, and the intercostal nerves and vessels run in the costal groove along the caudal edge of each rib. This arrangement is why a thoracocentesis needle is placed just cranial to a rib, so it avoids the neurovascular bundle. ### The Diaphragm The diaphragm is a dome-shaped musculotendinous sheet that separates the thoracic cavity from the abdominal cavity. Its thoracic surface is covered by the diaphragmatic pleura. The diaphragm is the principal muscle of inspiration, and its contraction increases thoracic volume. In dogs, the coupling between each hemidiaphragm and the lungs is not perfectly symmetric. When one phrenic nerve is stimulated, the pressure change in the contralateral lung is only about 65% of that in the ipsilateral lung, and the mediastinum stiffens during hemidiaphragmatic contraction, with mediastinal elastance estimated to be about 25 times greater during hemidiaphragmatic contraction than during unilateral intercostal contraction [2]. This means a single hemidiaphragm expands both lungs, but it also stiffens the partition between them. ### The Mediastinum The mediastinum is the partition between the two pleural cavities. It is a composite structure, not a single membrane. It contains the heart and pericardium, the trachea and mainstem bronchi, the esophagus, the thoracic aorta and other great vessels, the thoracic duct, lymph nodes, nerves, and variable amounts of fat and connective tissue. The mediastinal pleura covers its lateral surfaces and is the part of the parietal pleura that faces each pleural cavity. The mediastinum is often described as a single space, but that is misleading. It is a region, and its contents are not hollow. The pleural cavities are the two spaces on either side of it. The mediastinum is not a cavity at all in the usual sense. ### The Thoracic Inlet Cranially, each pleural cavity is bounded by the thoracic inlet, the opening formed by the first thoracic vertebra, the first pair of ribs, and the manubrium of the sternum. The cupula of the pleura projects a short distance through this opening into the root of the neck. This is why injuries or surgical approaches at the thoracic inlet can enter the pleural space. ## Pleural Cavity, Mediastinum, and Thoracic Cavity: Distinguishing the Terms Students often use these three terms interchangeably, and that causes real confusion in clinical reasoning. - **Thoracic cavity** is the entire space inside the rib cage, bounded by the thoracic wall, the diaphragm, and the thoracic inlet. It contains the lungs, the heart, the mediastinal structures, and the pleural cavities. - **Mediastinum** is the central partition within the thoracic cavity. It is a region of tissue and organs, not an empty space. - **Pleural cavity** is the potential space within one pleural sac, between the parietal and visceral pleura. There are two, one on each side. A useful way to keep them straight is to think of the thoracic cavity as the building, the mediastinum as the central dividing wall and its contents, and the pleural cavities as the two rooms on either side. ## Pleural Recesses The pleural recesses are the places where the parietal pleura reflects from one wall to another and where the lung does not fully fill the space during normal breathing. They are the sites where fluid collects first and where a needle can enter without hitting lung tissue. ### Costodiaphragmatic Recess The costodiaphragmatic recess is the space formed where the costal pleura meets the diaphragmatic pleura. It is the lowest part of the pleural cavity in a standing animal and the most dependent part in lateral recumbency. Because of gravity, pleural fluid accumulates here first. This is why the costodiaphragmatic recess is a common target for thoracocentesis. ### Costomediastinal Recess The costomediastinal recess is the space formed where the costal pleura meets the mediastinal pleura, along the ventral aspect of the thorax near the sternum. It is narrow and becomes more prominent when the lung is collapsed or when the heart is enlarged. ### Vertebral Recess A smaller vertebral recess exists dorsally where the costal pleura reflects onto the vertebral column. It is less commonly used clinically but is part of the complete picture of the pleural space. | Recess | Location | Clinical importance | |--|--|--| | Costodiaphragmatic | Where costal pleura meets diaphragmatic pleura, caudoventral thorax | Most dependent site, fluid collects here, common thoracocentesis target | | Costomediastinal | Where costal pleura meets mediastinal pleura, ventral thorax near sternum | Narrow space, widens with lung collapse or cardiac enlargement | | Vertebral | Dorsal reflection of costal pleura onto vertebrae | Less commonly used clinically | ## Species Differences in Mediastinal Completeness and Lung Lobation The completeness of the mediastinum varies across domestic species, and this has direct clinical consequences for how air and fluid distribute between the two pleural cavities. In dogs and cats, the mediastinum is incomplete. There are fenestrations, especially in the ventral part, that allow communication between the right and left pleural cavities. This is why a unilateral pneumothorax or pleural effusion in a dog can become bilateral. A computed tomography study in dogs showed that when saline was infused into one side of the pleural space, the effusate was distributed bilaterally because of the incomplete canine mediastinum [3]. Despite this incompleteness, the canine mediastinum is not functionless. When the diaphragm contracts, the mediastinum stiffens and sustains a significant pressure gradient between the two sides [2]. In horses, the mediastinum is also incomplete, but the fenestrations are smaller and less numerous than in dogs. In ruminants, the mediastinum is more complete, and the two pleural cavities are more effectively separated. This means that in cattle and sheep, a unilateral lesion may remain unilateral for longer. Lung lobation also differs by species. Dogs and cats have well-developed interlobar fissures that divide the lungs into distinct lobes. The right lung in the dog has four lobes (cranial, middle, caudal, and accessory), and the left lung has two (cranial and caudal). The cat has a similar pattern, though the left lung may have a partial separation of the cranial lobe. Horses have a right lung with cranial, middle, caudal, and accessory lobes and a left lung with cranial and caudal lobes. Ruminants have a similar lobation pattern to dogs but with differences in the relative size and shape of the lobes. | Species | Mediastinal completeness | Right lung lobes | Left lung lobes | Clinical note | |--|--|--|--|--| | Dog | Incomplete, fenestrated ventrally | 4 (cranial, middle, caudal, accessory) | 2 (cranial, caudal) | Unilateral air or fluid can become bilateral [3] | | Cat | Incomplete, similar to dog | 4 | 2 (cranial lobe may be partially divided) | Unilateral lesions often spread | | Horse | Incomplete but less fenestrated | 4 | 2 | Unilateral lesions may remain unilateral longer | | Ruminant | More complete | 4 | 2 | Cavities more effectively separated | ## How the Pleural Cavity Is Examined in Practice Several imaging and sampling methods are used to evaluate the pleural cavities in veterinary patients. ### Thoracic Radiography Radiography is the most common first-line imaging method. It can show increased soft tissue opacity in the pleural space, loss of the normal sharp outline of the heart and diaphragm, retraction of lung lobes from the chest wall, and elevation of the trachea. In a study of postmortem thoracic radiographs in dogs, gas accumulation in the pleural cavity and mediastinum was detectable within hours after death at tropical ambient temperatures, and severe pneumothorax developed by 16 hours [4]. This confirms how readily pleural air is seen on radiographs. ### Computed Tomography Computed tomography provides detailed cross-sectional images of the pleural cavity, lung parenchyma, and mediastinum. A helical CT study in normal dogs using intravenous iodine contrast medium and different CT windows showed that lung windows gave a good view of the parenchyma, bronchial tree, vascular structures, and pleural cavity [5]. CT is especially useful for detecting small volumes of pleural fluid, characterizing mediastinal masses, and planning surgical approaches. ### Thoracocentesis Thoracocentesis is the insertion of a needle or catheter through the chest wall into the pleural cavity to remove air or fluid. The site is chosen based on the suspected location of the fluid or air and the anatomy of the recesses. In dogs and cats, the costodiaphragmatic recess is a common target. The needle is placed just cranial to a rib to avoid the intercostal vessels and nerve. In a clinical case, nearly 2 liters of pleural fluid were removed by thoracocentesis from a dog with a plasma cell tumor [6]. Thoracocentesis is both diagnostic and therapeutic. ### Pleural Fluid Analysis Fluid removed from the pleural cavity is submitted for cytology, protein concentration, cell count, and sometimes culture. The fluid is classified as a transudate, modified transudate, or exudate based on these parameters. Transudates form from alterations in the hydrostatic-osmotic pressure relationship that normally keeps the pleural cavity dry [1]. Exudates usually involve more than one factor, including increased vascular permeability and impaired lymphatic drainage [1]. Proteins, particles, and cells in pleural exudates are removed mainly through preformed stomas and lymphatic lacunae in the lower mediastinum and subcostal region [1]. ## Clinical Relevance, Limitations and Common Mistakes ### Pneumothorax Pneumothorax is the presence of air in the pleural cavity. It can be spontaneous, traumatic, or iatrogenic. A case of congenital lobar emphysema in a dog led to air trapping, alveolar hyperinflation, and pleural rupture, causing fatal tension pneumothorax [7]. The "bloat line" seen on radiographs in that case, along with compressive atelectasis of all other lung lobes and lack of negative pressure within the thoracic cavity, signified markedly elevated intrathoracic pressure [7]. Tension pneumothorax is a life-threatening emergency that requires immediate decompression. ### Pleural Effusion Pleural effusion is the accumulation of fluid in the pleural cavity. Causes include congestive heart failure, neoplasia, infection, and inflammation. Neoplastic effusions can be challenging to diagnose. A dog with an ovarian papillary adenocarcinoma developed ascites and pleural effusion, and immunohistochemistry on cell blocks from the effusion revealed PR-positive epithelial cells, while activated mesothelial cells from dogs without neoplastic lesions were negative for PR [8]. In another case, a dog with a clonal IgG lambda plasmacytoma presented with dyspnea and pleural effusion, and nearly 2 liters of fluid were removed by thoracocentesis [6]. A Portuguese water dog with malignant melanoma developed neoplastic melanocytic pleural effusion 232 days after initial presentation [9]. ### Thoracocentesis Sites The choice of thoracocentesis site depends on the species, the suspected location of the fluid or air, and the position of the patient. In dogs and cats, the costodiaphragmatic recess is commonly used for fluid. For air, a higher site may be chosen. In horses, the site is often in the lower third of the thorax at the level of the costodiaphragmatic recess. In cattle, the more complete mediastinum means that a unilateral tap may not drain the opposite side. ### Common Mistakes - Confusing the pleural cavity with the mediastinum. The mediastinum is a partition, not a cavity. - Assuming that a unilateral pneumothorax in a dog will stay unilateral. The incomplete mediastinum allows air to cross [3]. - Placing a thoracocentesis needle too close to the caudal border of a rib, risking the intercostal vessels and nerve. - Forgetting that the costodiaphragmatic recess is the most dependent part of the pleural cavity and therefore the first place fluid collects. - Overlooking the possibility of neoplastic effusion when routine cytology shows atypical cells. Immunohistochemistry and cell block techniques can improve diagnostic yield [8]. ## Quick Review - The pleural cavities are two potential spaces, one around each lung, lined by parietal and visceral pleura. - The boundaries are the ribs and intercostal muscles, the diaphragm, the mediastinum, and the thoracic inlet. - The thoracic cavity is the whole space inside the rib cage. The mediastinum is the central partition. The pleural cavities are the two spaces on either side. - The costodiaphragmatic recess is the most dependent part of the pleural cavity and the first site of fluid accumulation. - The mediastinum is incomplete in dogs and cats, so unilateral air or fluid can become bilateral [3]. - Pneumothorax and pleural effusion are the two most common pleural space emergencies. - Thoracocentesis is both diagnostic and therapeutic and is guided by the anatomy of the recesses. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Frequently Asked Questions ### What is the difference between the pleural cavity and the thoracic cavity? The thoracic cavity is the entire space inside the rib cage. The pleural cavities are the two potential spaces within it, one around each lung. ### Are the right and left pleural cavities connected in dogs? Yes, the mediastinum in dogs is incomplete and fenestrated, so air or fluid can move between the two sides [3]. ### Where is the best site for thoracocentesis in a dog with pleural effusion? The costodiaphragmatic recess is a common site because it is the most dependent part of the pleural cavity and fluid collects there first. ### What is a pleural recess? A pleural recess is a space where the parietal pleura reflects from one wall to another and the lung does not fully fill the space during normal breathing. ### Can pleural effusion be caused by cancer? Yes, neoplastic pleural effusion occurs in dogs with several types of cancer, including plasma cell tumors and ovarian adenocarcinoma [6][8]. ### What happens if air enters the pleural cavity? Air in the pleural cavity is called pneumothorax. It disrupts the coupling between the lung and chest wall, causing the lung to collapse and leading to difficulty breathing. ## Related Articles - [Clinical Research Notes Templates](/blog/research-skills/clinical-research-notes-templates-balancing-detail-with-patient-privacy-and-compliance) - [Veterinary Anatomy Notes: How to Create and Use Them for NAVLE](/knowledge/veterinary-medicine/navle-exam-prep/veterinary-anatomy-notes-how-to-create-and-use-them-for-navle) - [The Best Note-Taking Tools for Clinical Research](/blog/research-skills/the-best-note-taking-tools-for-clinical-research-compliance-security-and-collaboration) - [Project-Based Note Organization for Clinical Research](/blog/research-skills/project-based-note-organization-for-clinical-research-compliance-and-collaboration) - [Protein Domain Boundary Prediction](/blog/guides/protein-domain-boundary-prediction) - [Bovine Hoof Anatomy and Lameness Examination: A Clinical Approach](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/bovine-hoof-anatomy-lameness-examination-clinical-approach) - [Thoracic Cavity Anatomy: Organs and Boundaries](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/thoracic-cavity-anatomy-organs-and-boundaries) - [Peritoneal Cavity Anatomy: Boundaries, Organs, and Fluids](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/peritoneal-cavity-anatomy-boundaries-organs-and-fluids) - [Epiphyseal Plate: Growth, Anatomy and Clinical Notes](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/epiphyseal-plate-growth-anatomy-and-clinical-notes) ## Sources 1. 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