# Lecture 2: Pleura and Lungs ## Unit 1.4: Human Gross Anatomy II - Thorax and Abdomen --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the structure and function of the pleura (parietal and visceral) 2. Identify the pleural recesses and their clinical significance 3. Describe the surface projections of the lungs and pleura 4. Identify the gross anatomy of the lungs (lobes, fissures, surfaces, borders) 5. Describe the bronchopulmonary segments and their clinical importance 6. Explain the blood supply, innervation, and lymphatic drainage of the lungs --- ## Lecture Content ### I. The Pleura The pleura consists of a thin serous membrane that forms a closed sac around each lung. This membrane has two continuous layers that create the pleural cavity, a potential space essential for respiratory mechanics. The visceral pleura intimately covers the lung surface, extending into the depths of the fissures that separate the lobes. This layer is inseparable from the underlying lung tissue and reflects onto the parietal layer at the hilum where structures enter and exit the lung. The visceral pleura receives its sensory innervation from autonomic fibers, rendering it insensitive to painful stimuli. Inflammation or pathology affecting only the visceral pleura therefore does not produce sharp pain. The parietal pleura lines the inner surface of the thoracic cavity and is named according to the surface it covers. This layer is sensitive to pain because it receives somatic sensory innervation, making pleural inflammation intensely painful when the parietal layer is involved. The parietal pleura has four named portions. The costal pleura lines the inner surface of the ribs and intercostal muscles, receiving sensory innervation from the intercostal nerves. The mediastinal pleura covers the lateral aspect of the mediastinum, innervated by the phrenic nerve. The diaphragmatic pleura covers the superior surface of the diaphragm, with the central portion innervated by the phrenic nerve and the peripheral portion by the lower intercostal nerves. The cervical pleura, also called the cupola, extends above the first rib into the root of the neck, receiving innervation from the cervical plexus via contributions from C3 through C5. Panel A: Visceral pleura intimately covering the lung surface and extending into the fissures between lobes. Panel B: Parietal pleura with costal portion lining the thoracic wall and mediastinal portion covering the lateral mediastinum. Panel C: Diaphragmatic pleura on the superior diaphragm surface and cervical pleura (cupola) rising above the first rib. Panel D: Pleural cavity as a potential space between the layers with the reflection at the hilum and pulmonary ligament extending inferiorly. --- ### II. Pleural Cavity and Fluid The pleural cavity represents the potential space between the visceral and parietal pleural layers. Under normal conditions, this space contains only a thin layer of serous fluid, approximately fifteen to twenty milliliters. The cavity maintains a negative pressure of approximately negative five centimeters of water relative to atmospheric pressure. The pleural fluid serves several critical functions. It provides lubrication between the two pleural surfaces, reducing friction during the constant movement of respiration. The surface tension created by this fluid holds the lung surface against the chest wall, coupling lung expansion to chest wall movement. This coupling ensures that when the thoracic cavity expands during inspiration, the lung expands with it. Several pathological conditions affect the pleural cavity. Pleural effusion describes the accumulation of excess fluid within the pleural space, which may result from heart failure, infection, malignancy, or pulmonary embolism. Pneumothorax occurs when air enters the pleural cavity, disrupting the negative pressure and causing lung collapse. Hemothorax refers to blood accumulating in the pleural space, typically from trauma or malignancy. Pyothorax, also called empyema, indicates pus within the pleural cavity from infection. Chylothorax results from lymphatic fluid or chyle entering the cavity, usually from thoracic duct injury. Panel A: Normal pleural cavity state with thin fluid layer maintaining lung expansion against the chest wall. Panel B: Pleural effusion with fluid collection compressing the lung from below in the dependent portion. Panel C: Pneumothorax with air in the pleural cavity and collapsed lung falling away from the chest wall. Panel D: Hemothorax with blood accumulation in the dependent portion of the pleural space. --- ### III. Pleural Recesses The pleural recesses represent areas where the parietal pleura reflects on itself, creating potential spaces that the lungs do not fully occupy during quiet breathing. These recesses accommodate lung expansion during deep inspiration and serve as sites where pathological fluid collections accumulate. The costodiaphragmatic recess forms where the costal pleura reflects onto the diaphragmatic pleura. This recess represents the deepest portion of the pleural cavity, extending approximately five centimeters between the inferior lung margin and the inferior pleural reflection during quiet breathing. The lowest point of this recess lies at the midaxillary line. Because fluid follows gravity, pleural effusions first accumulate in this dependent recess, making it the standard site for diagnostic and therapeutic thoracentesis. The costomediastinal recess forms where the costal pleura reflects onto the mediastinal pleura, located behind the sternum. This recess is more prominent on the left side due to the cardiac notch, where the lung margin deviates laterally to accommodate the heart. The anterior junction line visible on chest radiographs represents where the two costomediastinal recesses nearly meet behind the sternum. Panel A: Sagittal view of the costodiaphragmatic recess as a potential space between the inferior lung margin and diaphragmatic pleura reflection. Panel B: Coronal view showing recess depth during quiet breathing versus deep inspiration with measurements indicated. Panel C: Costomediastinal recess behind the sternum with prominence on the left side due to the cardiac notch. Panel D: Fluid accumulation pattern in the costodiaphragmatic recess with thoracentesis needle approach from the posterior chest wall. --- ### IV. Surface Projections Understanding the surface projections of the pleura and lungs enables accurate physical examination and safe procedural approaches. The pleura extends both higher and lower than the lung margins, creating the pleural recesses. The cervical pleura rises two to three centimeters above the medial third of the clavicle, reaching the level of the T1 spinous process posteriorly. This superior extent places the pleural apex at risk during procedures in the root of the neck, particularly subclavian venous catheterization. The anterior pleural borders follow slightly different courses on the two sides. On the right, the pleura descends from the cupola behind the sternoclavicular joint, continuing along the sternum to reach the sixth costal cartilage. On the left, the course is similar until the fourth costal cartilage, where the pleura deviates laterally to create the cardiac notch before continuing to the sixth cartilage. The inferior pleural borders extend well below the lung margins. At the midclavicular line, the pleura reaches the eighth rib while the lung reaches only the sixth rib. At the midaxillary line, the pleura reaches the tenth rib and the lung the eighth rib. Posteriorly at the scapular line, the pleura reaches the twelfth rib and the lung the tenth rib. A useful mnemonic remembers that the pleura follows the pattern 8, 10, 12 while the lungs follow 6, 8, 10 at the midclavicular, midaxillary, and scapular lines respectively. Panel A: Anterior chest view with pleural borders and lung borders marked, showing the cardiac notch deviation on the left at the fourth costal cartilage. Panel B: Lateral chest view with numbered rib levels at midclavicular line (pleura 8, lung 6) and midaxillary line (pleura 10, lung 8). Panel C: Posterior chest view showing scapular line projections (pleura 12, lung 10) with the cervical pleura rising above the clavicle. Panel D: Color-coded legend distinguishing pleural from pulmonary boundaries with the 6-8-10 and 8-10-12 rule indicated. --- ### V. Gross Anatomy of the Lungs The two lungs occupy the lateral compartments of the thoracic cavity, separated by the mediastinum. Each lung is light, spongy, and elastic, floating if placed in water due to its air content. The right lung is larger and heavier than the left, containing three lobes compared to two on the left. The left lung is smaller because the heart occupies more space on that side, creating the cardiac impression. Each lung has three surfaces. The costal surface is convex and contacts the ribs and intercostal spaces, often showing impressions from the ribs on its surface. The mediastinal or medial surface is concave and contains the hilum where structures enter and exit the lung. This surface also bears impressions from adjacent mediastinal structures. The diaphragmatic surface or base is concave and rests on the dome of the diaphragm, with the right base sitting higher than the left due to the liver beneath. The borders of each lung define the transitions between surfaces. The anterior border is sharp and separates the costal from the mediastinal surface. On the left lung, this border contains the cardiac notch where the lung margin deviates laterally. The inferior border separates the base from the costal surface and follows the pleural reflection at a higher level. The posterior border is rounded and occupies the paravertebral gutter alongside the thoracic vertebrae. Panel A: Lateral view of both lungs showing the convex costal surfaces with rib impressions visible. Panel B: Medial view displaying the mediastinal surfaces with hilum structures and impressions from adjacent organs. Panel C: Inferior view of the diaphragmatic surfaces showing the concave bases with the right base sitting higher. Panel D: Borders and lobes with the anterior sharp edge, cardiac notch on the left, and the right lung with three lobes compared to the left with two lobes. --- ### VI. Lobes and Fissures The fissures divide each lung into lobes, with the fissures extending deeply into the lung substance to separate relatively independent portions. The right lung contains three lobes separated by two fissures. The oblique fissure separates the inferior lobe from the superior and middle lobes. The horizontal fissure separates the superior lobe from the middle lobe at approximately the level of the fourth rib. The superior lobe occupies the upper anterior portion of the lung, the middle lobe lies anteriorly between the horizontal and oblique fissures, and the inferior lobe occupies the posterior and inferior portions. The left lung contains two lobes separated by a single oblique fissure. The superior lobe lies above and anterior to this fissure, while the inferior lobe lies below and posterior. The lingula, a tongue-like projection of the superior lobe, represents the anatomical equivalent of the right middle lobe and occupies a similar anterior position. The surface projections of the fissures help localize pathology on physical examination and imaging. The oblique fissure on both sides follows a similar course, running from the T3 spinous process posteriorly, along the course of the sixth rib, to the sixth costochondral junction anteriorly. This course can be approximated by the medial border of the scapula when the arm is raised above the head. The horizontal fissure, present only on the right, extends from the oblique fissure at the midaxillary line, running horizontally along the fourth rib to reach the sternum at the fourth costal cartilage. Panel A: Right lung with three lobes (superior, middle, inferior) separated by oblique and horizontal fissures. Panel B: Left lung with two lobes (superior including lingula, inferior) separated by the oblique fissure only. Panel C: Surface projection of the oblique fissure from T3 spinous process to the sixth costochondral junction following the sixth rib. Panel D: Horizontal fissure along the fourth rib on the right side, with the lingula labeled as the middle lobe equivalent. --- ### VII. Hilum and Root of the Lung The hilum represents the wedge-shaped depression on the mediastinal surface of each lung where structures enter and exit. A sleeve of pleura surrounds these structures, reflecting from the mediastinal parietal pleura onto the visceral pleura. Below the hilum, this pleural sleeve extends as the pulmonary ligament, anchoring the lung to the mediastinum and allowing passage of vessels between lobes. The root of the lung comprises all the structures connecting the lung to the mediastinum at the hilum. These structures include the pulmonary artery carrying deoxygenated blood from the right ventricle to the lung for oxygenation, two pulmonary veins returning oxygenated blood to the left atrium, the main bronchus providing the airway, bronchial vessels supplying nutritional blood to the lung tissue itself, lymphatic vessels draining to the bronchopulmonary nodes, and autonomic nerves from the pulmonary plexus. The arrangement of structures at the hilum follows predictable patterns that differ between sides. From superior to inferior on the right, the eparterial bronchus lies highest, followed by the pulmonary artery, then the main bronchus, and finally the pulmonary veins at the lowest level. The mnemonic BAPV, or "Boys Attract Pretty Violets," helps remember this right-sided arrangement. On the left, the pulmonary artery lies highest, followed by the bronchus, then the veins, following the pattern APV. From anterior to posterior on both sides, the relationship is veins anteriorly, then artery, then bronchus most posteriorly. Panel A: Medial view of the right hilum showing the eparterial bronchus highest, pulmonary artery below, main bronchus, and veins lowest in the BAPV arrangement. Panel B: Medial view of the left hilum with pulmonary artery highest, bronchus below, then veins in the APV arrangement. Panel C: Cross-section showing the anterior-to-posterior relationship with veins anteriorly, artery in the middle, and bronchus posteriorly. Panel D: Pulmonary ligament extending below the hilum with bronchopulmonary lymph nodes at the hilum indicated. --- ### VIII. Impressions on Mediastinal Surface The mediastinal surfaces of the lungs bear impressions from adjacent structures, providing evidence of the close anatomical relationships within the thorax. The right lung bears several characteristic impressions. The cardiac impression on the anterior mediastinal surface is relatively small compared to the left. The groove for the superior vena cava runs vertically along the anterior mediastinal surface. The arch of the azygos vein creates an impression as it arches over the root of the right lung to join the superior vena cava. The esophageal impression lies posterior to the root where the esophagus contacts the lung. The left lung shows different impressions reflecting its relationship to left-sided structures. The cardiac impression is large and deep, creating the obvious concavity on the anterior mediastinal surface. The aortic groove marks the course of the aortic arch superiorly and the descending aorta inferiorly, creating a continuous curved impression. Above the aortic arch, the left subclavian artery creates a separate impression. The esophageal impression lies posterior to the root, similar to the right side. Panel A: Right lung mediastinal surface showing the cardiac impression anteriorly and SVC groove as a vertical line. Panel B: Right lung with the azygos arch impression curving over the root and esophageal impression posteriorly. Panel C: Left lung mediastinal surface with the large, deep cardiac impression and continuous aortic groove from arch to descending portions. Panel D: Left lung showing the subclavian artery impression above the arch and esophageal impression posteriorly. --- ### IX. Bronchopulmonary Segments Each lung is divided into bronchopulmonary segments, which represent functionally independent units of lung tissue. Each segment receives its own segmental bronchus and segmental artery, and is separated from adjacent segments by connective tissue septa. This independence allows surgical resection of individual segments while preserving the remainder of the lung, a procedure called segmentectomy. The right lung contains ten bronchopulmonary segments. The superior lobe has three segments: apical, posterior, and anterior. The middle lobe contains two segments: lateral and medial. The inferior lobe has five segments: superior, medial basal, anterior basal, lateral basal, and posterior basal. The left lung typically contains eight to ten segments, with some variation due to fusion of certain segments. The superior lobe contains four to five segments: the apicoposterior segment often represents fusion of the apical and posterior segments, followed by the anterior segment, and the superior and inferior lingular segments corresponding to the right middle lobe. The inferior lobe contains four to five segments: the superior segment, the anteromedial basal segment often representing fusion of the anterior and medial basal segments, the lateral basal segment, and the posterior basal segment. The clinical importance of bronchopulmonary segments extends beyond surgical planning. Aspirated foreign bodies tend to lodge in specific segments based on bronchial anatomy, most commonly the posterior segment of the right upper lobe in supine patients or the posterior basal segments in upright patients. Pneumonia may be limited to specific segments. Understanding segmental anatomy helps interpret chest imaging and localize pathology. Panel A: Right lung lateral and medial views showing 10 bronchopulmonary segments with distinct color-coding for superior (3), middle (2), and inferior (5) lobes. Panel B: Left lung with 8-10 segments including superior lobe with lingular segments and inferior lobe segments. Panel C: Bronchial tree diagram showing the segmental bronchi branching pattern from main bronchi to segmental levels. Panel D: Table listing segment names for each lobe with notation of common aspiration sites in the posterior segments. --- ### X. Blood Supply and Lymphatics The lungs receive blood from two distinct circulations serving different functions. The pulmonary circulation provides the functional blood supply responsible for gas exchange. The pulmonary arteries carry deoxygenated blood from the right ventricle, branching to follow the bronchial tree into progressively smaller vessels until reaching the capillary networks surrounding the alveoli. After gas exchange, oxygenated blood returns via the pulmonary veins to the left atrium. Despite carrying oxygenated blood, the pulmonary veins are named veins because they carry blood toward the heart. The bronchial circulation provides the nutritional blood supply to the lung tissue itself. The bronchial arteries arise from the thoracic aorta or its branches, typically one artery on the right and two on the left. These vessels supply the bronchi, connective tissue septa, visceral pleura, and lymph nodes. The bronchial veins have a complex drainage pattern. The right bronchial vein drains to the azygos vein, while the left bronchial veins drain to the hemiazygos or left superior intercostal veins. Importantly, some bronchial venous blood drains into the pulmonary veins, contributing to the normal physiologic shunt that slightly reduces arterial oxygen saturation. Lymphatic drainage follows a predictable pathway from the lung periphery to the mediastinum. Lymph from the lung parenchyma drains first to pulmonary nodes within the lung substance, then to bronchopulmonary or hilar nodes at the hilum, then to tracheobronchial nodes at the tracheal bifurcation, and finally to paratracheal nodes along the trachea. The efferent vessels eventually form the bronchomediastinal lymph trunk. Notably, lymph from the right lung may cross to left-sided tracheobronchial nodes, which has implications for the spread of malignancy. Panel A: Pulmonary arteries carrying deoxygenated blood and branching with the bronchial tree to reach alveolar capillaries. Panel B: Pulmonary veins carrying oxygenated blood returning to the left atrium from the lung periphery. Panel C: Bronchial arteries from the aorta providing nutritional blood supply to bronchi and lung tissue. Panel D: Lymphatic drainage pathway from pulmonary nodes to hilar nodes to tracheobronchial nodes to paratracheal nodes to the bronchomediastinal trunk. --- ### XI. Innervation The pulmonary plexus, located at the hilum of each lung, provides autonomic innervation to the airways and vessels. This plexus contains both sympathetic and parasympathetic fibers that produce opposing effects on bronchial smooth muscle and glandular secretion. Parasympathetic innervation arrives via the vagus nerve. These fibers cause bronchoconstriction by contracting bronchial smooth muscle, increase mucus secretion from bronchial glands, and produce vasodilation of pulmonary vessels. Vagal tone predominates at rest, maintaining a degree of baseline bronchial constriction. Sympathetic innervation originates from the T2 through T4 segments of the spinal cord, with postganglionic fibers reaching the lung via the sympathetic trunk. Sympathetic activation causes bronchodilation by relaxing bronchial smooth muscle, decreases glandular secretion, and produces vasoconstriction of pulmonary vessels. These effects support increased ventilation during exercise or stress. Sensory innervation differs between the visceral and parietal pleura. The visceral pleura receives only autonomic afferent fibers and is therefore insensitive to painful stimuli such as needle puncture. The parietal pleura receives somatic sensory innervation: the costal and peripheral diaphragmatic portions via intercostal nerves, and the mediastinal and central diaphragmatic portions via the phrenic nerve. This difference explains why pleuritic chest pain occurs only when the parietal pleura is inflamed, and why central diaphragmatic irritation may be referred to the shoulder via the phrenic nerve's C3-C5 dermatomes. --- ### XII. Clinical Correlations Pneumothorax results from air entering the pleural cavity, disrupting the negative pressure that maintains lung expansion. The lung collapses away from the chest wall, impairing ventilation. Spontaneous pneumothorax occurs without trauma, typically from rupture of a subpleural bleb. Traumatic pneumothorax results from penetrating or blunt chest injury. Tension pneumothorax represents a medical emergency where a one-way valve effect allows air to enter but not escape the pleural space, progressively increasing pressure, shifting the mediastinum, and compromising venous return to the heart. Pleural effusion, the accumulation of excess fluid in the pleural space, has multiple causes. Transudative effusions result from altered hydrostatic or oncotic pressures, as in heart failure or hypoalbuminemia. Exudative effusions result from increased capillary permeability due to infection, malignancy, or inflammation. Effusions first accumulate in the dependent costodiaphragmatic recess and appear as blunting of the costophrenic angle on chest radiograph. Thoracentesis, the needle drainage of pleural fluid, requires knowledge of surface anatomy to avoid complications. The procedure is typically performed posteriorly, entering the pleural space above the ninth rib at the midscapular line to remain below the lung margin but above the diaphragm. The needle must pass immediately above the rib to avoid the intercostal neurovascular bundle running in the costal groove along the inferior border of the rib above. Knowledge of bronchopulmonary segments aids surgical planning for lung cancer resection. Segmentectomy removes only the affected segment, preserving more lung function than lobectomy. Understanding lymphatic drainage patterns predicts sites of metastatic spread and guides surgical lymph node dissection. Lung auscultation locations correlate with underlying anatomy. Anterior chest examination primarily assesses the upper lobes. Posterior examination primarily assesses the lower lobes. The right middle lobe and left lingula are best heard on the anterior chest at the cardiac notch level. Panel A: Pneumothorax showing collapsed lung with tension pneumothorax demonstrating mediastinal shift and tracheal deviation. Panel B: Pleural effusion with blunted costophrenic angle on chest X-ray and layering fluid on decubitus view. Panel C: Thoracentesis procedure showing patient position with entry site above the ninth rib at the midscapular line avoiding the neurovascular bundle. Panel D: Auscultation zones mapped to underlying lung lobes on anterior and posterior chest views. --- ## Summary - The pleura consists of visceral (lung surface) and parietal (cavity lining) layers creating the pleural cavity - Parietal pleura has costal, mediastinal, diaphragmatic, and cervical portions with distinct sensory innervation - Pleural recesses (costodiaphragmatic, costomediastinal) are potential spaces for lung expansion and fluid accumulation - Surface projections follow the rule: pleura at ribs 8, 10, 12; lungs at ribs 6, 8, 10 at midclavicular, midaxillary, and scapular lines - The right lung has three lobes; the left has two lobes plus the lingula equivalent to the middle lobe - Bronchopulmonary segments are functionally independent units that can be surgically resected individually - Pulmonary circulation provides gas exchange; bronchial circulation provides nutrition to lung tissue - Parietal pleura is pain-sensitive; visceral pleura is not --- ## Key Terms | Term | Definition | |------|------------| | Visceral pleura | Serous membrane intimately covering lung surface, insensitive to pain | | Parietal pleura | Serous membrane lining thoracic cavity, pain-sensitive with somatic innervation | | Costodiaphragmatic recess | Pleural space between ribs and diaphragm; deepest recess and site of fluid accumulation | | Bronchopulmonary segment | Functionally independent lung unit supplied by segmental bronchus and artery | | Hilum | Site on mediastinal surface where structures enter and exit lung | | Lingula | Tongue-like projection of left superior lobe; equivalent to right middle lobe | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*