Comprehensive Anatomy Study Guide: Lungs, Pleura, Thoracic Duct, and Azygos System

Overview and General Anatomy of the Lungs

Anatomical Definition and Role:

• The lungs function as the primary organs of respiration.

• They are conical in shape and located on either side of the mediastinum within the thoracic cavity.

• Texture is soft, spongy, and highly elastic.

• Color Changes Across Lifespan:

  • In children, the lungs present a pink color.

  • With advancing age, they become mottled due to the progressive accumulation of atmospheric dust particles.

• Anatomical Attachments and Pleural Coverings:

  • Each lung is enveloped by visceral pleura.

  • Each lung is suspended free within its respective pleural cavity, except at its root (hilum) where it attaches to the mediastinum.

• General Structural Differences and Features:

  • The right lung is wider and heavier than the left lung.

  • Each lung features $1$ apex, $3$ surfaces (costal, medial, and diaphragmatic), and $3$ borders (anterior, posterior, and inferior).

• The Apex of the Lung:

  • Form: Rounded superior portion projecting superiorly into the root of the neck.

  • Location: The apex summit is situated $2.5\,\text{cm}$ above the medial third of the clavicle or $3\,\text{cm}$ above the $1\,\text{st}$ costal cartilage.

Surfaces, Borders, Lobes, and Fissures of the Lungs

• Diaphragmatic Surface (Base):

  • Concave surface directly related to the upper dome of the diaphragm.

  • The base of the right lung exhibits greater concavity than the left lung because the right dome of the diaphragm is pushed higher by the underlying right lobe of the liver.

• Costal Surface:

  • Convex surface related directly to the sternum, ribs, and costal cartilages.

• Medial Surface:

  • Subdivided into two distinct regions:

  • Vertebral (Posterior) Part: Related to the lateral aspects of the vertebral bodies.

  • Mediastinal (Anterior) Part: Related to mediastinal structures.

  • Features a cardiac impression for the pericardium, which is noticeably deeper on the left lung because the heart projects predominantly to the left.

  • Situated superior and posterior to the cardiac impression is the hilum (hilus), through which neurovascular and airway structures constituting the root of the lung pass to enter or exit.

• Borders of the Lungs:

  • Anterior Border: Sharp margin separating the medial and costal surfaces. The anterior border of the left lung contains the cardiac notch.

  • Posterior Border: Rounded margin separating the costal surface from the medial surface.

  • Inferior Border: Margin separating the diaphragmatic base from the costal surface.

• Right Lung Lobes and Fissures:

  • Divided into $3$ lobes: superior, middle, and inferior.

  • Right Oblique Fissure:

  • Originates at the posterior border of the lung at the level of the spine of $T4$ or slightly lower.

  • Crosses the $5\,\text{th}$ intercostal space at the mid-axillary line.

  • Follows the anatomical course of the $6\,\text{th}$ rib down to the $6\,\text{th}$ costo-chondral junction.

  • Is mapped by the vertebral border of the scapula when the upper limb is abducted above the head.

  • Right Horizontal Fissure:

  • Separates the superior lobe from the middle lobe.

  • Extends horizontally from the oblique fissure near the mid-axillary line to the anterior border of the lung at the level of the $4\,\text{th}$ costal cartilage.

• Left Lung Lobes and Fissures:

  • Divided into $2$ lobes: superior and inferior.

  • Left Oblique Fissure:

  • Follows a path similar to the right oblique fissure, but originates at a higher level in the interval between the spines of $T3$ and $T4$.

  • Runs more vertically than the right oblique fissure.

• Surface Markings of the Lungs:

  • In the supraclavicular region and along the costal wall, lung surface markings match those of the pleura.

  • Left Anterior Border: Curves laterally from the $4\,\text{th}$ costal cartilage to the apex beat located in the mid-clavicular line.

  • Lower Borders of Both Lungs: Run horizontally $2$ ribs higher than the corresponding pleural boundary:

  • $6\,\text{th}$ rib along the mid-clavicular line.

  • $8\,\text{th}$ rib along the mid-axillary line.

  • $10\,\text{th}$ rib posteriorly.

Neurovascular Supply and Applied Anatomy of the Lungs

• Vascular Supply Pathways:

  • Pulmonary Vascular System: Conveys deoxygenated blood via pulmonary arteries to alveolar wall capillary networks and drains oxygenated blood back to the left atrium via pulmonary veins.

  • Bronchial Vascular System: Smaller systemic vessels supplying oxygenated blood directly to pulmonary tissues that lack direct exposure to atmospheric oxygen.

• Innervation of the Lungs:

  • Sympathetic Innervation:

  • Derived from spinal segments $T2$ to $T5$.

  • Functional effect: Inhibitory to smooth muscle and glands of the bronchial tree, resulting in bronchodilation.

  • Parasympathetic Innervation:

  • Derived from the vagus nerve.

  • Functional effect: Motor to bronchial smooth muscles (causing bronchospasm) and secretomotor to mucous glands.

• Applied Clinical Anatomy:

  • Segmental Pathologies and Surgical Interventions:

  • Infections are typically restricted to an individual bronchopulmonary segment, though specific chronic infections like tuberculosis ($TB$) can spread across segmental boundaries.

  • Surgical procedures include resections of individual bronchopulmonary segments, lobectomy (removal of one entire lobe), or pneumonectomy (removal of the entire lung).

  • Mass Effects of Upper Lobe Tumors:

  • Tumors located in the upper lobe produce local pressure effects leading to venous engorgement, facial and upper extremity edema, and hoarseness of voice.

  • Auscultation Landmarks:

  • Upper lobe breath sounds: Best heard on the anterior chest wall.

  • Lower lobe breath sounds: Best heard on the posterior chest wall.

  • All lung lobes: Breath sounds can be auscultated along the mid-axillary line.

  • Environmental Lung Diseases and Aspiration:

  • Conditions such as silicosis, asbestosis, lung cancer, and pneumonia interfere with normal lung expansion during inspiration.

  • Foreign body aspiration preferentially enters specific bronchial pathways based on structural alignment.

Anatomy, Layers, and Subdivisions of the Pleura

• Definition and Cavity Characteristics:

  • The pleura is a double-layered serous membrane surrounding each lung.

  • The potential space enclosed between the visceral and parietal layers is the pleural cavity.

  • It is a closed space containing a thin film of serous fluid that lubricates pleural surfaces and reduces friction during respiration.

• Visceral Pleura:

  • Covers the external surfaces of the lungs and lines the interlobar fissures.

  • Directly continuous with the parietal pleura around the root of the lung.

• Parietal Pleura and Subdivisions:

  • Lines the inner surface of the thoracic cage and superior aspect of the diaphragm.

  • Subdivided into four named regions based on local anatomy:

  • Costal Pleura: Covers internal aspects of the ribs and transversus thoracis muscle; separated from these structures by endothoracic fascia (which corresponds to the transversalis fascia of the abdominal wall). Represents the strongest part of the parietal pleura.

  • Diaphragmatic Pleura: Covers the thoracic surface of the diaphragm.

  • Cervical Pleura (Dome of Pleura): Extends superiorly into the root of the neck; reinforced by the suprapleural membrane (Sibson's fascia), a localized fibrous thickening of the endothoracic fascia.

  • Mediastinal Pleura: Covers the lateral boundary of the mediastinum.

• Pleural Recesses and Pulmonary Ligament:

  • Recess Anatomy in Respiration:

  • During quiet respiration, the lower border of the lung is positioned approximately $5\,\text{cm}$ superior to the lower limit of the pleura, placing lower regions of costal and diaphragmatic parietal pleurae in direct apposition.

  • Costo-diaphragmatic Recess: Slit-like potential space formed between the costal and diaphragmatic parietal pleurae at the lower thoracic margin.

  • Costomediastinal Recess: Slit-like space formed between costal and mediastinal parietal pleurae behind the sternum.

  • Pulmonary Ligament:

  • A tubular cuff of parietal pleura hanging inferiorly below the root of the lung as a loose fold.

  • The term is a misnomer, as it is not a structural ligament and does not attach to bone or support the lung directly.

  • Functional significance: Functions as dead space allowing for expansion of the pulmonary veins during elevated venous return (such as during exercise) and facilitates vertical movement of the lung root during respiration.

Surface Markings, Neurovascular Supply, and Clinical Pathology of the Pleura

• Surface Markings of the Pleura:

  • Cervical Pleura: Projects into the neck along a curved line from the sternoclavicular joint to a apex point $2.5\,\text{cm}$ superior to the junction between the medial and intermediate thirds of the clavicle.

  • Anterior Margins:

  • Passes inferiorly from behind the sternoclavicular joint to meet the contralateral pleura at the sternal angle ($2\,\text{nd}$ rib level), where right and left pleurae may overlap.

  • Extends vertically behind the sternum down to the $4\,\text{th}$ costal cartilage.

  • At the $4\,\text{th}$ costal cartilage, the right pleura continues vertically downward to the $6\,\text{th}$ costal cartilage, whereas the left pleura arches laterally halfway to the cardiac apex beat.

  • Inferior Margins:

  • At the $6\,\text{th}$ costal cartilage, both pleural borders turn laterally.

  • Crosses the mid-clavicular line at the $8\,\text{th}$ rib.

  • Crosses the mid-axillary line at the $10\,\text{th}$ rib.

  • Crosses the lateral border of the sacrospinalis muscle at the $12\,\text{th}$ rib.

  • Passes horizontally to terminate at the inferior border of the $T12$ vertebra.

• Nerve Supply of the Pleura:

  • Parietal Pleura (Somatic Innervation, Pain Sensitive):

  • Costal Pleura: Segmentally innervated by intercostal nerves.

  • Mediastinal Pleura: Innervated by the phrenic nerve.

  • Diaphragmatic Pleura: Innervated centrally over the domes by phrenic nerves, and peripherally by intercostal nerves.

  • Visceral Pleura (Autonomic Innervation, Pain Insensitive):

  • Receives autonomic vasomotor supply only.

  • Completely insensitive to somatic sensations like pain and touch.

• Vascular Supply and Lymphatic Drainage of Pleura:

  • Parietal Pleura Arterial Supply:

  • Costovertebral part: Branches of intercostal and internal thoracic arteries.

  • Mediastinal part: Branches from bronchial, superior diaphragmatic, internal thoracic, and mediastinal arteries.

  • Cervical part: Branches from the subclavian artery.

  • Diaphragmatic part: Microcirculation of the diaphragmatic musculature.

  • Parietal Venous and Lymphatic Drainage:

  • Veins join systemic thoracic wall veins draining into the Superior Vena Cava ($SVC$).

  • Lymph from costovertebral parietal pleura drains anteriorly into internal thoracic nodes and posteriorly into intercostal nodes.

  • Lymph from diaphragmatic pleura drains into mediastinal, retrosternal, and coeliac axis nodes.

  • Visceral Pleura Supply and Drainage:

  • Supplied and drained by bronchial vessels as an integral part of lung tissue.

  • Lymph drains into the deep pulmonary plexus situated within interlobar and peribronchial spaces.

• Clinical Conditions of Pleura:

  • Pleurisy (Pleuritis): Inflammation of the pleural membranes secondary to underlying pulmonary parenchymal infection.

  • Pleural Effusion: Abnormal fluid collection in the pleural space secondary to elevated fluid formation or impaired lymphatic clearance.

Anatomy, Relations, and Pathologies of the Thoracic Duct

• Overview and General Features:

  • The thoracic duct is the largest lymphatic vessel in the human body.

  • Measures approximately $45\,\text{cm}$ ($18\,\text{inches}$) in length.

  • Extends from the upper abdominal cavity to the base of the neck, traversing the posterior and superior mediastinum.

  • Features a beaded external appearance due to numerous internal semilunar valves.

• Path and Course of the Thoracic Duct:

  • Begins as the superior continuation of the cisterna chyli near the lower border of the $T12$ vertebra.

  • Enters the thoracic cavity through the aortic opening of the diaphragm.

  • Ascends through the posterior mediastinum from level $T12$ to $T5$.

  • At the $T5$ level, it crosses the midline from the right side to the left side of the vertebral column.

  • Ascends through the superior mediastinum along the left margin of the esophagus into the neck.

  • In the neck, it arches laterally at the level of the transverse process of the $C7$ vertebra.

  • Descends anterior to the first part of the left subclavian artery.

  • Terminates by emptying into the junction of the left internal jugular and left subclavian veins (the left venous angle).

• Anatomical Relations of the Thoracic Duct:

  • At the Aortic Opening of the Diaphragm:

  • Anterior: Diaphragm.

  • Posterior: Vertebral column.

  • To the Right: Azygos vein.

  • To the Left: Aorta.

  • In the Posterior Mediastinum:

  • Anterior: Diaphragm, esophagus, right pleural recess.

  • Posterior: Vertebral column, right posterior intercostal arteries, terminal parts of hemiazygos veins.

  • To the Right: Azygos vein.

  • To the Left: Descending thoracic aorta.

  • In the Superior Mediastinum:

  • Anterior: Arch of aorta, origin of left subclavian artery.

  • Posterior: Vertebral column.

  • To the Right: Esophagus.

  • To the Left: Pleura.

  • In the Neck:

  • Forms an arch projecting $3\text{ to }4\,\text{cm}$ above the clavicle.

  • Anterior Relations of the Arch: Left common carotid artery, left vagus nerve, left internal jugular vein.

  • Posterior Relations of the Arch: Vertebral artery and vein, sympathetic trunk, thyrocervical trunk and its branches, left phrenic nerve, medial border of scalenus anterior muscle, prevertebral fascia, first part of left subclavian artery.

• Drainage Territory and Tributaries:

  • Drains lymph from both halves of the body inferior to the diaphragm, and the left half of the body superior to the diaphragm.

  • Thoracic Tributaries: Receives efferents from posterior mediastinal nodes and small intercostal nodes.

  • Cervical and Upper Limb Tributaries:

  • Left Jugular Trunk: Drains lymph nodes of the left side of the neck.

  • Left Subclavian Trunk: Drains lymph nodes of the left axilla and upper limb.

  • Left Bronchomediastinal Trunk: Drains the left thoracic cavity.

• Clinical Applied Anatomy:

  • Chylothorax:

  • Rupture or erosion of the thoracic duct leading to chyle accumulation within the pleural cavity.

  • Etiologies:

    • Traumatic Iatrogenic: Surgery, radiation therapy.

    • Traumatic Non-Iatrogenic: Penetrating/blunt chest trauma, severe emesis, violent coughing.

    • Non-Traumatic: Malignancy, tuberculosis, sarcoidosis, superior vena cava obstruction.

    • Congenital: Associated with Down, Noonan, and Turner syndromes.

  • Diagnosis: Chest radiograph reveals pleural effusion; pleural fluid cholesterol and triglyceride measurements confirm chylous origin.

  • Virchow's Node and Troisier's Sign:

  • Virchow's Node: A prominent lymph node in the left supraclavicular region at the venous angle junction of the thoracic duct and left subclavian vein.

  • Troisier's Sign: Palpable enlargement of Virchow's node, serving as an early clinical sign of intra-abdominal malignancy (typically gastric carcinoma) via thoracic duct tumor embolization.

  • Thoracic Duct Fistula:

  • Occurs following surgical procedures in the lower left neck.

  • Causes loss of lymph, leading to fluid, electrolyte, and protein imbalances.

  • Clinical manifestation: Persistent discharge of milky white fluid from the neck incision.

Anatomy and Tributaries of the Azygos Venous System

• Azygos Vein Overview:

  • Unpaired vein ("azygos" = unpaired) occupying the upper posterior abdominal wall and posterior mediastinum.

  • Drains the thoracic wall and upper lumbar region.

  • Forms a venous channel linking the Superior Vena Cava ($SVC$) and Inferior Vena Cava ($IVC$), and connects the portal and systemic venous networks.

• Formation and Course of Azygos Vein:

  • Formed by the union of:

  • Lumbar azygos vein.

  • Right subcostal vein.

  • Right ascending lumbar vein.

  • Enters the thorax by passing through the aortic hiatus of the diaphragm.

  • Ascends along the posterior mediastinum to the level of the $4\,\text{th}$ thoracic vertebra ($T4$).

  • Arches forward over the root of the right lung.

  • Ends by entering the posterior aspect of the Superior Vena Cava ($SVC$) right before the $SVC$ pierces the pericardium.

• Relations of the Azygos Vein:

  • Anterior: Esophagus.

  • Posterior: Lower $8$ thoracic vertebrae, right posterior intercostal arteries.

  • To the Right: Right lung, right pleura, greater splanchnic nerve.

  • To the Left:

  • Lower part: Thoracic duct and descending thoracic aorta.

  • Upper part: Esophagus, trachea, and vagus nerve.

• Tributaries of the Azygos Vein:

  • Right superior intercostal vein (formed by union of $2\,\text{nd}$, $3\,\text{rd}$, and $4\,\text{th}$ posterior intercostal veins).

  • $5\,\text{th}\text{ to }11\,\text{th}$ right posterior intercostal veins.

  • Hemiazygos vein (at the level of the lower border of the $T8$ vertebra).

  • Accessory hemiazygos vein (at the level of the upper border of the $T8$ vertebra).

  • Right bronchial vein (near the terminal entry of the azygos vein).

  • Esophageal, mediastinal, and pericardial veins.

• Hemiazygos Vein (Inferior Hemiazygos Vein):

  • Mirror image of the lower part of the azygos vein.

  • Formation: Union of the left lumbar azygos, left ascending lumbar, and left subcostal veins.

  • Course:

  • Pierces the left crus of the diaphragm.

  • Ascends along the left side of the vertebral column behind the aorta.

  • At the level of the $T8$ vertebra, turns right and passes behind the esophagus and thoracic duct to join the azygos vein.

  • Tributaries: $9\,\text{th}\text{ to }11\,\text{th}$ left posterior intercostal veins and esophageal veins.

• Accessory Hemiazygos Vein (Superior Hemiazygos Vein):

  • Mirror image of the upper part of the azygos vein.

  • Course:

  • Begins at the medial end of the $4\,\text{th}$ or $5\,\text{th}$ intercostal space.

  • Descends along the left side of the vertebral column.

  • At the level of the $T8$ vertebra, turns right and passes behind the aorta and thoracic duct to join the azygos vein.

  • Tributaries: $5\,\text{th}\text{ to }8\,\text{th}$ left posterior intercostal veins, and occasionally the left bronchial veins.