Pleural Cavity and Layers of Pleura

Pleural Cavity

• Bilateral regions of the thoracic cavity surround the lungs bilaterally
• It is considered the region between the external surface of the lung and the internal surface of the thoracic wall
• The pleural cavity is mostly a potential space that contains a small volume of serous fluid to limit friction
• The pleural cavity is bordered by the pleural sac, a double layer of serous pleural membrane continuous at the hilum of the lung
• During the embryonic period, the lungs grow into pericardioperitoneal canals, forming structures that resemble a fist pushing into a balloon
• As the lung grows into the cavity, it pushes in on the serous membrane, and excess fluid in the pleural cavity is reabsorbed as the lung expands
• Ultimately, 2 layers of pleura remain: visceral and parietal pleura

Visceral and Parietal layers of Pleura

• Visceral Pleura is the portion of serous membrane which adheres to the surface of the lung
• Parietal Pleura is the internal covering of the pleural cavity
• It is anchored to the wall with endothoracic fascia
• Parietal Pleura is divided into costal, cervical, mediastinal, and diaphragmatic portions, according to respective locations
• The Cervical portion extends superior to the clavicle, posterior to the sternocleidomastoid muscle and is susceptible to injury, breaks, and subclavian lines

Pleural Recesses

• During expiration, the diaphragm recedes superiorly and portions of the thoracic cavity fold upon themselves Costomediastinal Recess
• Located between the sternum and pericardium Costodiaphragmatic Recess
• Found in the posteroinferior region, where the costal parietal pleura is in contact with the diaphragmatic parietal pleura
• It is a common site for sampling pleural fluid, done with minimal risk of puncturing the lung
• In disease, effusions may accumulate in the costodiaphragmatic recess and be visible on medical imaging

Pleural Sac

• It generates hydrostatic pressure necessary for expansion/compression of lungs during ventilation
• The diaphragm and external intercostals contract to expand pleural cavity
• Hydrostatic tension results in a pull on the visceral pleura by the parietal pleura
• When the diaphragm relaxes, elastic recoil of the lung

Lung Collapse

• Hydrostatic pressure maintains distention in the lungs, pressure in pleural space is lower than atmospheric pressure
• When the parietal pleura tears after a stab wound, the lung recoils as air is sucked into the pleural space, resulting in pneumothorax
• A lung collapse can occur with tearing of visceral pleura from a rupture of pulmonary blebs
• If blood accumulates in the pleural space after a parietal tear, the lung collapses due to hemothorax
• Lungs are reinflated through the surgical placement of a chest tube in the 5th/6th intercostal space, close to the nipple level

Pleurisy and Pleural Effusions

Pleurisy

• Also known as pleuritis, defined as inflammation of the visceral pleura surrounding the lungs
• It's typically bacterial in origin and can result in a pleural friction rub
• Can be identified on auscultation as a grating sound or stepping on fresh snow Pleural Effusions:
• Defined as increased serous fluid accumulation in the pleural space
• Typically results from seepage of excess interstitial fluid from lung lobules
• Greater volume of fluid enters the interstitium surrounding alveolar sacs, ultimately pooling into the pleural cavity
• Common in pneumonia, COPD, heart failure, other conditions
• Medical imaging can identify fluid accumulation in the costodiaphragmatic recess
• Infection of effusions results in empyema; development of lung abscesses

Lungs

• Organs serve as the site of gas exchange via an air-blood interface which are highly pliable and elastic
• Natural recoil is sufficient to retract pleural cavity during passive ventilation

Lung Oddities

• In embalmed cadavers, lungs turn rigid, which leaves distinct impressions on the medial surface
• Ridges form from alternating ribs and intercostal spaces
• Grooves mark the right lung for the vena cava and esophagus
• Grooves mark the left lung for the cardiac notch for the heart and groove for the arch of the aorta
• The lungs have concave mediastinal and diaphragmatic surfaces with a convex costal surface

Lung Hilum

• It is the region of pleural reflection and encapsulates the root of the lung with numerous structures entering and exiting the lung
• The pulmonary artery is the most superior portion of the root and carries deoxygenated blood to the lung for gas exchange
• The pulmonary veins are paired vessels carrying reoxygenated blood back to the heart for circulation throughout the body; they sit anteroinferior in the root
• The primary bronchus lies posteroinferior in the root and allows atmospheric air into lungs for gas exchange

Trachea and Bronchi

• The trachea bifurcates into the right and left primary bronchi
• The right bronchus is wider, shorter, and more vertical, so most aspirated objects that pass into the trachea tend to lodge in the right main bronchus, instead of the left
• The primary Bronchi immediately split into secondary bronchi
• Bronchiole arteries/veins are small vessels lying on the surface of the bronchus
• Arteries branch from the thoracic aorta, delivering oxygen-rich blood to regions of the lung for its own gas exchange
• Veins drain into the azygous/hemiazygous system for return to the heart

Lung Lobes

• Lungs are divided into lobes, separated by fissures
• The right lung has 3 lobes: a superior lobe separated from the middle lobe by the horizontal fissure and an inferior lobe separated from both superior and middle lobes by the right oblique fissure
• The left lung has 2 lobes: a superior lobe separated from the inferior lobe by the left oblique fissure, with the smaller size of the 2 lobes due to asymmetric positioning of heart to the left of the body
• Each lobe is supplied by secondary (lobar) bronchi and vessels
• The lungs are further subdivided into bronchopulmonary segments, with 10 in the right lung and 8 in the left lung
• Each BP segment gives rise to 20-25 terminal bronchioles

Lobules and Alveoli

• Terminal bronchioles are surrounded by fibrous septa, forming bag-like lobules
• Within lobules, terminal bronchioles branch into 20-30 respiratory bronchioles
• Each respiratory bronchiole supplies one acinus (alveolar sac), with each acinus surrounded by a small volume of interstitial fluid
• Capillary networks collect into pulmonary venules that enter interlobular septa and pass back towards the hilum
• In adults, approximately 300 million alveoli are present for gas exchange

Lower Respiratory Infections

• It is a general term for an infection below the larynx Bronchitis
• Inflammation of the upper divisions of the bronchial tree is characterized by a productive cough with mucous secretions
• Fever is not a consistent finding
• Acute bronchitis is a common condition, typically viral in origin that resolves within 3 weeks
• Chronic bronchitis: symptoms last >3 months for at least 2 years

Pneumonia

• Inflammation of the alveolar sacs is most commonly from a bacterial or viral infection
• Typically results from microaspiration of infectious agent
• The condition is normally cleared through microvilli and an innate immune response
• Precipitating factors that may result in pneumonia include age, smoking, alcoholism, immunocompromised status, and the volume of aspirate/virulence of the pathogen
• Pneumonia presents with coughing, fever, chills, labored breathing, and stabbing chest pain; treated with antibiotics and corticosteroids

Lymphatics of the Lung

• Inflammation leads to increased capillary permeability and pleural effusions
• There are 2 major pulmonary lymph plexuses in the lung
• Superficial plexus drains superficial structures in the lung into bronchopulmonary lymph nodes
• Deep plexus drains internal structures branching from root of the lung into intrinsic lymph nodes, which continue to bronchopulmonary lymph nodes
• Lymph drains into superior/inferior tracheobronchial nodes, up into bronchomediastinal trunks and ultimately into right/left subclavian veins either directly or indirectly through the right lymphatic/thoracic duct
• Bronchogenic carcinoma can metastasize to tracheobronchial nodes, resulting in deviation of terminal portion of bronchus which can be observed with a bronchoscope

Nerves of the Lung

• Derived from the pulmonary plexus, it runs mainly posterior within the root of the lung
• They convey post-synaptic sympathetic fibers from the sympathetic trunk, presynaptic parasympathetic fibers from the vagus nerve, and visceral afferents for pain detection

Lung Surface Anatomy

• The lungs and pleural cavities have predictable boundaries related to surface anatomy through rib segments, with pleural boundaries extending past the lung boundaries, creating pleural recesses
• The anterior border of the pleura and lung projects to sternum at rib 2
• Cardiac notch is between rib segments 4 through 6 anteriorly
• The inferior-most extent of the lung is at rib 6 anteriorly, rib 8 laterally, and rib 10 posteriorly
• The inferior-most extent of the pleural cavity is at rib 8 anteriorly, rib 10 laterally, and rib 12 posteriorly
• There are 2 levels between the lung and pleural cavity at each location

Thoracentesis

• Defined as the insertion of a hypodermic needle through intercostal space to obtain a sample or drain fluid or air from the lungs
• When sitting, fluid drains posteriorly into the costodiaphragmatic recess, aiming for the midscapular line between ribs 10 and 12
• Used for removal of larger quantities of fluid and air
• Aim for the midaxillary line between ribs 8 and 10, slightly higher up to avoid liver/spleen with initial incision

Percussions

• Tapping on fingers pressed firmly on various regions of the thorax allows determination of the state of the lungs
• Resonating sounds indicate air
• Dull sounds indicate fluid
• Flat sounds indicate no air or fluid