Pleural Cavity and Pleural Layers

Pleural Cavity

• Bilateral regions of the thoracic cavity that surround the lungs
• Considered the region between the lung's external surface and the thoracic wall's internal surface
• It's primarily a potential space that contains a small amount of serous fluid, which limits friction

Boundaries and Formation

• Bordered by the pleural sac, a double layer of serous pleural membrane that is continuous at the lung's hilum
• Forms during the embryonic period as the lungs grow into pericardioperitoneal canals
• This is similar to a fist pushing into a balloon
• As the lung grows into the cavity, it pushes in on the serous membrane
• Excess fluid is resorbed as the lung expands

Pleural Layers

• The pleural sac contains two layers
• The visceral pleura adheres to the lung's surface as a serous membrane
• Parietal pleura is the internal covering of the pleural cavity
• Anchored by endothoracic fascia like "wallpaper glue"
• The parietal pleura is divided into costal, cervical, mediastinal, and diaphragmatic portions

Cervical Portion

• The cervical portion extends superior to the clavicle
• It goes posterior to the sternocleidomastoid muscle
• It is susceptible to injury, such as breaks and subclavian lines

Pleural Recesses

• During expiration, the diaphragm moves superiorly
• Portions of the thoracic cavity fold upon themselves
• The costomediastinal recess is located between the sternum and pericardium
• Costodiaphragmatic recess is in the posteroinferior region
• Costodiaphragmatic recess is where the costal parietal pleura contacts the diaphragmatic parietal pleura
• The costodiaphragmatic recess is a common site for sampling pleural fluid, due to minimal risk of puncturing the lung
• Pleural effusions can accumulate in this recess during disease; visible via medical imaging

Pleural Sac and Lung Ventilation

• The pleural sac generates the hydrostatic pressure for lung expansion and compression during ventilation
• Contraction of the diaphragm and external intercostals expands the pleural cavity
• Hydrostatic tension results in a pull on the visceral pleura by the parietal pleura
• Elastic recoil of the lung occurs when the diaphragm relaxes
• Hydrostatic tension results in a pull on the parietal pleura by the visceral pleura
• This is assisted by passive expansion of abdominal contents after compression
• Allows surfaces to smoothly slide along each other

Lung Collapse

• Hydrostatic pressure maintains lung distention and the pressure in the pleural space is lower than atmospheric pressure
• Air is drawn into the pleural space, causing the lung to recoil, in the event of parietal pleura torn from a stab wound or broken rib
• Results in pneumothorax such as from a sucking chest wound, or a collapsed lung
• Occurs with tearing of the visceral pleura from rupture of pulmonary blebs
• The lung will collapse from hemothorax due to blood accumulation from parietal tears or tearing of thoracic vessels
• Lungs are reinflated through surgical placement of a chest tube in the 5th/6th intercostal space

Pleurisy

• Also known as pleuritis
• Inflammation of the visceral pleura surrounding the lungs, typically bacterial in origin
• Can lead to a pleural friction rub, which can be identified during auscultation as a "grating sound" or "stepping on fresh snow"

Pleural Effusions

• Increased serous fluid accumulates in the pleural space
• Seepage of excess interstitial fluid from lung lobules causes this
• Greater fluid in the interstitium surrounding the alveolar sacs ultimately flows into the pleural cavity
• Common in pneumonia, COPD, heart failure, and other conditions
• Medical imaging can identify fluid accumulation in the costodiaphragmatic recess
• Empyema and lung abscesses develop from infection of effusions

Lungs

• These facilitate gas exchange at the air-blood interface
• Highly pliable, elastic organs with natural recoil to retract the pleural cavity during passive ventilation
• During embalming, the lungs become rigid and leave impressions on the medial surface
• Ridges form between alternating ribs and intercostal spaces
• The right lung has grooves for the vena cava and esophagus
• The left lung has a cardiac notch for the heart and a groove for the arch of the aorta
• Concave mediastinal and diaphragmatic surfaces; convex costal surface

Hilum

• The hilum is the region of pleural reflection that encapsulates the root of the lung (transition from visceral to parietal)
• The passage point for numerous structures into or out of the lung

Pulmonary Artery

• The most superior portion of the root
• Carries deoxygenated blood to the lung for gas exchange

Pulmonary Vein

• Located anteroinferior in the root
• Paired vessels carry reoxygenated blood back to the heart for circulation throughout the body

Primary Bronchus

• Lies posteroinferior in the root
• It is the point where the trachea bifurcates, which allows atmospheric air into the lungs for gas exchange
• Right bronchus is wider, shorter, and more vertical
• Aspirated objects tend to lodge in the right main bronchus due to this structural difference
• Splits almost immediately into secondary bronchi

Bronchiole Artery/Vein

• Small vessels on the surface of the bronchus
• Arteries branch from the thoracic aorta to deliver oxygen-rich blood to the lungs for its own gas exchange
• Veins drain into the azygous/hemiazygous system for return to the heart

Divisions of the Lung

• Lungs are divided into lobes that are separated by fissures
• The right lung has 3 lobes: superior (separated from the middle by the horizontal fissure), and inferior (separated from both, superior and middle, by the right oblique fissure)
• The left lung has 2 lobes: the smaller size relates to the asymmetric positioning of heart to the left of the body
• The superior lobe separated from the inferior lobe by the left oblique fissure
• Each lobe is supplied by secondary (lobar) bronchi and vessels
• Lungs are further subdivided into bronchopulmonary segments

Segments and Lobules

• The right lung contains 10 segments: 3 in the superior lobe, 2 in the middle lobe, and 5 in the inferior lobe
• The left lung contains 8 segments: 4 in the superior lobe and 4 in the inferior lobe
• Each bronchopulmonary segment gives rise to 20 to 25 terminal bronchioles
• Terminal bronchioles are surrounded by fibrous septa, which forms a bag-like lobule
• Terminal bronchioles branch into 20-30 respiratory bronchioles within lobules
• Each respiratory bronchiole supplies one acinus (alveolar sac) and is surrounded by a small volume of interstitial fluid
• Pulmonary arteriole branches follow terminal bronchioles, branching into a capillary network around alveoli
• Capillary networks collect into pulmonary venules that enter interlobular septa, then pass back towards the hilum
• Adults have around 300 million alveoli for gas exchange

Lower Respiratory Tract Infections

• This is a general term for an infection below the larynx

Bronchitis

• This is inflammation of the upper divisions of the bronchial tree
• Defined by a productive cough (mucus secretions)
• Fever may not be consistent

Acute Bronchitis

• Common, affecting > 10 million/year in the US, or more than 5% of the population each year
• Is usually viral, >90%
• Resolves in 3 weeks

Chronic Bronchitis

• Symptoms that are lasting longer than 3 months for at least 2 years

Pneumonia

• Inflammation of the alveolar sacs, frequently caused by bacterial or viral infection
• Usually develops from microaspiration of an infectious agent during sleep
• Innate immune responses and microvilli typically clear this microaspiration

Pneumonia Predisposition

• Age less than 2 or over 65
• Smoking
• Alcoholism
• Immunocompromised
• Aspiration volume
• Virulence of the pathogen
• Presents with coughing, fever, chills, labored breathing, and stabbing chest pain
• Treated with antibiotics and corticosteroids
• Inflammation increases capillary permeability and leads to pleural effusions

Lymphatics

• The lung contains two major pulmonary lymph plexuses
• The superficial plexus drains superficial lung structures into bronchopulmonary lymph nodes
• The deep plexus drains internal structures, branching from the 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 the right and left subclavian veins

Bronchogenic Carcinoma

• Via the right lymphatic duct or thoracic duct, metastasis may occur to the tracheobronchial nodes either directly or indirectly
• Bronchogenic carcinoma can metastasize to tracheobronchial nodes, causing the terminal of the bronchus to deviate and is observable with a bronchoscope

Nerves of the Lung

• Derived from the pulmonary plexus
• Run mainly posterior within root of lung
• Post-synaptic sympathetic fibers from the sympathetic trunk
• Presynaptic parasympathetic fibers from the vagus nerve
• Visceral afferents for pain detection

Surface Anatomy

• Boundaries of both the lungs and pleural cavities can be predicted relative to the surface anatomy through rib segments
• Pleural boundaries extend beyond lung boundaries, creating pleural recesses

Rule of 2's

• The anterior border of the pleura and lung projects to the sternum at rib 2
• Cardiac notch occurs between rib segments 4-6 anteriorly
• The inferior-most extent of the lung is rib 6 anterior, rib 8 laterally, and rib 10 posterior
• The inferior-most extent of the pleural cavity is rib 8 anterior, rib 10 laterally, and rib 12 posterior
• There are 2 vertebral levels between the lung and pleural cavity at each location

Thoracentesis

• Insertion of a hypodermic needle through the intercostal space
• Used to obtain a sample, drain fluid, or drain air
• The fluid will drain posteriorly into the costodiaphragmatic recess in a sitting position
• Typically, the midscapular line between ribs 10 and 12 is the goal

Chest Tube Placement

• Used to remove large amount of fluid or air
• For example, this is relevant in severe, unresolved hemo- or pneumothorax
• The goal is midaxillary line between 8 and 10
• Aim slightly higher to avoid liver or spleen with initial incision
• To drain air (pneumothorax), the tube should be directed up, and to drain fluid (hemothorax), direct down

Percussions

• Tapping on fingers pressed firmly on various regions of the thorax
• Resonating is an indication of air
• Dull is an indication of fluid
• Flat is an indication of air