9 - Lungs and Pleura (Exam 2)

The Thoracic cavity

• The thoracic inlet is bordered by the manubrium, R1 and its costal cartilage, and T1 posteriorly
• The thoracic outlet is inferior and surrounded by T12, the costal margin, and the xiphisternal joint
• The muscular diaphragm closes the thoracic outlet
• The sternum, 12 pairs of ribs and the vertebral column, diaphragm and the suprapleural membrane, form the thoracic cavity
• The suprapleural membrane is a layer of fascia arching over each lung, extending up into the thoracic inlet
• The mediastinum in the thoracic cavity separates two pleural cavities

The Pleural Cavities

• The pleural cavities surround each lung and are lined by a serous membrane (pleura)
• The cavity itself remains empty
• The lung bud grows into a serous sac and becomes completely enveloped except at the point where the lung is anchored to the mediastinum
• The pleura on the lung surface is the visceral pleura
• The pleura on the body wall is called the parietal pleura

The Pleura

• The parietal pleura lines the thoracic wall inside, separated from the musculature by the endothoracic fascia
• The parietal pleura has specialized divisions based on the thoracic cavity portion it lines
• Divisions of the parietal pleura:
  • lines the back side of the thoracic rib cage - costal pleura
  • lines the mediastinal mass - mediastinal pleura
  • lines the diaphragm - diaphragmatic pleura
  • covers the apex of the lungs within the thoracic inlet, lining the inside of the reinforcing suprapleural membrane - cervical pleura
• The parietal pleura reflects off the mediastinum onto the lung surface as visceral pleura, at the root of the lung
• The parietal pleura sags below the root (like the cuff on a shirt) forming an unfilled potential space: the pulmonary ligament, a double pleura layer
• The visceral pleura intimately attaches to the lung surface and cannot be lifted to demonstrate
• It dives deep into the lung fissures before reflecting upon onto the adjacent lobe
• The intercostal nerves innervate the costal pleura of the intercostal space
• The diaphragmatic pleura is innervated peripherally by the intercostal nerves and centrally by the phrenic nerve sensory component
• These membranes are highly sensitive to pain
• The visceral pleura is not innervated and is insensitive to pain

Pleural Recesses

• Areas where the parietal pleura reflects off one thoracic cavity aspect onto another, forming potential spaces or recesses
• These recesses are never completely filled by the lung
• The mediastinal pleura reflects off the mediastinum onto the costal wall, forming a vertical strap-like recess on either side of the sternum: the costomediastinal recess
• The diaphragmatic pleura reflects off the diaphragm onto the costal wall, forming a semicircular recess, communicating anteriorly with the costomediastinal recess and running posteriorly to meet the vertebral column: the costodiaphragmatic recess

The Suprapleural Membranes

• The entire thorax is lined by endothoracic fascia, which adheres the parietal pleura to the body wall, mediastinum, and diaphragm aspects
• The thoracic inlet (operculum) sits at the neck root and is bounded by the first ribs, vertebral column, and manubrium
• The endothoracic fascia extends across these openings and thickens into the suprapleural membranes
• These membranes cover the superior (apical) portion of each lung

Holey Ghost of the Thorax

• During expiration, the tracheobronchial tree's elastic tension pulls the lung and its adherent visceral pleura away from the parietal pleural-lined thoracic cavity walls
• This creates a “negative pressure" within the pleural sac
• The pleural sac contains only a few milliliters of serous fluid and is a potential space
• The negative pressure within this potential space (between the two pleural layers) is 5 cm H20 lower than atmospheric pressure
• However, atmospheric pressure within the alveoli presses the lung and its adherent visceral pleura against the parietal pleura, with fluid-filled potential pleural cavity in between
• If either the visceral or parietal pleura is punctured, the elastic recoil of the tracheobronchial trees sucks air into the pleural cavity (sac)
• The two pleural surfaces separate and the lung “collapses” causing a pneumothorax
• Efforts to re-establish the workability of the lung during respiration involve redevelopment of the negative pressure
• The tear/puncture must be closed
• Once this is done a tube is left exiting the pleural cavity with the collapsed lung
• The tube passes into a bottle of water placed lower than the lung so that expired air from the pleural sac cannot re-enter nor can fluid flow into the lung
• Eventually, the atmospheric air is expelled from the lung, the two pleural surfaces again re-contact one another and negative pressure is re-established
• The tube is quickly withdrawn and the opening closed

Root (Hilum) of the Lungs

• Each root is surrounded by the parietal pleura reflection onto the lung surface as visceral pleura
• This pleural reflection extends inferiorly into the pulmonary ligament
• The cut surface of each root has pulmonary vv., pulmonary aa., bronchi, and bronchopulmonary lymph nodes
• The bronchial aa. supply oxygen-laden blood to the lung tissue, as branches directly off the aorta
• The bronchial vv. drain carbon dioxide-laden blood from the lung tissue into the azygous system
• The pulmonary aa lie above the bronchi except for the secondary bronchus to the right upper lobe
• The pulmonary vv lie below the bronchi
• Pulmonary aa follow the bronchi all the way to their termination level as alveoli
• Intersegmental pulmonary vv drain the alveoli and diverge from the bronchi
• These veins extend through the intersegmental connective tissue septa
• Eventually these coalesce and rejoin the tracheo-bronchial tree at the level of the tertiary bronchi
• The veins are often used as surgical guidelines for resection of a bronchopulmonary segment

Bronchi

• The trachea bifurcates at the sternal angle (T4/5) into two main bronchi
• The right main bronchus is short and diverges little from the trachea, so most inhaled objects end up in this bronchus
• The left main bronchus is longer than the right and more horizontal, leaving the trachea at a distinct angle
• Each main bronchis divides into secondary bronchi (supplying a lobe of the lung) within the lung substance
• In turn these divide into tertiary bronchi that supply an individual portion of each lobe, known as a bronchopulmonary segment (BPS)
• A BPS is the smallest resectable lung portion and is supplied by a tertiary bronchus
• Each lung contains 10 BPSs (the left lung utilizes an upper and lower division with 4 quaternary bronchi to get 10)
• Their location is important with reference to positioning the body for draining regions of the lung by percussion (postural drainage)
• Within each BPS the bronchi will eventually get narrower until they form bronchioles that continue taking in inhaled/exhaled air to the alveoli
• Clinically (radiologically) - and in the lab - the bronchial tree pattern is more important as they are demonstrable on x-ray
• A knowledge of the actual BPS position and extent is surgically important

Questions about the anatomy of the thorax and pleura, including boundaries of the thoracic inlet, pleural membranes, relationships between the lung and pleura, and structures surrounding the root of the lungs. Includes surgical scenarios involving incisions and tumor locations.