Thoracic cavity

The space between the ribs, sternum and vertebrae radially; the thoracic inlet superiorly; and diaphragm inferiorly.

Key relationships

• Tracheal bifurcation at T7
• Thoracic inlet
  • Connects the root of the neck with the thorax
  • Boundaries
    • Posteriorly - T1 vertebral body and costovertebral joints
    • Laterally - first ribs and costal cartilages
    • Anteriorly - superior border of manubrium
  • Contents
    • Viscera
      • Thymus
      • Trachea
      • Oesophagus
      • Lung apices
    • Vessels, nerves and lymphatics
      • CCAs
      • Subclavian arteries
      • Confluences of IJV and subclavian veins
      • Phrenic nerves
      • Vagus nerves and RLNs
      • Thoracic duct
      • Sympathetic trunk
    • Prevertebral fascia
    • Muscles
      • Sternohyoid
      • Sternothyroid

Mediastinum

• Anatomical divisions (no specific boundaries)
  • Anterosuperior
    • Thymus gland
      • Atrophies to a variable degree after puberty
      • Contained in a strong fibrous capsule
      • Lies in front of brachiocephalic vein and the great vessels in root of neck; and behind the upper sternum and attachment of sternohyoid and sternothyroid strap muscles
    • Brachiocephalic veins
    • Adipose tissue
  • Middle
    • Heart, pericardium, great vessels, phrenic, vagus and RLN nerves
  • Posterior
    • Oesophagus and descending vessels

Heart

• Pericardium
  • Outer fibrous pericardium - 1-2mm thick
  • Visceral pericardium - monolayer of mesothelial cells, continuous with epicardium
  • Pericardial space usually contains 50mL of serous fluid
  • Functions to maintain the cardiac structures in a fixed position, barrier to infection, and lubrication between the beating heart and surrounding tissue
  • Once the outer pericardium has stretched to its maximum size, intra-pericardial pressure rises abruptly

Pleura

Parietal pleura

• • The internal lining of the chest wall
  • Covers the chest wall, mediastinum, diaphragm and pericardium
  • Blood supply from systemic arteries and veins (posterior intercostal, internal mammary, anterior mediastinal and superior phrenic arteries/veins)
  • Rich nerve endings from the intercostal nerves

Pleural space

• • Potential space
  • Right and left pleural spaces are separated by the mediastinum

Visceral pleura

• • Covers the lung
  • Separates the lobes from one another
  • Blood supply is both systemic and pulmonary
  • Lymphatic drainage into regional nodes - follows superficial lung lymphatics into the mediastinal nodes
  • Innervated by vagal branches and the sympathetic system

Lungs

• Components
  • Air is 80% of lung volume, blood 10% and solid tissue 10%
• Anatomical segments
  • Divided into five lobes, and multiple segments within each lobe
  • A segment is described as 'that portion of the lung substance that represents the total branching of a major segmental subdivision of a lobar bronchus'
  • 10 segments on the right; 8 on the left
  • Right lung - larger of the two - upper, middle and lower lobes.
  • Left lung - upper lobe and lower lobe. Lingula is part of the right ML embryologically. Single oblique fissure.
  • Variations in fissures occur - often as failure to fully develop. >50% have incomplete horizontal fissures on right. Accessory fissures can also occur.
• Hilum
  • Bronchus most posterior structure, artery middle, vein most anterior

Right lung

• • Right bronchial tree
    • Right main bronchus (1.2cm to point of RUL branch). Arises at less of an angle than the left, meaning that aspirated material typically passes to the right.
    • RUL bronchus branches from lateral aspect and gives off three segmental bronchi - apical, posterior, anterior
    • Bronchus intermedius - between RUL branch and RML branch - 2cm long. Crossed by mainstem pulmonary artery branch anteriorly - hence names 'eparterial bronchus'.
    • RML bronchus - arises from anterior surface bronchus intermedius. Bifurcates into lateral and medial branches after 1.2-2.2cm.
    • RLL superior segmental bronchus then branches off posteriorly
    • Basal stem bronchus sends off branches to medial, anterior, lateral and posterior basal segments. Each of these bronchi then typically divide into two major subdivisions.
  • RUL
    • Right PA anterior and inferior to RMB, and posterior and superior to right PV. Right PA gives off truncus anterior superolaterally which divides into two branches: superior and inferior. Superior to apical and posterior segments, inferior to anterior segment, but may also give off a branch to the apical segment.
    • 9/10 people also have branches to RUL from ascending vessels from the interlobar part of the pulmonary artery, which crosses over the bronchus intermedius - this branch is called the posterior ascending artery and generally supplies the posterior RUL segment.
    • Superior PV lies anterior and inferior to PA, and is usually made of four branches. The top three branches (apical anterior, anteroinferior, and posterior) drain RUL.
  • RML
    • RML branch of PA arises anteromedially from the interlobar part of right main PA, at about the same level as the posterior ascending branch. Usually at the junction of the horizontal and oblique fissures.
    • Venous drainage via the fourth and inferior-most trunk of the superior pulmonary vein, generally made up of two branches.
  • RLL
    • Inferior PV is inferior and posterior to the superior vein, and is made up of two major trunks (superior segmental and common basal)
  • Oblique (major) fissure
    • Separates LL from UL and ML
    • Begins posteriorly at the level of the 5th rib or ICS, runs downward and forward approximating the course of the 6th rib, and ends at the diaphragm at level of 6th costochondral junction
  • Horizontal (minor) fissure
    • Separates UL from ML
    • Begins in the oblique fissure at the region of the mid-axillary line at level of 6th rib, and runs anteriorly to the costochondral junction at the 4th rib

Left lung

• • Left bronchial tree
    • Left main bronchus: longer than the right (4-6cm), sharper angle
    • LUL bronchus - arises anterolaterally - divides into superior and inferior (lingular) branches. 1-1.5cm long.
    • LLL stem bronchus. Branches into superior segmental bronchus (posterior), then bifurcates into anteromedial basal segmental bronchus and a common stem bronchus for the lateral basal and posterior basal bronchi.
  • LUL
    • 2-7 branches to LUL from left PA, but median of 4
      • Usually first branch arises from the anterior part of the artery to supply the anterior segment
      • Second branch from the main artery as it passes over the LUL bronchus and into the interlobar fissure in 80% of cases
    • Superior pulmonary vein applied closely to the anteroinferior aspect of the PA; often obscuring the anterior branches of the artery. 3-4 tributaries draining the LUL.

• LLL
  • PA passes into interlobar fissure, giving off a branch to the superior segment of LLL. Lingular artery usually originates from the interlobar PA distal to this artery and is the sole artery to lingula in 80% of patients.
  • PA then becomes the common basal trunk, which divides into anterior and posterior branches.
  • Inferior PV is inferior and posterior to superior PV and is made up of two tributaries.

• Oblique (major) fissure
  • Begins between 3rd and 5th ribs and runs downward and forward to end at the region of the 6th or 7th costochondral junction

• Blood supply
  • Unoxygenated blood from RV, through the pulmonary artery to each lung
  • PA divides into right and left main pulmonary arteries anterior to the left main bronchus. These two vessels lie on a line parallel and slightly superior to the pulmonary veins
  • PA branching pattern is very variable, although arteries tend to lie closely adjacent to segmental bronchi
  • Pulmonary venous pattern is even more variable than arterial. Usually two main trunks from each lung - superior and inferior pulmonary veins
  • Blood supply to bronchi from the systemic circulation (bronchial arteries from the superior thoracic aorta between T3 and T8 or the aortic arch)
    • Bronchial vessels form a communicating around each main bronchus, entering at the hilum
    • Closely applied to the bronchial wall, and generally have an anterior and posterior division
• Lymphatics
  • Intra-pulmonary
    • Present throughout the parenchyma, gradually coalescing toward the hilar areas
    • Mostly located just beneath the visceral pleura
    • Only infrequently seen
    • More common in older patients - appear to be environmentally stimulated
    • Can be mistaken for metastatic disease - on CT have sharp borders, ovoid shapes, subpleural location, with high homogenous density
  • Broncho-pulmonary
    • Lobar (level 11)
      • Found at the angles formed by the origins of the various lobar bronchi, in close association with the bronchus or adjacent pulmonary vessels
      • Common locations in right lung: right bronchial sump (area between RUL bronchus and RML bronchus, adjacent to bronchus intermedius - superior interlobar lymph node of Rouviere) and the region just below RML bronchus (inferior interlobar lymph node of Rouviere).
      • Common locations in left lung: left lymphatic sump (angle of LUL and LLL bronchi - left interlobar node of Rouviere)
        • Mostly found in the main fissure
        • Constant node near origin of lingular branch

• Hilar (level 10)
  • Found alongside the lower portions of the main bronchi or the respective pulmonary artery/vein, lying within the visceral pleural reflections
  • Contiguous with lobar nodes distally, and mediastinal nodes proximally (fuzzy boundaries)
  • Right:
    • Termed hilar nodes superior right mainstem bronchus up to the level of azygos
    • Become subcarinal nodes once outside the visceral pleural sheath
  • Left:
    • Become mediastinal nodes proximal to a line connecting the lateral surfaces of the ascending and descending aorta
    • Mostly found medial to LMB
• Variable number in each location, tending to reduce in number with age
• Greatly increased number seen with infection or malignancy

• Mediastinal
  • Generally stays in ipsilateral nodes, but lymph can flow from LLL to right mediastinal (paratracheal) nodes
  • Lymphatic drainage within mediastinum moves cephalad
  • Normal size 8-10mm paratracheal; 10-12mm subcarinal
  • 2: upper paratracheal
  • 4: lower paratracheal
  • 5: subaortic
  • 6: para-aortic
  • 7: subcarinal
    • Lie in the angle of tracheal bifurcation
    • Some lie within the paratracheal fascial envelope
    • Contiguous with hilar nodes
  • 8: paraoesophageal
  • 9: pulmonary ligament
    • Usually 2-3 small nodes
    • There is a relatively constant node in close proximity to the inferior border of the inferior pulmonary vein, and is often termed the sentinel node of the pulmonary ligament

• Nerve supply
  • None

Other arteries

Internal thoracic

• • Arises from subclavian artery and passes downwards with accompanying veins, about 1-2cm lateral to the sternal edge
  • Separated from the pleura through most of its course by transversus thoracis
  • Divides at 6th intercostal space to superior epigastric artery and musculophrenic artery
  • Gives off mediastinal branches, perforating branches (seen in mastectomy), and two anterior intercostal branches for each intercostal space (which anastomose with posterior branches)

Other veins

• Brachiocephalic veins
  • See separate topic under 'vascular system'
• Azygos system
  • See separate topic under 'vascular system'

Nerves:

Vagus nerves

• • Right: in contact with trachea, passing down behind lung root, forming a plexus on oesophagus
  • Left: held away from trachea by great arteries. Crosses aortic arch (and gives off recurrent laryngeal branch) medial to left superior intercostal vein, passes down behind lung root, then onto oesophagus.

Thoracic sympathetic trunk and splanchnic nerves

• • See separate topic under 'autonomic nervous system'

Phrenic nerves

• • See 'diaphragm'

Lymphatics

Thoracic duct:

• • The main lymphatic channel for the return of chyle to the venous system. It drains all lymph of the body except the right arm, and right halves of thorax, head and neck.
  • Commences at upper end of the cisterna chyli (T12), between aorta and azygos vein. Enters through diaphragm between these structures. Lies against right side of oesophagus while passing upwards, then passes behind it and to left at T5 (about aortic arch). Moves into superior mediastinum, posterior to aortic arch and carotids. Then arches forwards, crossing over dome of pleura and left subclavian artery, to enter the point of confluence of the left internal jugular and subclavian veins.
  • Has several valves.
  • Relations
    • Anterior - oesophagus, left atrium, carina
    • Posterior - vertebral column, terminal hemiazygos, accessory hemiazygos, right posterior intercostal arteries
    • Left lateral - descending aorta, left lung, pleura
    • Right lateral - azygos, right lung, pleura
  • Variant anatomy
    • Variable in 40%
    • Double thoracic ducts in 20%
    • Aberrant termination - left or right IJV, azygos, brachiocephalic vein, or left subclavian vein
    • Multiple terminal channels from single duct with typical course