Lower Respiratory Tract Anatomy

Lower Respiratory Tract and Thoracic Wall Anatomy

• The respiratory bud originates from the foregut
• Gas exchange begins during the canalicular phase of lung development

Trachea

• Located at the lower border of the cricoid cartilage of the larynx, spanning C6-T4/5
• Carina is located inside of the bifurcation
• C-shaped hyaline cartilage rings are positioned anteriorly
• Trachealis smooth muscle is located posteriorly, innervated by CN X, and just anterior to the oesophagus
• Blood supply is provided by the inferior thyroid arteries
• Divides into the Right Main Bronchus (RMB) and Left Main Bronchus (LMB)

Bronchial Tree

• RMB branches into 3: the right upper, middle, and lower lobe bronchus
• LMB branches into 2: the left upper and lower lobe bronchus
• Bronchopulmonary segment is a pyramid-shaped segment supplied by a tertiary bronchus
• Supplied by one branch of the pulmonary artery
• Drained by multiple pulmonary veins
• The left upper lobe features a tongue-like projection, known as the lingula with two segments

Pleura

• Visceral pleura lacks sensitivity
• Parietal pleura is innervated by intercostal nerves and the phrenic nerve (diaphragmatic pleura)
• Continuous at the hilum of the lung, forming the pleural reflection
• Mediastinal pleura connects to the heart and trachea via the inferior pulmonary ligament
• The cupola represents the apex of the pleura, above rib 1
• The pleural cavity exists between the visceral and parietal layers
  • Pathological fluid accumulation in this potential space is known as pleural effusion
  • Types of pleural effusion include hydrothorax, haemothorax, chylothorax, and pneumothorax
• Costo-diaphragmatic recess is present

Lungs - Medial View

• The right lung features both oblique and horizontal fissures, while the left lung only has an oblique fissure
• Relationship of pulmonary arteries to the bronchi follows the pattern RALS (right anterior, left posterior)

Lungs - Blood Supply

• Dual blood supply
  • Pulmonary arteries carry deoxygenated blood for gas exchange
  • Bronchial arteries supply oxygenated blood to lung tissues
    • One bronchial artery is on the right
    • Originates from the thoracic aorta, superior bronchial artery on the left side, or the right third posterior intercostal artery
    • Two bronchial arteries are on the left (superior and inferior)
      • Arise from the thoracic aorta

Chest Wall - Sternoclavicular Joint

• The sternoclavicular joint is a saddle synovial joint with limited mobility (elevation and depression of the clavicle)
• Components include:
  • Articular disc
  • Interclavicular ligament
  • Costoclavicular ligament (between the clavicle and rib I), has anterior and posterior parts
  • Anterior and posterior sternoclavicular ligaments

Clavicle

• The sternal end is bulbous, while the acromial end is flatter
• The superior surface is smoother, while the inferior surface is bumpy
  • Conoid tubercle and trapezoid line near the acromial end
  • Impression for costoclavicular ligament near the sternal end
• Medial 2/3 is convex anteriorly, lateral 1/3 concave anteriorly

Ribs

• True ribs (1-7): directly attach to the sternum
• False ribs (8-12)
  • Ribs 8-10 join the costal margin
  • Ribs 11-12 are floating ribs, lacking anterior attachment
• Inspiration Movements
  • ribs 1-6 exhibit a pump handle motion (anteroposterior expansion)
  • ribs 7-10 exhibit a bucket handle motion (more lateral expansion)

First Rib

• Has a scalene tubercle for the anterior scalene muscle
• Groove for the subclavian vein runs anterior to the scalene tubercle
• Groove for the subclavian artery runs posterior to the scalene tubercle

Thoracic Vertebrae

• The neural arch is composed of 2 pedicles and 2 laminae
• Features a facet for the head of the rib on the body
• Features a facet for the tubercle of the rib on the transverse process

Atypical Thoracic Vertebrae

• Atypical thoracic vertebrae exhibit a superior demifacet, inferior demifacet, and a facet on the transverse process
• T1 has a complete superior facet and an inferior demifacet
• T9 has only a superior demifacet
• T10 has a complete circular facet
• T11 has a complete circular facet but lacks a facet on the transverse process
• T12 has a complete circular facet that encroaches on the pedicle, without a facet on the transverse process

Costotransverse and Costovertebral Joints

• Both are synovial plane joints
• Costotransverse Joint
  • Has 3 Ligaments
    • Costotransverse Ligament: Connects neck of rib to transverse process
    • Lateral Costotransverse Ligament: Connects non-articular part of rib to tip of transverse process
    • Superior Costotransverse Ligament - Connects neck of rib to transverse process above
• Costovertebral Joint
  • Has a radiate ligament

Intercostal Muscles

• Three layers of muscles
  • External intercostals
    • Begin posteriorly, run anteriorly, becoming a membrane at costal cartilages
    • "Hands in pockets"
  • Internal intercostals
    • Begin anteriorly, run posteriorly, becoming a membrane at the angle of the rib
  • Innermost layer consists of 3 muscles
    • Innermost intercostals
    • Transversus thoracis anteriorly
      • Attaches to the sternum like a starfish
    • Subcostalis posteriorly
      • Skips ribs
• The neurovascular bundle runs between internal and innermost layers

Chest Wall - Blood Supply and Drainage

• Primarily supplied by posterior intercostal arteries
  • Arise from the descending aorta, except for the first 1 or 2 (from the superior intercostal artery, a branch of the subclavian artery)
• Anastomose with anterior intercostal arteries
• Which come from the internal thoracic arteries
• Drainage occurs in the opposite direction of the arteries
  • Mainly into the azygos vein on the right, or hemiazygos/accessory hemiazygos vein on the left
  • The 1st intercostal vein drains into the brachiocephalic vein
  • The 2nd and 3rd intercostal veins unite and drain into the azygos on the right and brachiocephalic on the left

Diaphragm

• Dome-shaped
• The central tendon has the caval opening for the inferior vena cava
• The right crus originates from L2-4, and the left crus originates from L1-3
• The median arcuate ligament lies between the right and left crura
  • Forms the aortic hiatus for the aorta, azygos vein, and thoracic duct
• The medial arcuate ligament lies posterior to the psoas major
• The lateral arcuate ligament lies posterior to the quadratus lumborum
• Innervated by the phrenic nerve (C3,4,5)
  • Irritation can cause referred pain to the shoulder tip due to the dermatome of C4

Muscles of Respiration