and Function of Lung



The carina is normally at the level of T4/T5.

Embryology of Airways & Maldevelopment

first 5 weeks GA lung buds grow from ventral aspect of primitive foregut (from caudal end of laryngotracheal groove of primitive pharyngeal floor)
          abnormal: pulmonary agenesis
5th week GA separation of trachea + esophagus
5–16 weeks formation of tracheobronchial tree with bronchi, bronchioles, alveolar ducts, alveoli
          abnormal: bronchogenic cyst ← abnormal budding;
  pulmonary hypoplasia ← fewer than expected bronchi
16–24 weeks dramatic increase in number + complexity of airspaces and blood vessels
          abnormal: small airways ← reduction in number and size of acini

Anomalous Bronchial Division

Branching anomalies:

(a) displaced (replaced) bronchus

√  bronchus with abnormal origin while normal bronchus ventilating corresponding parenchyma is absent

(b) supernumerary (accessory) bronchus

√  may end blindly in parenchyma of corresponding normal bronchus = congenital bronchial diverticulum

√  may ventilate additional lung parenchyma, possibly delineated by an accessory fissure

Tracheal Bronchus (0.1–2.0%)

=  bronchus of variable length arising from lower trachea

Frequency:   0.1–1.3% in adults; 1.5–2% in childhood

In 78% associated with:

Down syndrome; malformation of thoracic cage / foregut / lung; tracheal stenosis; other tracheobronchial branching anomalies

Type:  displaced in 75%; supernumerary in 25% → ventilating intra- / extralobar tracheal lobe (NOT related to azygos lobe)

Location:   almost invariably on right; bilateral (rare)

Site:  distal trachea < 2 cm from carina

•  recurrent pneumonia, respiratory distress in childhood

•  almost invariably asymptomatic in adults

√  blind-ending pouch (= congenital right tracheal diverticulum) / aeration of a portion or all of RUL

√  early origin of apicoposterior LUL bronchus (less common)

√  “pig bronchus” = entire RULB displaced on trachea

Right Preeparterial Bronchus

=  any bronchus directed toward RUL that arises abnormally from RMB above level of right eparterial ULB

Frequency:   0.9%

Type:  82% displaced

•  mostly asymptomatic

DDx:  accessory cardiac bronchus

Right Posteparterial Bronchus

=  any bronchus directed toward RUL that arises abnormally from right bronchial tree below level of right eparterial ULB

Left Eparterial Bronchus

=  any bronchus directed toward LUL that arises from posterolateral / lateral wall of LMB above level where left pulmonary artery crosses LMB

Left Prehyparterial Bronchus

=  anomalous bronchus directed toward LUL that arises from LMB between level of left pulmonary artery crossing and hyparterial LULB

Accessory Cardiac Bronchus (ACB)

=  true supernumerary anomalous bronchus

The only bronchus originating from medial wall of either RMB or IMB (occasionally on left side)

M÷F = 2.8÷1

√  arises from medial wall of bronchus intermedius prior to origin of apical segmental RLL bronchus

√  caudal course toward pericardium

√  blind-ending pouch / ventilation of an accessory lobe

Bridging bronchus

=  aberrant bronchus that partially / totally supplies the right lung but originates from LMB

√  carina at T4–T5

√   pseudocarina in the shape of an inverted T at T6-T7

Paracardiac Bronchus

=  normal bronchus arising from medial aspect of lower lobe

Prevalence:   5% of patients


=  conducting branches for the transport of air; ~ 300,000 branching airways from trachea to bronchiole with an average of 23 airway generations


bronchus  =   cartilage in wall

bronchiole  =   absence of cartilage (after 6–2 divisions of segmental bronchus)

›  membranous bronchiole = purely air conducting

›  respiratory bronchiole = contains alveoli in its wall

›  lobular bronchiole = supplies secondary pulmonary lobule; may branch into 3 or more terminal bronchioles

›  terminal bronchiole = last generation of purely conducting bronchioles without alveoli; each supplying one acinus

small airways = internal diameter < 2 mm = small noncartilaginous membranous and respiratory bronchioles; account for 25% of airway resistance
large airways = diameter > 2 mm; account for 75% of airway resistance

HRCT of normal lung (window level –700 HU, window width 1,000–1,500):

√  –875 ± 18 HU at inspiration

√  –620 ± 43 HU at expiration

√  8th order bronchi visible = bronchi > 2 mm in diameter

◊  Normal lobular bronchioles NOT visible!


◊  Functionally most important subunit of lung!

=  all parenchymal tissue distal to one terminal bronchiole comprising 2–5 generations of respiratory bronchioles + alveolar ducts + alveolar sacs + alveoli

•  gas exchange

√  radiologically NOT visible

Cells of Lung Parenchyma 75% of all lung cells

1.   Air-blood barrier

›  epithelial cells (25%) = lining of air space

›  endothelial cells (25%) = lining of vessels

›  interstitial cells (35%), collagen fibres (15%)

2.   Alveolar epithelium

›  lining cells (type I pneumocyte) → tight junctions, no mitosis

›  secretory cells (type II pneumocyte) → synthesis + storage + secretion of surfactant

›  brush cells

[Primary Pulmonary Lobule]

=  alveolar duct + its connected air spaces

Secondary Pulmonary Lobule


[Lynne McArthur Reid (1923–?), experimental pathologist and dean of Cardiothoracic Institute, London University, Harvard Medical School, pathologist-in-chief emeritus at Children’s Hospital in Boston]

=  smallest portion of lung surrounded by connective tissue septa; supplied by 3–5 terminal bronchioles

√  basic anatomic + functional pulmonary unit appearing as an irregular polyhedron containing 3–24 acini

√  separated from each other by thin fibrous interlobular septa (100 µm)

Size:     10–25 mm in diameter

•  visible on surface of lung


›  centrally = lobular core:

»  branches of terminal bronchioles with a 0.1 mm wall thickness = below the resolution of HRCT

√pulmonary arterioles (1 mm)

›  peripherally (within interlobular septa):

√pulmonary vein + lymph vessels


√  barely visible fine lines of increased attenuation in contact with pleura (= interlobular septa); best developed in subpleural areas of

UL + ML: anterior + lateral + juxtamediastinal
LL: anterior + diaphragmatic regions

√  dotlike / linear / branching structures (= pulmonary arterioles)

Site:   near center of secondary pulmonary lobule; 3–5 mm from pleura

Interstitial Anatomy

1.   Bronchovascular interstitium

surrounding bronchovascular bundle

2.   Centrilobular interstitium

surrounds distal bronchiolovascular bundle

√  line extending to the center of a lobule

3.   Interlobular septal interstitium

√  lines perpendicular to pleura surrounding a lobule

4.   Pleural interstitium

Lung Development

›  embryonic phase

respiratory diverticulum (= laryngotracheal bud) originates from ventral wall of primitive foregut

→  elongation of lung bud → lateral invagination of mesoderm → tracheoesophageal septum

→  bifurcation of laryngotracheal bud at 5–7 weeks EGA → R + L mainstem bronchi

→  mainstem bronchi branch further into lobar bronchi

→  pulmonary arteries arise from 6th aortic arch

Time:   26 days to 7 weeks EGA

›  pseudoglandular phase development of segmental + subsegmental bronchi, respiratory bronchioles + terminal bronchioles, alveolar ducts + alveoli

Time:   7–16 weeks EGA

›  canalicular / acinar phase development of distal acinar units + canalization of further airspaces; airspaces are approximated by network of capillaries; type II alveolar cells capable of surfactant synthesis

Time:   16–24 weeks EGA

›  saccular phase increase in number of terminal sacs + thinning of intervening interstitium + beginning of alveolar septation

Time:   24–36 weeks EGA

›  alveolar phase development of true fully mature alveoli with progressive formation throughout first 2 years of life

Time:   36 weeks EGA – 18th postnatal month

mnemonic:   Every Premature Child Takes Air

Embryonic phase

Pseudoglandular phase

Canalicular phase

Terminal sac phase

Alveolar phase


=  surface-active material essential for normal pulmonary function

Substrate:   phospholipids (dipalmitoylphosphatidylcholine, phosphatidylglycerol), other lipids, cholesterol, lung-specific proteins

Production:   type II pulmonary alveoli synthesize + transport + secrete lung surfactant; earliest production around 18th week of gestation (in amniotic fluid by 22nd week of gestation)

Action:   increases lung compliance, stabilizes alveoli, enhances alveolar fluid clearance, reverses surface tension, protects against alveolar collapse during respiration, protects epithelial cell surface, reduces opening pressure + precapillary tone


Primary Pulmonary Circulation

⇒  supplies 99% of blood flow to lungs pulmonary arteries travel along lobar + segmental bronchi down to subsegmental level matching caliber of airways

(a)  large elastic pulmonary arteries (500 to > 1,000 µm) accompany lobar + segmental bronchi matching caliber of airways

›  main pulmonary artery / trunk: ≤ 28 mm

›  right / left pulmonary artery

›  lobar pulmonary artery

›  segmental pulmonary artery

(b)  muscular arteries (50–1,000 µm) accompany subsegmental airways + terminal bronchioles

√  provide active vasodilatation + constriction

(c)  arterioles (15–150 µm) accompany respiratory bronchioles + alveolar ducts

(d)  capillary network in alveolar walls

(e)  venules

(f)  pulmonary veins course through interlobular fibrous septa

Function:   gas exchange

Bronchial Circulation

⇒  supplies 1% of blood flow to lungs = 1% of cardiac output

Pressure:  systemic high-pressure system (6 x that of normal pulmonary circulation); bronchial arteries are resistant to arteriosclerosis


(a)  orthotopic bronchial artery (64%): anteriorly from proximal to mid-descending thoracic aorta at level of left main bronchus between superior endplate of T5 and inferior endplate of T6

Angio landmark:   1 cm above / below level of left main bronchus as it crosses descending thoracic aorta

(b)  at least one ectopic bronchial artery (36%):

›  from undersurface of aortic arch (15%)

›  distal descending thoracic aorta, subclavian artery, thyrocervical trunk, costocervical trunk, brachiocephalic trunk, internal mammary artery, pericardiophrenic a., inferior phrenic a., coronary a.

(c)  left bronchial artery: most commonly directly from aorta toward left side of esophagus

(d)  right bronchial artery: most commonly originating from another artery, typically intercostal artery toward right side of esophagus

Variants of vascular anatomy (9 types):

(1)  1 right bronchial a. arising posteromedially from a common InterCostal Bronchial Artery Trunk (ICBAT) + 2 left bronchial a. anteriorly (41%)

(2)  1 bronchial artery on each side, the right bronchial artery originating from an ICBAT (21%)

(3)  2 bronchial aa. on each side, 1 right bronchial artery originating from an ICBAT (21%)

(4)  1 right bronchial a. + 1 right ICBAT + 2 left bronchial arteries (10%)

Course:   behind trachea and main-stem bronchi; enter lung via hila; tortuous path along peribronchial sheath of mainstem airway to terminal bronchioles


⇒  nourishment for supporting structures

›  extra- and intrapulmonary airways

›  vasa vasorum of pulmonary arteries

›  nerves, pulmonary veins, lymph nodes within thorax

⇒  systemic blood supply to

›  trachea, bronchi, bronchial branches, visceral pleura

›  esophagus

The bronchial circulation + other collateral vessels (eg, intercostal, internal mammary, inferior phrenic aa.) respond to chronic pulmonary ischemia and ↓ pulmonary blood flow → vessel hypertrophy / enlargement → maintenance of blood flow to affected lung + participation in gas exchange through systemic-pulmonary arterial anastomoses beyond the pulmonary artery obstruction.

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Jun 29, 2017 | Posted by in GENERAL RADIOLOGY | Comments Off on and Function of Lung
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