and Function of Lung



ORDER OF BRONCHOPULMONARY SEGMENTS OF RIGHT LUNG:


AIRWAYS



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


Airway


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


Definition:


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!


Acinus


◊  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


=  REID 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


Contents:


›  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


HRCT:


√  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


Surfactant


=  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


PULMONARY CIRCULATION


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


Origin:


(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


Function:


⇒  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.

Only gold members can continue reading. Log In or Register to continue

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

Full access? Get Clinical Tree

Get Clinical Tree app for offline access