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8. The anatomy and histology of the trachea and lung. The development of the respiratory tract.

8 Dec

8. The anatomy and histology of the trachea and lung. The development of the respiratory tract.

Lung - Parts, surfaces, a/v

Lung - Parts, surfaces, function, a/v

Lung - Parts, Impressions

Lung - Parts, Impressions

Lung - lymph, nerves, development

Lung - lymph, nerves, development

Lung 2 - development, action

Lung 2 - development, action

Anatomy of the trachea and lung


fibrocartilaginous tube supported by incomplete hyaline cartilage rings
back of ring is completed by trachealis m
Location: from lower end of larynx @ C6 –> Sternal angle @ border b/w T4-5, in superior mediastinum


lat to trachea = common carotid a, lobes of thyroid glands,
below the thyroid isthmus = jugular arch, inf thyroid v
R to trachea = brachicephalic thrunk

Trachea bifurcates @ level of sternal angle
R bronchus – wider, shorter and runs more vertically than L
L bronchus – passes inf to arch of aorta and ant to esophagus and thoracic aorta



Surface Projection:

R lung:

  • Apex @ parasternal line , 2 fingers above clavicle
  • descends till body of 5th rib, then runs lat to make inf border

L lung:

  • Apex @ parasternal line , 2 fingers above clavicle  (Instead of parasternal line, some would prefer that you say,1/3 of the way b/w midline and mid clavicular line)
  • @ 4th rib, goes a littl lateral , and then comes back towards midline @ 6th rib = cardiac impressions

Inf border of both lungs:

  • parasternal line = top of 6th rib
  • @ midinguinal line = bottom of 6th rib
  • @ant axillary line = body of 7th rib
  • @ mid axillary line = body of 8th rib
  • @post axillary line = body of 9th rib
  • @ para vertebral line = 11th rib

Function: major organs of respiration
Musc = Diaphragm, Ext/Int/Innermost IC m, SCM, Levator scapulae, Serratus Ant, Scalenus m, Pectoral Major & Minor, Levator costarum, Serratus Post sup m

1. Diaphragm contracts (inc vert diameter of thorax)
2. Pleural cavity & lung expands – dec intrapulmonary pressure (neg) –> air rushes into lungs passively as result of atmospheric pressure
3. Forced inspiration – involves contraction of IC m & elevation of ribs (superiolat movement) w/ sternum moving ant (bucket handle movement), Ribs move up and ant –> results in inc transverse and antero-post diameteres of thoracic cavity

Ab volume is decreased as ab pressure is increased

Musc = m of ant ab wall, int IC m, serratus post inf m
1. Overall process – involves relaxation of diaphragm, int IC m, and other m
dec in thoracic volume, inc in intrathoracic pressure, Ab pressure decreased & ribs depressed
2. Elastic recoil of Lungs – produces subatmospheric pressure in pleural cavities –> much of air is expelled
3. Forced Expiration – contraction of ant ab m and int IC m

* Quiet inspiration results from movement of diaphram mainly,
* Quiet expiration is passive process caused by elastic recoil of lungs

Apex – projects into scalenus tent, 1-2cm above clavicle, poorly ventilated
Base – rest on diaphragm, lowers in inspiration, rises in expiration
Root – place where a/v/n, main bronchi enter lung, one place NOT covered by pleura

  • = main bronchus, pul a/v, bronchial a/v,lymph vessles, Hilus lymph nodes, Pulmonary plexus
  • root of lung w/in sleeve of pleura = mesopneumonium (mesentary of lung),
  • unites around root to form pulmonary ligament, that runs inferiorly and attaches to the diaphragm

1. Costal surfaces – faces the ribs, sternum, costal cartilages, intercostal m

  • has costal pleura b/w it and ant thoracic wall,

2. Mediastinal surfaces – faces the pericardial sac, and vertebral bodies

  • Has a # of impressions on this surface made by structures b/w lungs
  • R Lung has 10 impressions: R subclavian a, Brachiocephalic truck, 1st rib, Thymus, Groove for azygos v, cardiac impression, groove for IVC, trachea, esophagus, partially the aorta
  • L Lung has 7 impressions: L subclavian a, L Brachiocephalic v, 1st rib, Trachea, Esophagus, Thymys, arch of Aorta,
Impressions of R lung

Impressions of R lung

Impressions on L Lung

Impressions on L Lung

3. Diaphragmatic surfaces – inf surface, rest upon the diaphragm

  • The R lung has deeper impression, due to the dome of diaphragm on R side being higher w/ liver underneath it.

R Lung has 3 lobes, and L lung has 2 lobes
The 3 lobes of R are made of the horizontal and oblique fissure, and the L has just the oblique fissure

The anatomical, functional and structural unit of the Lung = BronchoPulmonary Segment
each BP segment has:

  • a tertiary bronchus,
  • segmental br of pul a/v,
  • bronchiol a
  • pyramid structure w/ apex facing root, and base facing external surface
  • surgically removable w/o disrupting surrounding tissue

Pul acinus – structural unit w/ alveolar sac of 1 terminal bronchiole

Blood Supply of Lung:
Vasa Publica – pul a/v, called the respiratory circuit = CO2/O2 exchange

  • Pul Trunk = BP < aorta, covered by pericardium, extends from infundibulum of R ventricle = conus arteriosus
  • R pul a – behind asc aorta & SVC and ant to R bronchus
  • L pul a – connected to aorta via ligamentum arteriosum, remnant of ductus arteriosus, & shorter, narrower than R pul a
  • Pul v – intersegmental in drainage, leave lung as 5 pul v (1 for each lobe), but R upper & middle join so only 4 –> heart

Vasa Privata – bronchiol a/v, called the systemic circuit = supply viscera of lung itself

  • Bronchiol a – O2 –> visceral lungs & pleura,
  • connects to pul a that arises from aorta,
  • L bronchiol a from thoracic aorta
  • R bronchiol a from sup/post IC a, thoracic aorta, or L bronchiol a
  • Bronchiol v – rec blood from larger subdivisions of bronchi
    • drains to azygos v on R side, and acc hemiazygos, and hemiazygos on L side.
    • Acc hemiazygos –> hemiazygos v
    • The azygos system drains into SVC

Lymph Drainage:
Drains in 2 directions:
1. Superficial (Subserosal) lymph vessels = Superficial lymph plexus

  • drain surface and regions close to surface
  • runs on surface, deep to visceral pleura
  • –> bronchopul lymph nodes

2. Deep (Pericanalicular) lymph vessels = Deep lymph plexus

  • drain lymph from deep parts of lung
  • run along bronchiol tree
  • drain –> (Intra)pulmonary l.n. –> bronchopul lymph nodes

1. Pulmonary nodes –> 2 Bronchopulmonary –> 3. Sup/Inf tracheobronchiol nodes –> 4. Paratracheal nodes –> 5. Bronchomediastinal nodes & trunks @ birfucation of trachea

On R side –> R lymph duct
On L side –> Thoracic duct
Both empty in respective angulus venosus

Nerve Supply:
Pulmonary plexus – aff/eff fibers (PNS pre ggl) from CN X br, SNS post ggl fibers from SNS trunk, & cardiac plexus
Ant pul plexus = in front of root of lung
Post pul plexus = lies behind root of lung
br w/ blood vessesl & bronchi into lung

PNS – Eff = VM to bronchiol smooth m = Broncho-constrictor, bronchiol glands = secretomotor, Inhibitory to pul a/v = VD
Runs to:

  • Bronchiol mucosa – cough reflex
  • Bronciol m – stretch receptors
  • Interalveolar CT – Herring -Breuer reflex
  • Pul a as baroreceptors
  • Pul v as chemoreceptors

SNS – from SNS trunk
(-) Inhibitory for bronchiol m = broncho dilators
motor to pul a/v – VC
(-) Inhibitory for glands of bronchiol tree (type II epith cells)

Histology of the trachea and lung.

Development of the respiratory tract.


  • Int lining from endoderm, as well as the laryngeal epithelium & glands
  • musc & cartilage from 4th & 6th pharyngeal arch = thyroid, cricoid, arytenoid cartilages – therefore innervated by CN X
    • superior laryngeal n – above the vocal fold
    • recurrent laryngeal n = below the vocal fold
  • @ wk 4, on the ventral side of the primitive gut, a pocket forms that bulges out from the gut = laryngotracheal diverticulum
  • distal end of diverticulum to form lung bud
  • then, 2 folds of tracheo-esophageal folds, push medially and push together to midline to form a wall “septum”
    • ant (ventrally) = laryngealtracheal tube
    • post (dorsally) = esophageal tube


  • endoderm = epith + glands
  • mesoderm = smooth m, CT , cartilage

Bronchial/ Lung development:

  • from the lung bud –> forms 2 bronchiol buds
  • wk 5 = primary bronchi
  • primary bronchi –> branch into secondary (3 on R, 2 on L) –> branch into tertiary (10 on R, 8-9 on L), which become bronchopulmonary segment
  • these branches will expand into area  on either side of the foregut = primitive pleural cavity (or before that, pericardiocanal)
  • visceral mesoderm = visceral pleura
  • somatic mesoderm = parietal pleura

Phases of Development:

1. Glandular phase (5-16 wk)

  • branching continues
  • no respiratory bronchioles or alveoli
  • future airways are narrow with little lumens and a pseudostratified squamous epithelium.
  • embedded within a rapidly proliferating mesenchyme.
  • The structure has a glandular appearance.

2. Canalicular phase (13/16 – 26 wk)

  • beginning of respiratory bronchiole formation
  • primitive alveoli begin to form (terminal sacs)
  • Increased # a/v & capillaries start forming =extensive angiogenisis within the mesenchyme to form a dense capillary network.
  • Diameter of the airways increases with a consequent decrease in epithelial thickness to a more cuboidal structure.
  • Terminal bronchioles branch to form several orders of respiratory bronchioles.
  • Differentiation of the mesenchyme progresses down the developing respiratory tree, giving rise to chondrocytes, fibroblasts and myoblasts.

3. Terminal Sac phase (wk 24/26 – birth)

  • terminal sacs form
  • capillaries branching forming around terminal sac
  • # sacs and a/v increased enormously
  • Continued thinning of the stroma brings the capillaries right next to alveoli
  • Functional type-II pneumonocytes differentiate via several intermediate stages from pluripotent epithelial cells in the prospective alveoli.
  • Type I pneumonocytes differentiate from cells with a type-II like phenotype.
  • These cells then flatten, increasing the epithelial surface area by dilation of the saccules, giving rise to immature alveoli.
  • Type I & II pneumocytes seen – beginning of Blood brain barrier

4. Alveolar phase (wk 28/29 – 8 yrs)

  • mature alveoli continue to form till well developed
  • continue into childhood
  • Mature Blood Air Barrier forms