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

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

Lung - Parts, surfaces, function, a/v

Lung - Parts, Impressions

Lung - lymph, nerves, development

Lung 2 - development, action

Anatomy of the trachea and lung

Trachea:

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

Topography:

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

Lung

Location:

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
Inspiration
Musc = Diaphragm, Ext/Int/Innermost IC m, SCM, Levator scapulae, Serratus Ant, Scalenus m, Pectoral Major & Minor, Levator costarum, Serratus Post sup m

Action
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

Expiration
Musc = m of ant ab wall, int IC m, serratus post inf m
Action
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

Parts:
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

Topography
Surfaces:
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 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.

Structure:
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.

Larynx:

• 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

Trachea:

• 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.
• NO SURVIVAL

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.
• RARE SURVIVAL

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
• CAN SURVIVE

4. Alveolar phase (wk 28/29 – 8 yrs)

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