L27: Lungs

Question 1

Origin, germ layer of the laryngotracheal diverticulum and tube

Answer 1

• Laryngotracheal diverticulum (pouch) forms at the ventral and caudal part of the forgegut. It develops from endodermal lining, which takes with it the splanchnic mesoderm as it grows
• Distal end enlarges to form respiratory bud
• It separates from primordial pharynx, but maintains communication via primordial laryngeal inlet
• The posteroinferior aspect of the laryngotracheal diverticulum has developing tracheoesophageal folds
• They approach each other and fuse to become the tracheoesophageal septum
• This septum divides ventrally into laryngotracheal tube and dorsally into oropharynx and esophagus

Question 2

Define and describe the structure which divides the laryngotracheal tube

Answer 2

• This structure is the tracheoesophageal septum
• Divides ventrally into laryngotracheal tube, which is primordium for larynx, trachea, bronchi and lungs
• Divides dorsally into oropharynx and esophagus

Question 3

Describe how the birth defect, tracheoesophageal fistula, develops

Answer 3

• Tracheoesophageal folds develop in the posteroinferior aspect of the laryngotracheal diverticulum, they fuse to become the tracheoesophageal septum
• This septum then divides ventrally into laryngotracheal tube and dorsally into oropharynx and esophagus
• Failure of this to occur, leads to tracheoesophageal fistula, esophagus and trachea are linked in some way – many variants.

Question 4

Describe the evolution of the respiratory (aka tracheal) and bronchial buds

Answer 4

• Respiratory bud arises when the distal end of the laryngotracheal tube (tracheal portion) differentiates at the ~ week 4.
• As the tracheal bud continues growing inferiorly, it forms two outpouchings known as primary bronchial (lung) buds

Question 5

Describe the adult derivatives of the endodermal tube and the overlying mesenchyme

Answer 5

• The endodermal lining of the laryngotracheal groove (caudal to 4th pharyngeal arch) develops into the epithelium and glands of larynx, trachea, bronchi and pulmonary epithelium
• Overlying splanchnic mesenchyme in these developing areas become cartilage, CT, muscles and visceral pleura (if any) associated with the lower respiratory structures

Question 6

Define and describe the space the body provides, in which, the bronchial (lung) buds grow into

Answer 6

• Bronchial buds grow laterally into mesenchyme surrounding pleuroperitoneal canals, which are the primordial of the pleural cavities. These cavities are always ahead to the growth of the lungs

Question 7

Discuss development of bronchi

Answer 7

• As tracheal bud grows inferiorly, it forms primary bronchial buds (endoderm)
• Bronchial buds grow laterally into mesenchyme surrounding
• Endodermal lining becomes epithelium and glands of bronchial branches, splanchnic mesenchyme overlying becomes cartilage, CT , muscles and visceral pleura
• Early in 5th week, mainstem bronchi form, these develop into secondary lobar bronchi
• Around 7th week, segmental bronchi develop with surrounding mesenchyme that separates bronchopulmonary segments
• By 24 weeks, there are 17 orders of branches, respiratory bronchioles have developed, additional 7 orders after birth

Question 8

Be able to describe the important developmental structures associated with the four stages in the maturation of the lungs. When do most mature alveoli form? Viability of fetus?

Answer 8

1. ) Pseudoglandular period: 6-16 weeks
   - early in this period, lungs resemble exocrine glands
   - by 16 weeks, all major elements have formed except those necessary for gas exchange
   - birth in this period not viable
2. ) Canalicular period: 16-26 weeks
   - lumina of bronchi and terminal bronchi become larger, more highly vascularized
   - by 24 weeks, terminal bronchiole have given rise to respiratory bronchioles each with alveolar ducts
   - Some primordial alveoli (terminal saccules) may have developed
   - Fetuses may survive with ICU, but this system with others are quite immature
3. ) Terminal sac period: 26 weeks-birth
   - More terminal saccules develop, epithelium becomes thinner, capillaries starting moving in more
   - Blood-air barrier better established
   - By 26 weeks: type I pneumocytes (sq epi) and type II (surfactant producers) fully present
4. ) Alveolar period: 32 weeks-8 years
   - about 95% of mature alveoli develop postnatally
   - increase in size and number
   - complete alveolar development by age 3, more may be added up to age 8

• most mature alveoli form in alveolar period 32 weeks to 8 years. In terms of viability, latter part of canalicular period with ICU ~ 26 weeks.

Question 9

Define relationship of tracheal bifurcation to the arch of the aorta and esophagus

Answer 9

• Trachea bifurcates at the carina, which is at the sternal angle (T4-5 vertebra)
• Trachea is behind the aorta, infront of the esophagus
• Trachea and esophagus are the structures furthest back

Question 10

What landmark indicates end of larynx and beginning of trachea? End of trachea?

Answer 10

• Cricoid cartilage at C6, C-shaped cartilages that open posteriorly after this
• End of trachea at sternal angle = carina

Question 11

Describe branching of the trachea and respiratory tree

Answer 11

• Trachea branches into the left and right main bronchi at the carina (sternal angle)
• Primary bronchi branch into secondary (lobar) bronchi (2 left to 2 lobes, 3 right to 3 lobes) and then into segmental, brochopulmonary segments)
• Bronchopulmonary segments divide segmental bronchi into terminal bronchioles, respiratory bronchioles, alveolar ducts and alveolar sacs

Question 12

What is the significance of the bronchopulmonary segments?

Answer 12

• These are the surgical units of the lungs that have own CT avascular septa that separate them from the larger lobes of the lungs. They have their own segmental arteries, bronchi and veins. As a result, these can be removed from the lung without disturbing function of other parts.

Question 13

Vasculature to the lungs

Answer 13

• Left bronchial arteries (two of them, inf and sup) come off aorta and run with left primary bronchus into left lung
• Right bronchial artery comes off aorta, then divides into two (inf and sup), which run with right primary bronchus into right lung

Question 14

Where are FB more likely to get lodged in the respiratory system? Why?

Answer 14

• In the right primary bronchus

- Right primary bronchus is wider, shorter and more vertical than left

Question 15

What vascular structure passes over the right primary bronchus? Over left primary bronchus?

Answer 15

• Over right: azygos arch connecting to the SVC

- Over left: aortic arch

Question 16

How many lobes does each lung have? Names of lobes. Fissures that divide each lung?

Answer 16

• 3 right, 2 left
• Right: upper(sup), middle, lower(inf)
• Left: upper(sup), lower(inf)
• Both have oblique(primary) fissures separating inf from sup lobes
• Right has horizontal(secondary or transverse) fissure separating sup from middle lobes

Question 17

Unique features of left lung

Answer 17

• Cardiac notch
• Cardiac fossa
• Lingula

Question 18

Lingula of left lung is projection from what lobe?

Answer 18

• Upper lobe of left lung

Question 19

Borders and margins of lungs

Answer 19

Borders and margins of lungs

Question 20

Surfaces of lung

Answer 20

• Cervical surface
• Costal surface
• Mediastinal
• Diaphragmatic

Question 21

Root vs hilum of lung

Answer 21

• Root = structures that go into lung

- Hilum = indentation of organ where structures go in and out from lung

Question 22

List the structures passing through the root of the lung

Answer 22

• Pulmonary ligament
• Pulmonary arteries and veins
• Bronchi
• Bronchial arteries and veins
• Nerves
• Lymphatics

Question 23

How to differentiate root structures of the lung in cadaver lab?

Answer 23

• Pulmonary artery is typically more thicker and more superior than pulmonary veins, even more so than bronchial arteries and veins
• Bronchi are thickest tubes in hilum

Question 24

List the margins of the lobes of the lung in relation to the ribs and the vertebral level

Answer 24

• ????
• Anteriorly: midclavicular line down 6th rib (T6 and T5) bilaterally = rib, below to rib 8 (T8 and T7) = pleura
• Posteriorly: scapular line down to 10th rib (T10 and T9) bilaterally = rib, below to rib 12 (T12 and T11) = pleura

Question 25

List the subdivisions of the parietal pleura and their locations

Answer 25

1. ) Cervical surface = surface emerging through thoracic inlet
2. ) Costal surface = surface in contact with ribs
3. ) Mediastinal surface = surface in contact with mediastinal structures
4. ) Diaphragmatic surface = surface in contact with domes of diaphragm

Question 26

Define: cardiac notch, cardiac fossa, pulmonary ligament and costodiaphragmatic recess

Answer 26

• Cardiac notch = anterior margin of left lung indentation as a result of heart placement
• Cardiac fossa = area in left lung where heart lives
• Pulmonary ligament = reflection of parietal pleura as it becomes visceral pleura
• Costodiaphragmatic recess = potential space where peripheral diaphragmatic pleura is in contact with the lowest part of the costal pleura.
• These are named for the surface they are on

Question 27

What is the pleural cavity? What fluid is found here? Function?

Answer 27

• Space between two pleural membranes (visceral and parietal)
• Filled with serous fluid, which reduces friction between these layers as they move over each other

Question 28

What attaches the parietal pleura to the thoracic wall?

Answer 28

• Endothoracic fascia

Question 29

Two recesses found in thoracic cavity?

Answer 29

• Costodiaphragmatic recess
• Costomediastinal recess
• Reflections where specific pleural surface becomes another pleural surface

Question 30

Describe the nerve supply to the parietal pleura

Answer 30

• Costal and cervical pleura = local intercostal nerves
• Mediastinal pleura = phrenic nerve
• Diaphragmatic pleura = central part (phrenic nerve) and peripheral part = local intercostal nerves

Question 31

Which pleura is insensitive to pain and pressure?

Answer 31

• Visceral pleura

Question 32

Nerve supply to lungs. Function?

Answer 32

• Via pulmonary plexus (ant and post) at carina. Post = main supply to lung.
• Sympathetics: vasoconstrictors and bronchodilators
• Parasympathetics: vasodilators, bronchoconstrictors, secretomotor and afferents (cough reflex/irritant receptor at carina, stretch receptors in bronchial tree for respiratory control and touch pain in respiratory epithelium)

Question 33

Discuss lymphatic drainage of lung

Answer 33

• Two divisions: superficial lymphatic plexus and deep lymphatic plexus
  1. ) superficial lie deep to visceral pleura (chicken wire): drain into superior and inferior tracheobronchial lymph nodes
  2. ) deep lie within lamina propria of bronchi (none in alveoli) drain into pulmonary lymph nodes (within lung), then to bronchopulmonary (hilar) lymph nodes (outside lungs) into superior and inferior (carinal) tracheobronchial lymph nodes and then into bronchomediastinal lymph trunks, which respectively drain into thoracic duct (left side) and right lymphatic duct (right side)