Lecture 22 - Lung Development, Structure And Gas Movement Flashcards
What are the five phases of lung development
Phase 1 - Embryonic Phase 2 - Pseudoglandular Phase 3 - Canalicular Phase 4 - Saccular Phase 5 - Alveolar
What doe the five phases of lung development lead to
The formation of the alveolar blood-gas barrier
What is established during phase 1
The basic lung structure as a template for further growth
When does the formation of the right and left lobes occur
Between 26 days and 6 weeks
What does the primordial lung develop as
Buds which extend outwards from the foetal foregut
What is established in phase 2
The branched network of gas conducting airways
When does phase 2 occur
Between weeks 6-16 of gestation
What creates a distending pressure
Fluid secretion into the airway
What does the distending pressure give
Mechanical support for the growth of the airway in three dimensions
At the end of phase 2 what has happened to the airways and vasculature
They have developed to completely fill the space available in the chest cavity
What is the developmental outcome of phase 2
The formation of the conducting airways of the lungs and accompanying blood vessel
What is the airways and accompanying blood vessels of the lung known as collectively
The respiratory tree
What type of branching does the airway of the lung follow
Irregular dichotomous branching
What are the advantages of irregular dichotomous branching
It achieves even dispersion of gas among terminal airways branches and the mechanical strain is dispersed evenly among units
What does the regulated increase of the number of airways at each branch allow
The dispersion of airflow resistance
What drives fluid movement into the airway lumen
A chloride gradient
What happens in phase 3
The airways and blood vessels meet to form an interface known as the blood-gas barrier
When does phase 3 occur
Between weeks 16-24 of gestation
What is the onset of phase 3 marked by
The extensive angiogenesis within the mesenchyme that surround the more distal reaches of the embryonic respiratory system which forms dense capillary network
The increase of the diameter of the air is accompanied by
A decrease in epithelial thickness to a more cuboidal structure
When does epithelial cell differentiation occur
Canalicular phase (phase 3)
What does differentiation of the mesenchyme progressing down the developing respiratory tree give rise to
Chondrocytes, fibroblasts and myocytes
What is the earliest stage of lung development that a premature baby could survive at
Canalicular (phase 3)
During phase 4 the first what occurs
The first septal fold of the early alveolus occurs
What does phase 4 define
The gas exchange zone of the lungs
What is the gas exchange zone of the lungs known as
The respiratory acinus
When does phase 4 occur
Between weeks 24-36 of gestation
What happens during phase 4
The branching and growth of the terminal sacs or primitive alveolar ducts and continued thinning of the storm bringing the capillaries into apposition with the prospective alveoli
The completion of what differentiation occurs during phase 4
The pneumocyte differentation
What do type I pneumocytes differentiate from
Cells with type-II like phenotypes
What happens to type I pneumocytes
They flatten, increasing the epithelial surface area by dilation of the saccules
The production of what is fully functional in phase 4
Surfactant
What has formed by 26 weeks
A rudimentary though functional blood-gas barrier
Through what process does further maturation the alveoli occur
Further enlargement of the terminal sacs, deposition of elastic foci and development of vascularised septae around the foci
What continues to thin to allow the capillaries to protrude into the alveoli
The stroma
When the septa forms what happens
Bifurcation of the airway terminus
What does the septum contain
Two closely apposed capillary networks
What is the patency of the thin-wall airway maintained by
Cl- driven secretion into the luminal space
What happens during phase 5
The gas exchange surface area is increased
When does phase 5 occur
36 weeks to 6 years
What clears the lungs of fluid
Na+ driven fluid adsorption in the lung lumen
When does pulmonary circulation become fully established
When the umbilical cord is cut off
How does enlargement of the gas exchange surface occur
By pocketing on the wall of the saccular lung
What are the basic anatomical features of the lungs
The trachea, pleura, bronchi and alveoli
Where does the trachea run from/to
It runs from below the larynx (C6/7) to the carina
What is the trachea composed of
C-shaped cartilage and small muscles that connect the cartilage
Which direction do the C-shaped cartilage face
Anteriorly
When the trachea bifurcates what does it give
The left and right main bronchi
What is the main function of the lungs
To oxygenate the blood
What does the parietal pleura line
The pulmonary cavity
What are the different parts of the parietal pleura
Cervical, costal, diaphragmatic and mediastinal
What is the pleura that covers the lungs
Visceral pleura
What is the name for the sleeve of pleura that hangs from the lung root
The pulmonary ligamnet
What do the main bronchi go onto form
Lober bronchi
What do lober bronchi go onto form
Segmental bronchi
How many lober bronchi are in the left lung
Two
How many lober bronchi are in the right lung
Three
What are attached to the bronchi
Alveoli
What are the respiratory surfaces within the lung
The alveoli
What are alveoli
Air-filled spaces that are closely associated with capillaries within the lungs
What type of gradient does gas follow
A partial pressure gradient
What does partial pressure tell you
The direction of movement of gas
What is the ENaC
A Na+ selective ion channel
Where is ENaC found
In all secretory epithelia
What increases during the third trimester of pregnancy
The maternal cortisol levels
What happens when cortisol enters the foetal circulation
It induces the ENaC subunit gene expression and membrane insertion in epithelial cells lining the foetal airways
During labor what activates the ENaC
The rise in the level of adrenaline within the mum which passes into foetal circulation
What does the ENaC channel allow for
Fluid to be rapidly cleared from the foetal lung in preparation for the first breath
What are the two advantages of airway branching
It causes an increase in the surface area for gas exchange and dissipates resistance to air flow as airway diameter narrows towards the respiratory zone
What drives the bulk of gas into the conductive zone
Net pressure caused by expansion and relaxation of the chest cavity
When will gas move into the alveoli
When the alveolar pressure is less than the atmospheric pressure and the airway is open
How does gas movement in the airways arise
By convection
When will gas move out of the alvoli
When the alveolar pressure is greater than the atmospheric pressure and the airway is open
What is alveolar pressure the sum of
Elastic recoil pressure and pleural pressure
What muscles are involved in inspiration
The diaphragm, external intercostal and the accessory muscles
What nerves control the diaphragm
The phrenic nerves
What happens to the diaphragm during inspiration
It flattens on contraction
What are external intercostal muscles innervated by
The intercostal nerves
What are the accessory muscles
The sternomastoid, scalenes anterior, medius and posterior, and the pectoralis major and minor
What happens to the intrapleural pressure as the chest expands
It falls
What muscle are involved during forced expiration
The internal intercostal muscles
