lung structure, gas movement and volumes Flashcards
what is the anatomy of the lung (basic)
3 tubular systems - airway, pulmonary blood supply and bronchial blood supply which are packed into a low resistance, high surface area, blind-ended gas exchanging organ with a 6 litre volume
what are the five stages of lung development
lead to the formation of the alveolar blood-gas barrier
1. embryonic
2. pseudoglandular
3. canalicular
4. saccular
5. alveolar
what is the embryonic stage of lung development (basic)
establishes basic lung structure as a template for further growth
what is the pseudoglandular stage of lung development (basic)
establishes the branched network of gas conducting airway
what is the canalicular stage of lung development (basic)
formation of the blood-gas barrier
what is the saccular stage of lung development (basic)
formation of the respiratory acinus - the zone of gas exchange
what is the alveolar stage of lung development (basic)
formation of the alveolus and high surface area for gas exchange
what is the embryonic stage of lung development
formation of left and right lung lobes
26 days > 6 weeks gestation
the primordial lung (lung anlage) develop as buds which extend outwards from the foetal fore-gut (oesophagus)
what is the pseudoglandular stage of lung development
formation of the gas conducting airway of the respiratory tree
gestation weeks 6 -16
branching of the airway and vascular duct system for up to 21 further generations beyond embryonic stage
fluid secretion into the airways creates a distending pressure which gives mechanical support for the growth of the airway in 3 dimensions
at the end of this stage, the airways and vasculature have developed to completely fill the space available in the chest cavity
developmental outcome of this stage is the formation of the conducting airways of the lung and accompanying blood vessels, together known as the respiratory tree
airway growth follows a programme of irregular dichotomous branching to allow the airways to fill spaces of varied dimension
what is irregular dichotomous branching
achieves even dispersal of gas among terminal airway branches. mechanical strain dispersed evenly amoung units
regulated increase in the number of airways at each branch disperses air flow resistance which would otherwise increase with distance into the lung
what is the chloride gradient in the pseudoglandular stage of lung development
a chloride gradient drives fluid movement into the airway lumen giving mechanical support for 3D growth
Cl- is accumulated against its electrochemical gradient
what is the canalicular stage of lung development
airways and blood vessels meet to form an interface; the blood-gas barrier
gestation weeks 16-24
the onset of this phase is marked by extensive angiogenesis within the mesenchyme that surrounds the more distal reaches of the embryonic respiratory system to from a dense capillary network
the diameter of the airways increase with a consequent decrease in epithelial thickness to a more cuboidal structure
epithelial cell differentiation begins
the terminal bronchioles branch to form the respiratory acini around which the alveoli will develop
differentiation of the mesenchyme progresses down the developing respiratory tree, giving rise to chondrocytes, fibroblasts and myoblasts
earliest stage of lung development at which premature infants can survive
what is the formation of the blood gas barrier
pseudoglandular - epithelial cells line the airway
early canilicular - epithelial cells begin to differentiate, basement membrane forms, capillaries adhere beneath basement membrane
late canalicular - blood gas barrier forms
what is a type I pneumocyte
flattened
forms thin barrier to capillary making the gas exchange surface
what is a fibroblast cell
fibroblasts; myofibroblasts, chondrocytes of the mesenchyme
what are endothelial cells
form capillary and blood side of the gas exchange surface
what are type II pneumocytes
cuboidal
rich in surfactant - containing lamellar bodies
maintains fluid homeostasis by vectoral ion transport
what is the saccular stage of lung development
formation of the first septal fold of the early alveolus
defines the gas exchange zone of the lung: the respiratory acinus
gestation weeks 24-36
branching and growth of the terminal sacs or primitive alveolar ducts
continued thinning of the stroma brings the capillaries into apposition with the prosepective alveoli
completion of pneumocyte differentiation
surfactant production is fully operational
by 26 weeks, a rudimentary though functional blood/gas barrier has formed
maturation of the alveoli continues by further enlargement of the terminal sacs, deposition of elastin foci and development of vascularised septae around these foci
the stroma continues to thin until the capillaries protrude into the alveolar spaces
septa from bifurcating airway terminus
septum contains two closely opposed capillary networks, one for each saccule
thin-walled airways are maintained patent by Cl- driven fluid secretion into the luminal space
what is pneumocyte differentiation
type I pneumocytes 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
what is the alveolar stage of lung development
increase in gas exchange surface area
gestation weeks 36 to approx 6 years postnatal
maturation of the lung indicated by the appearence of fully mature alveoli begins at 36 weeks, though new alveoli will continue to form for up to 6 years
a decrease in the relative proportion of parenchyma to total lung volumes still contributes significantly to growth for 1 to 2 years after birth, thereafter, all components grow proportionally until adulthood
Na+ driven fluid absorption from the lung lumen clears the lung of fluid and maintains a thin film of liquid on the surface of the airways throughout adult life
pulmonary circulation becomes fully established as the umbilical blood supply is cut off
enlargement of the gas exchange surface area, occurs by pocketing on the wall of the saccular lung
a single capillary network occupies the septal walls between each alveolus
the membranes thin to 200nm
how is preparation for the first breath undertaken
fluid absorption in the foetal lung is driven by the epithelial Na+ channel (ENaC)
maternal cortisol increases in the last trimester and crosses foetal circulation to induce gene expression of the ENaC subunit, and, membrane insertion in epithelial cells lining the foetal airways
during labour, a rise in maternal adrenaline crosses into foetal circulation to activate the channel
fluid is rapidly cleared from the foetal lung in preparation for the first breath
what is the epithelial Na+ channel (ENaC)
a Na+ selective ion channel found in all secretory epithelia - lung, kidney, gut, salivary duct, sweat duct
what does fractal mean
a repeated geometric shape, repeated at different magnitudes
what is the fractal design of the airway and vasculature
packs colossal proportions into a confined space
chest volume = 6 litres
generations of airway branching = 23
combined airway length = 8km
300 million alveoli
gas exchange surface area = 130m^2
combined vascular length = 2-6000km
average distance travelled by gas = 25cm
tensile pressure to rupture = 0.5MN/m^2
