Microscopic Anatomy of the Airways Flashcards
Respiratory system components
1) Pair of lungs
2) Airways that lead to and from the lungs
Functions of respiratory system
1) Inspiration and exhalation of air
2) Gas exchange
3) Olfaction (smell)
4) Phonation (speech)
Two divisions of structures of the respiratory system
1) Conducting structures
2) Respiratory structures
Conducting airways
1) Nasal cavities
2) Nasopharynx, oropharynx, larynx
3) Trachea
4) Bronchi
5) Bronchioles
Function of conducting airways
- warm and humidify air
- remove foreign particles so don’t damage the delicate structures where gas exchange occurs
Respiratory structure - composition
- respiratory bronchioles
- pulmonary alveoli (alveolar ducts, alveolar sacs, alveolus)
Function of respiratory structures
-gas exchange
Histological plan for conducting airways - components
- mucosa (inner most layer)
- submucosa
- adventitia
Composition of mucosa
-respiratory epithelium on top of a basement membrane and a lamina propria
Composition of submucosa
-loose connective tissue containing seromucous glands
Adventicia
- outer, connective tissue layer
- binds airways to adjacent tissues
Respiratory epithelium other name
Pseudostratified ciliated columnar epithelium with goblet cells
Requirements of air before enters respiratory epithelium
-must be conditioned (needs to be humidified and cleaned of particles/pathogens)
What carries out the process of conditioning
- cells in respiratory epithelium
- airway lining fluid
Cells of respiratory epithelium
- ciliated columnar epithelial cells
- goblet cells
- basal cells
Role of goblet cells in respiratory epithelium
-produce mucous that line all but the smallest airways
Role of basal cells
- sit along basement membrane
- act as stem cells – renewal population for both columnar epithelial and goblet cells
Role of ciliated columnar epithelial cell
-cilia propels mucous and debris out of airways
Origin of mucous that lines the airways
- goblet cells
- seromucous glands
Why lungs are susceptible to damage by inhaled pathogens/particles/toxic chemicals
Gas exchange dependent on very thin and delicate blood air barrier
How gas exchange regions in the lungs are protected from inhaled pathogens/particles/toxic chemicals
Airway lining fluid
- comprised of:
a) peri-cilliary layer (low viscosity - secreted by epithelial cells)
b) mucous blanket -thick on top of pericilliary layer - pathogens get caugh in this plane
Mucociliary escaltor -explain what is happening
Peri-cilliary layer = layer that cilia will do majority of beating in
-only the tops of the cilia grab onto mucous blanket to propel it up the airways moving to the larynx for clearance
Respiratory diseases associated with impaired ciliary function-what’s going on?
A problem with the peri-ciliary layer
a) too viscous and cilia can’t beat
b) disipeared completely and cilia can’t function
Metaplasia -definition
Replacement of one type of epithelium with another
Where does metaplasia of the respiratory tract typically occur
-Where pattern of airflow is altered i.e. when forceful airflow occurs
Smoking –> how leads to metaplasia
- respiratory epithelium will compensate for constantly being under attack by increasing the number of goblet cells by producing more mucous to try and trap particulates of smoke
- results in increase mucous that is difficult to clear and leads to chronic coughing
- chronic coughing = forceful airflow/change in pattern airflow
- this results in the replacement of ciliated columnar cells to squamous cells
- squamous = more sturdy = stronger where there is a change in airflow but at same time -reduced number of ciliated cells to clear mucous
- change is reversible if irritant is eliminated or can lead to metaplastic transformation
Function of the trachea
- conduit for air
- condition inspired air (lined with respiratory epithelium)
Four definable layers of the trachea
1) Mucosa
2) Submucosa
3) Cartilaginous layer
4) Adventitia layer
Mucosa of trachea composition
Respiratory epithelium
Submucosa of trachea composition
-Seromucous glands
Cartilagenous layer composition
-16-20 cartilagenous rings (C-shaped rings
Function of cartilagenous layer
-flexible, maintains patency of trachea
Adventitia composition and function
- loose connective tissue
- binds trachea to other structures
Tracheobronchial tree function
-air moves from trachea into lungs via this path
Number of generations of airways within tracheobronchial tree
-airway divides approximately 23 times
Average number of terminating pulmonar acini + alveoli
- 30, 000 pulmonary acini
- each containing more than 10,000 alveoli
Regular dichotomy model
Model that predicts that each airway will branch into 2 equally sized daughter airways
-in reality daughter bronchi are never actually the same size and sometimes there is trifurcations
Histology of bronchi
-initially similar histologically to trachea
-when enter the lung at hilum will several important histological differences
1) Rings of cartilage are replaced by irregular cartilage plates
2) Circular layer of smooth muscle
Five layers:
-mucosa
-muscularis
-submucosa
-cartilage layer
-adventitia
Branches of main bronchi
Intrapulmonary bronchi (characterized by successive, dichotomous branching)
Histological changes as bronchi become smaller
1) the amount of cartilage decreases (but still bronchi not directly embedded in lung tissue - do not rely directly on elastic component of lung to keep airways open)
2) respiratory epithelium becomes reduced in height
3) still have layer of smooth muscle
Somewhere around generation 11 what happens
- conducting air passage is < 1mm in diameter
- no cartilage -embedded directly in lung tissue and rely on its elastic properties to keep the airway open
- do not have submucosal glands - in rely small airways don’t want mucus
- still have smooth muscle (responsible for star-shaped lumen of bronchioles)
- epithelial lining changes
a) simple columnar epithelium (where airways are larger)
b) simple cuboidal epithelium (where airways are smaller) - goblet cells replaced by clara cells
Structure of clara cells
- non-ciliated
- dome shaped
Abundancy of clara cells
Represent 75-85% of cells lining the bronchioles
Function of clara cells
1) Secrete surfactant to reduce surface tension
2) Clara cells secretory protein (CCSP)**function?
3) Ingestion and breakdown of toxins
4) Produce enzymes to breakdown mucus
5) Lysozymes (antimicrobial)
Asthma
- intermittent reversible airway obstruction
- episodes of chest tightening, wheezing, coughing, shortness of breath (all the result of narrowing of airway)
- generally in response to inflammatory mediator that will induce contraction of smooth muscle in airway wall)
Bronchoconstriction
Contraction of airway smooth muscle
Allergen induced asthma
- one of the many types of asthma
- where allergens are the trigger for bronchoconstriction (inflammatory stimuli)
Inflammatory cells in airway walls
1) Eosinophils
2) Mast cells
3) Neutrophils
4) Lymphocytes
Role of mast cells
- have IgE receptors
- in response to allergen - IgE receptors activate the mast cells
- mast cells release spasmogens that induce contraction of airway smooth muscle
Role of eosinophils
-also release mediators that are spasmogens
Neutrophil role
-release proteases that damage epithelium
Airway changes in asthmatic
1) Goblet cells hyperplasia *(chronic)
2) Airway inflammation
3) Bronchoconstriction
Goblet cell hyperplasia - result
Mucous hypersecretion
Airway remodelling
-body tries to fix localized areas of damage
1) increase in connective tissue surrounding epithelium
2) increase in numbers of layers smooth muscle surrounding the airway (strengthens the contraction)
= subepithelial fibrosis?
Respiratory structures
1) Respiratory bronchioles
2) Alveoli
Respiratory bronchioles-function
- transition
- close to where branche with bronchioles still mainly function for air conduction
- further down greater role in gas exchange
Histology of respiratory bronchioles
- close to bronchioles:
- simple cuboidal epithelium (ciliated) and clara cells (non-ciliated)
- distal segments (closer to alveoli):
- clara cells predominate
- alveoli extend from lumen of respiratory bronchioles
Alveoli
- terminal air spaces in the lung
- where gas exchange occurs
Components alveolus
- alveolar septa
- alveolar epithelial cells (continuous lining)
- network of capillaries
- alveolar macrophages
Function of alveolar septa
- framework (for structure of alveoli to sit in)
- structure that blood vessels articulate with
Compositionn of alveolar septa
- type IV collagen fibers
- elastic fibers
Cell composition of alveolar epithelium
- type I pneumocytes
- type II pneumocytes
Type I pneumocytes
- thin squamous cell
- large surface area to faciliate gas exchange
- linked by tight junctions (restrict movement in and out of airspaces)
- cover 95% of alveola surface
Type II pneumocytes
- cuboidal cells
- more numerous but because of shape only cover 5% of alveolar surface
- do not function in gas exchange directly - secrete surfactant to prevent alveolar collapse
- usually located in septal junctions
Blood air barrier
- structure where gas exchange occurs
- represent areas of minimal thickness (have only thin type I pneumocytes + thin capillary endothelium - share basal lamina (fused) - creased barrier of minimal thickness = 0.3- 0.5 um which allows for rapid gas exchange)
Dust cells/alveolar macrophages location
Blood air barrier
Dust cell/alveolar mac function
-Scavenging foreign bodies (dust particles, pathogens, etc) and remove
How dust cells can be identified at
a) LM level
b) EM level
a) LM level:
- can see ingested particles (brown dots)
b) EM level:
- share features common to all macrophages
- primary and secondary lysosomes in cytoplasm
