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What's the main functional component of all aspects of the respiratory tract?
- the epithelium
- mostly what's described
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Conducting Portion
- includes structures that carry air to & from sites of gas exchange while warming, moistening, & cleansing it
- no gas exchange takes place IN the conducting portion because the walls of the structures are too thick
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What is the path air flows through in the conducting portion of the respiratory system?
nasal cavity → nasopharynx → larynx → trachea → (intrapulmonary) bronchi → bronchioles → terminal bronchioles
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Respiratory (Conducting) Epithelium
- the epithelium that covers the surfaces of the conducting portion of the respiratory tract
- it's a pseudostratified, ciliated, columnar epithelium with goblet cells
- very deep in the lung this epithelium will no longer be ciliated
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Respiratory Portion
where gas exchange takes place, specifically in the thin-walled alveoli
order: respiratory bronchioles → alveolar ducts → alveolar sac → alveoli
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What are the 3 types of cells in the respiratory portion (where gas exchange takes place) of the tract?
- type I pneumocytes
- type II pneumocytes
- alveolar macrophages
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Olfactory Epithelium
- not a huge region, comprises part of the upper lateral wall & septum of the nose
- is pseudostratified
- has stem cells (basal cells) that sit on the basal lamina
- bipolar neurons (olfactory cells) pass through the basal lamina
- they have cilia at the top of the epithelium that's bathed in serous fluid from Bowman's glands
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olfactory nerve cells
- bipolar neurons with modified, non-motile cilia present at knob-like endings
- receptors for odorants are present on the surface of the CILIA - such cells turn over constantly (unusual for CNS)
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How is something smelled?
- odors/molecules have to dissolve in the fluid from the Bowman's glands at the surface of the epithelium
- they then bind to odorant binding proteins
- the compound then binds to receptors on the olfactory cell (bipolar neuron) cilia
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What do axons from the olfactory cells become?
- CN I (cranial nerve I)
- any cells associated with these olfactory cell axons are Schwann cells (because they're part of the peripheral nervous system)
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Why are the bipolar neurons in the olfactory epithelium unusual?
- because they divide / turn over (@ ~the same rate as skin, olfactory epithelium)
- the capacity is diminished in Parkinson's & Alzheimer's - patient early on complain of anosmia
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- olfactory epith. is quite thick (especially compared to respiratory epithelium)
- upper most nuclei belong to the sustentacular (supportive) cells
- nuclei right along the basement membrane = basal cells
- all other nuclei in the middle belong to the bipolar neurons
- can see nerve bundles with the nuclei of associated Schwann cells
- everything else below = Bowman's glands
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Respiratory (Conducting) Epithelium
- can see pseudostratified, columnar epithelial cells
- can see the cilia at the top
- dark line under cilia = basal bodies; anchor them & organelles from which they develop
- also see goblet cells
- no respiration takes place where there is respiratory epithelium (too thick for gas exchange)
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Where would basal bodies not be visible?
it isn't present on villi
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- goblet cells visualized in respiratory epithelium
- goblet cells don't have cilia b/c they open up & exocytose their contents (mucus granules
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Kulchitsky (Enteroendocrine) Cells
- made lots of products, eg. NTs like catecholamine, serotonin, hormones like calcitonin
- are part of the DNES (diffuse neuroendocrine system)
- can't be observed using H&E
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CF as it relates to Respiratory Epithelium Cilia
- muco-cilliary 'escalator' doesn't work well
- CFTR is located ~ where the basal bodies are (found at the apical end of ciliated cells, not goblet cells)
- when CFTR doesn't properly transport Cl-, Cl- can't exit the cell, meaning Na+ stays in the cell to maintain electroneutrality, & WATER also goes into the cell FROM the mucus
- this dehydrates the mucus making it THICK
- thick mucus is hard for cilia to move, so pathogens that'd normally be cleared won't be
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Larynx
- this portion of conducting pathway isn't covered by respiratory epithelium
- - larynx contains stratified squamous epithelium (SSNKE)
- contains vocal chords & vocalis muscle that controls the vocal chords
- several cartilages (most hyaline, a few elastic) anchor the vocalis muscle
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epiglottis
- directs food away from the airway during swallowing
- has non-keratinized stratified epithelium on its superior surface and has respiratory epithelium on its inferior surface
- lamina propria contains some seromucous glands + an elastic cartilage plate
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vocal apparatus
- consists of:
- vestibular folds (false vocal cords) - respiratory epith.
- true vocal cords - stratified squamous epith, control phonation
- each true vocal cord has a large elastic fiber bundle, vocal ligament, & a parallel SKELETAL muscle bundle, the vocalis muscle
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Trachea
long tube extending from the larynx down to the point where it bifurcates into the 2 main-stem bronchi
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Trachea X-Section
- sits anterior to the esophagus
- lined by Respiratory (Conducting) Epithelium [pseudostratified ciliated columnar w/ goblet cells]
- Lamina Propria is the next layer after epithelium; it contains elastic fibers
- there is NO muscularis mucosa proximal to the lamina propria in the trachea
- instead there's Adventitia with Hyaline Cartilage that's C-shaped
- Trachealis Muscle connects the 2 ends of the C-shaped Hyaline Cartilage Adventitia
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What are the 2 functions of the Trachealis Muscle?
1. to contract when you cough & generate a lot of intrathoracic pressure that causes the trachea to expand (this muscle prevents over-expansion)
2. it allows the esophagus to expand into the trachea when you're swallowing something
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What happens to the hyaline cartilage of the trachea in older individuals?
it gets transformed into bone tissue
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Trachea Mucosa + Submucosa
- Mucosa (innermost layer, closest to lumen):
- - covered by respiratory epithelium
- - has a PROMINENT basement membrane
- - & underlying lamina propria
- Submucosa:
- - contains MANY mixed seromucous glands
- - is separated from mucosa by a layer of elastic fibers
*there is no muscularis mucosa separating the mucosa & submucosa in the trachea
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How are the mainstem bronchi different from the trachea?
- they're not - they're histologically identical to the trachea
- can identify changes once you get into the lung proper
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Which lung do aspirated foreign materials tend to enter more readily?
- the RIGHT one
- because the right primary bronchus is wider, shorter & more VERTICAL than the left, & it makes a smaller angle with the axis of the trachea than the left (more room for things to enter)
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Intra v. Extra Pulmonary
- Intra-anything means it's actually in the substance of the lungs
- everything before/outside of the lungs = extra pulmonary (trachea + main bronchi)
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How does cartilage exist in the lung?
- cartilage in the intra-pulmonary areas won't be C-shaped like it was extra-pulmonarily
- instead it'll exist as 'plates' in the aventitia
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Intrapulmonary (Secondary) Bronchi
- cartilaginous airways that arise from subdivisions of the primary bronchi via branching
- intrapulmonary bronchi themselves divide to give rise to lobar & segmental bronchi
- as bronchial diameter ↓ → thickness & complexity of the wall ↓
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Intrapulmonary (Secondary) Bronchi Characteristics
- prominent muscularis mucosa (folds - also characteristic of bronchioles & larger bronchi due to preparation)
- hyaline cartilage plates
- *anytime you see hyaline cartilage in the adventitia in any level, the passageway is an intrapulmonary bronchus
- (once the plates disappear the passageway is called a bronchiole)
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What causes asthma?
- the thick smooth muscle of intrapulmonary bronchi
- histamine release from mast cells due to an allergic reaction causes constriction of the smooth muscle layer, narrowing the passageway
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Bronchiole
- a passageway is by definition a bronchiole if it lacks hyaline cartilage in its adventitia
- still too thick for gas exchange
- are typically 1mm or less
- has no glands in the submucosa, lacks cartilage altogether
- contains only a few goblet cells
- epithelium is no longer really pseudostratified, mostly cuboidal at this point; most nuclei lie basally & a clear apical cytoplasm can be seen
- a muscularis mucosa is present
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Club cells
- non-ciliated, dome-shaped apex, with the ultrastructure of a protein secreting cell
- 1. make surfactant so bronchioles don't collapse in on themselves
- 2. act as progenitor stem cells of the bronchiole epithelium during regeneration & repair
- 3. protect against inflammation & oxidative stress
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How can club cells be used clinically?
proteins - such as CC16 - can be measured & if present in serum are indicative of epithelial damage in the lung
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Terminal Bronchioles
- you know a bronchiole is terminal because the region downstream has out-pouchings known as alveoli
- you can't differentiate a terminal from standard bronchiole via cross-section only
- is the smallest, last portion of the conducting pathway
- the epithelium contains Club cells & some cuboidal ciliated cells (to remove any mucous which might move this far)
- muscular layer is made up of an incomplete circular layer of smooth muscle
- each terminal bronchiole divides into 2 or more respiratory bronchioles
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Where is there both gas exchange & conducting epithelium?
- Respiratory Bronchiole
- once you get passed the terminal bronchiole you can have regions with thin walls (alveoli, where gas exchange occurs) & thick walls (conducting)
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Respiratory Bronchiole
- make up the first part of the respiratory portion
- has a cuboidal epithelium (with ciliated cells, goblet cells & clara/club cells) is similar to that of a terminal bronchiole BUT the wall is interrupted by alveoli
- can see smooth muscle (conducting) & out-pocketing (alveoli that contain capillaries in their walls for gas exchange)
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as the respiratory bronchioles divide, the # of alveoli in the walls _______, & the number of goblet & ciliated cells _______
- alveoli increase in number
- goblet cells disappear
- frequency of ciliated cells decreases
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Alveolar Duct
- lies downstream of respiratory bronchioles
- entire wall is composed of alveoli out-pocketings (thin not thick wall with no smooth muscle)
- ANY place along the duct is thin enough to have gas exchange
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Alveolar Sac
- where the alveolar ducts end
- multiple alveoli wrapped in elastic fibers
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How do you prevent alveoli surfaces from atelectasis (collapsing) & sticking to each other?
- surfactant, which reduces the surface tension of surface membranes
- surfactant is amphipathic: made up of lipids [dipalmitoylphosphatidylcholine, DPPC] & proteins [surfactant protein A - D]
- it also makes lungs more compliant
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What type of cell makes surfactant?
type II pneumocyte
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Respiratory Distress Syndrome
- non-functional lungs is a big problem for premature infants because surfactant production starts at 6 months in the fetus
- RDS may occur in premature babies that have insufficient surfactant
- can treat with corticosteroids to ↑ surfactant or just give exogenous surfactant
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Cells of the Alveoli
type I pneumocyte: stretch a lot; gas exchange takes place over them
type II pneumocyte: cover a smaller SA than type I; usually found in alveoli corners
Macrophages (Dust cells)
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Where are capillaries present in alveoli?
- in the middle of alveolar cell walls (septa)
- the barrier for gas exchange is between capillary endothelium & type I pneumocytes
- gas must traverse both cells + their shared basement membrane for exchange
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Caveolae
- take up inhaled molecules via pinocytosis
- can then be transported through to capillary endothelial cells & subsequently transported through the blood
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Type II Pneumocytes
chunkier, have microvilli extending out
contain lamellar bodies* in cytoplasm that contain surfactant (are lipid rich)
histologically type II cells/lamellar bodies look like little clearings
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Which pneumocytes are capable of division & which aren't?
- type II can divide
- type I cannot
- renewal of type I pneumocytes requires proliferation of type II cells
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Alveolar Pores (of Kohn)
- small pores that may interrupt alveolar septum & connect one alveolus to another
- they equalize pressure & allow collateral air flow
- are important when small airways are blocked
- can also play a role in the spread of infection from one alveolus to another
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Arteries v. Veins
- Veins bring blood back to the heart (usually deoxygenated)
- Arteries pump blood away from the heart (usually oxygenated)
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Lung Blood Vessels
1. Pulmonary Arteries: carry deoxygenated blood from the heart/pulmonary trunk; run with conducting airways (share adventitia); are a thin walled low pressure/resistance system that end as capillary networks (for gas exchange)
2. Pulmonary Veins: carry oxygenated blood back to the heart; usually run by themselves in the substance of the lung, aren't connected to the conducting airway (separated by a sea of alveoli)
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What do pulmonary arteries always follow?
- the bronchiole tree
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- pulmonary veins
- carrying oxygenated blood in isolation (sea of alveoli) back to the heart
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What makes up the lungs' own blood supply?
Bronchial Arteries & Veins
- arteries are small, branch directly off the aorta & supply the bronchiole tree itself
- have no function in gas exchange, just supply the lung organ
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