Respiratory System Notes

# Respiratory System 1A: Airway Wall Plan & Air Modification * Learning Objectives: * Identify the organs of the respiratory system and classify them according to location and function. * Identify the histological components (wall layers, tissues & cells) of the airways and correlate them with their roles in the modification of air. ## Respiratory System Anatomy * Upper respiratory system: * Nose * Pharynx * Lower respiratory system: * Larynx * Trachea * Bronchi * Lungs * Division based on function: * Air conduction pathways: * Nose * Pharynx * Larynx * Trachea * Bronchi * Terminal bronchioles * Gas exchange surfaces: * Respiratory bronchioles * Alveolar ducts * Alveoli ## Functions of the Respiratory System * Air modification: Making air suitable for gas exchange * Pulmonary ventilation: Movement of air into and out of the lungs * External respiration: Gas exchange between air and blood ## Airways: Basic Wall Plan * Mucosa * Epithelium * Basement membrane * Lamina propria (loose CT) * Glands * Smooth muscle * Submucosa * Hyaline cartilage * Adventitia * Connective tissue ## Respiratory Epithelium * Ciliated pseudostratified columnar with goblet cells * Ciliated cells * Goblet cells * Basal cells = stem cells * Brush cells - chemosensory receptors associated with nerve fibers * Small granule cell – release local hormones & cytokines ## Lamina Propria * Loose CT containing: * Defense cells * Macrophages * Lymphocytes * Plasma cells * Elastic fibers * Mixed serous & mucous glands = seromucous glands * Mucosal venules ## Lamina Propria to Submucosa * Seromucous glands * Gradual transition from loose to dense CT, with the presence of smooth muscle +/- cartilage ## Air Modification: Why is it necessary? * Problem 1: Air quality is highly variable; influenced by: * Climate & weather * Industrial activity * Social choices * Problem 2: Delicate gas exchange surfaces in the lungs are <1\mu m thick & are easily damaged by: * Debris or pathogens in the air * Fluctuations in air temperature * Air that is too dry à desiccation of surface * The Solution: * Change the properties of air before it reaches respiratory (gas exchange) surfaces * Remove debris and/or pathogens * Control air temperature to that of body temperature à optimal function * Humidify air so that gas exchange surfaces are not desiccated * Key Concept: Air that is conducive to the exchange process is a key requirement for efficient external respiration i.e. gas exchange between alveolar air & the pulmonary circulation. ## What modifications to air occur in the respiratory system? * Efficiency of gas exchange is maximised when air is: * Filtered or cleansed: * to remove impurities & pathogens that might clog up or damage the exchange surface * Humidified so that: * the exchange surface doesn’t dry out & become non-functional * mucus remains sticky * Warmed: * optimal exchange occurs at body temperature ## Where are modifications to air made? * Modification of inspired air is achieved via interactions between air & the mucosa of: * nasal cavity * trachea ## Structural Features of Nasal Passages that assist in Air Modification * Lateral wall has 3 conchae (bony ridges) & 3 meatuses (channels) * Increase surface area of nasal mucosa for better contact between air & modification mechanisms * Form a tortuous air pathway that creates turbulence in airflow è more contact with surfaces ## Components of Nasal Mucosa * Respiratory epithelium * Seromucous glands * Venous plexus * Hyaline cartilage ## Filtration of Air: Removal of Particles * Achieved via: * Vibrissae (nose hairs): * Remove coarse particles * Muco-ciliary escalator * Sticky mucus layer from goblet cells & seromucous glands traps fine particles * Cilia transfer mucus to pharynx * Dust cells (alveolar macrophages) * Engulf any particles reaching alveoli ## Removal of Pathogens * Achieved via: * Muco-ciliary escalator of respiratory epithelium * Wafts pathogens out of lungs * Defense cells in lamina propria * macrophages, lymphocytes, plasma cells * mucosal associated lymph tissue (MALT) ## Humidification of Air * Achieved by: * Secretions from mucosal seromucous glands * Saturate air with water vapor * Engorgement of mucosa * Reduces airflow through the nostril * Prevents eventual desiccation of tissues by incoming air, allows recovery enabling humidification of subsequent airflow ## Warming of Air * Achieved via countercurrent heat exchange * Superficial venous plexus in nasal mucosa * Heat transfer from warm blood in vessels to colder air in nostrils * Opposite direction of blood & air flow enables maximal heat exchange ## Summary of Key Points * Organs of the respiratory system are classified according to their location &/or function. * Air modification is necessary to maximise the efficiency of gas exchange between air & blood at the alveoli (= external respiration). * Modifications to inspired air include filtration, humidification & warming. * Modifications to air are facilitated by features of the respiratory mucosa of the nasal passages & trachea. ## Learning Summary * Identified the organs of the respiratory system & classified them according to location & function * Identified the histological components (wall layers, tissues & cells) of the airways & correlated them with their roles in the modification of air # Respiratory System 2: Histology of Lungs ## Learning Objectives * Identify the respiratory portion of the lungs * Distinguish between interalveolar septae, respiratory epithelium & and the respiratory membrane * Identify the requirements for efficient gas exchange within the lungs and describe how these requirements are achieved & compromised in lung disease ## Air Conduction Pathways (recap) * Upper Tracheo-bronchial tree * Trachea * Primary bronchi – outside lungs * Secondary bronchi – to lobes * Tertiary/segmental bronchi – to bronchopulmonary segments * Terminal Bronchioles – to lobules * All share the same general wall plan ## Wall Plan of Bronchus (Recap) * Ciliated pseudostratified to simple columnar epithelium (E) with goblet cells * Lamina propria (LP) with: * Incomplete band of smooth muscle (SM) * A few seromucous glands (G) * Plates & islands of cartilage (C) ## Wall Trends (Recap) | | Trachea/Bronchus | Bronchiole | Alveolus | | :-------------- | :--------------------------------------------------------------------------------------------------------------------------------------- | | | Mucus/Surfactant | | Surfactant | | Epithelium | Epithelium | | Type I epithelial cell
Type II epithelial cell | | Basement membrane | | | | | L.propria | | | Interalveolar
Septum | | Smooth muscle | Smooth muscle layer-> | | Site of gas
exchange | | Fibrocartilaginouslayer | Fibrocartilaginous
layer | | | | | -Capillary
-Gland
Cartilage
Air conduction | -Capillary
Alveolus
Capillary | | ## Lungs – Respiratory Portion * Terminal parts of the bronchial tree * Respiratory bronchioles; are transitional & involved in both air conduction & gas exchange * Alveolar ducts * Alveolar sacs * Alveoli * Much simplified wall plan ## Pulmonary Lobule & Vasculature * Area supplied by a single terminal bronchiole * Contains * Respiratory bronchioles * Alveolar ducts * Alveolar sacs * Alveoli * Pulmonary blood supply (branch of the pulmonary artery) accompanies the conduction airway ## Lungs: Respiratory Portions (sites of gas exchange surfaces) 1. Respiratory bronchioles 2. Alveolar ducts 3. Alveolar sacs 4. Alveoli (air spaces) ## Alveoli * Located at ends of alveolar ducts * Walls shared by adjacent alveoli = interalveolar septae ## Alveoli cont. * Lined by simple squamous epithelium * 2 cell types – type 1 (squamous pneumocytes) & type 2 (surfactant cells) * Basement membrane * Alveolar walls contain: * Very little smooth muscle * Abundant elastic fibres * Extensive capillary network ## Alveolar Epithelium * Type I alveolar cell/septal cell/ pneumocyte * Squamous for exchange * Type II alveolar cell/septal cell/ pneumocyte * Cuboidal, secretes surfactant * Other cells: * Alveolar macrophage = dust cell * Not part of the epithelium, sits on epithelium in the alveolus ## Type II Cells * Located at septal junctions, 5% air surface * Produce surfactant - secretory granules (lamellar bodies) contain phospholipids, neutral lipids, proteins * Secretion functions: * Decrease alveolar surface tension * Assist in clearing foreign materials * Modulate alveolar immune responses ## Dust Cells * Monocytes in the blood differentiate to become dust cells in the lungs * Can migrate into alveoli & back to septae * Scavenge alveolar surfaces to remove particulate matter * Remove escaped rbcs from septal CT * Fate: * Pass up the bronchial tree via the mucociliary escalator * Remain in situ in septal CT * Moved in lymph to pulmonary lymph nodes ## Alveolar Blood Supply * Capillary network within interalveolar septae * Embedded in connective tissue * Supplied by pulmonary arteries ## Interalveolar Septum (EM) * Partition/wall between adjacent alveoli * CT core contains an extensive pulmonary capillary network ## Interalveolar Septum (EM) CONT. * DO NOT CONFUSE THE INTERALVEOLAR SEPTUM WITH THE RESPIRATORY MEMBRANE (barrier between air and blood) * Epithelium composed of types I (squamous) & II (surfactant secreting) cells * Basement membrane * Connective tissue: * Has a high elastic fibre content * Contains numerous pulmonary blood capillaries ## Respiratory Membrane = Air-Blood Barrier * Average thickness of $0.5\mu m$ * 3 structural layers + surfactant ## Air-Blood Barrier * Air * Thin layer of surfactant * Type I alveolar epithelial cell * Fused basal laminae of 2 epithelial cell types = basement membrane * Capillary endothelial cell * Blood * No intervening CT between epithelial layers ## Air-Blood Barrier cont. * Site of most efficient gas exchange * Epithelial cells are attenuated (very thin) * Organelles confined to perinuclear location away from the exchange site * Numerous pinocytotic vesicles (arrow) assist with exchange ## Requirements for Efficient Gas Exchange * Air that is conducive to being exchanged * Thin barrier between environments * Large surface area * Mechanisms to drive exchange process * concentration gradients of gases * gas solubility * Way to maintain exchange gradient/prevent equilibrium ## How Requirements for Efficient Gas Exchange are Achieved in the Lungs * Requirement 1: Air that is conducive to being exchanged * Adaptations for the modification of air in the mucosa of the nose & trachea – see notes for previous presentations * Requirement 2: Thin barrier/short distance * Minimal number of layers to barrier between air and blood – squamous epithelium (type I cell), basement membrane, squamous epithelium (capillary endothelium) * Many pinocytotic vesicles assist exchange ## How Requirements for Efficient Gas Exchange are Achieved in the Lungs cont. * Requirement 3: Large surface area * Extensive branching of the bronchial tree to produce >300 million alveoli with a surface area of $70m^2$ * Requirement 4: Mechanisms to drive exchange * Movement of gases across semipermeable barrier due to different concentrations of gas in environments (move from higherà lower concentration) * The ability of a gas to cross barrier dependent on its molecular weight & solubility ## How Requirements for Efficient Gas Exchange are Achieved in the Lungs cont. again * Requirement 5: Way to maintain exchange gradient & prevent equilibrium Achieved via ventilation-perfusion coupling (ventilation = air flow; perfusion = blood flow) * Rate of circulation of blood through pulmonary capillary beds is matched with the rate of movement of air into & out of the lungs (ventilation) **Lung Disease & Gas Exchange** Gas exchange is compromised by: * ↑ membrane thickness * ↓alveolar surface area | | Normal | Pneumonia | Emphysema | | :----- | :-------------------- | :-------------------------------------------- | :------------------------------------------------------------------------------------------- | | | | Fluid and blood cells | Confluent alveoli | | | | Alveolar walls thickened by edema | | ## Pneumonia * Alveoli fill with fibrinous exudate from blood à thicker air-blood barrier * Rbcs & neutrophils in alveoli à reduced amount of alveolar air * Interalveolar septae thicken (oedema) à thicker air-blood barrier * Capillaries congested with rbcs à impaired perfusion & gas exchange ## Emphysema * Interalveolar septae destroyed à enlarged air spaces & reduced surface area for exchange * Reduced elasticity à less recoil & impaired exhalationà air trapping & compromised exchange gradient * Activation of dust cells à further destruction of alveolar walls ## Learning Summary * Identified the respiratory portion of the lungs * Distinguished between interalveolar septae, respiratory epithelium & and the respiratory membrane * Identified the requirements for efficient gas exchange within the lungs and described how these requirements are achieved & compromised in lung disease # Respiratory System 1B: Trends in Trachea & Bronchial Tree Structure ## Learning Objectives * Correlate the histological structure of the trachea & bronchial tree with their role in the movement of air to & from the lungs * Differentiate between the trachea, bronchi & bronchioles according to their structural & functional characteristics ## Recap: Basic Wall Plan of Airways * Mucosa * Epithelium * Basement membrane * Lamina propria (loose CT) * Glands * Smooth muscle * Submucosa * Hyaline cartilage * Adventitia * Connective tissue ## Trachea & Bronchial Tree * = conducting pathways; all share the same basic wall plan * Trachea * Primary bronchi – outside lungs * Secondary bronchi – to lobes * Tertiary bronchi – to lobules * Bronchioles – terminal, respiratory * Alveolar ducts, sacs, alveoli ## Movement of Air through Trachea & Bronchial Tree * Requires patent (open) air conduction pathways * Largely independent of wall structure * Cartilage - patency * Elastic fibres – recoil in passive expiration * Relies on generation of a reversible pressure gradient * Air movement from higher to lower pressure ## Trachea * Respiratory epithelium * Ciliated cells * Goblet cells * Abundant seromucous glands * Trachealis (smooth) muscle joins posterior ends of: * C-shaped cartilage rings (16 along length) – keep airway patent (open) ## Bronchus * Respiratory epithelium * Ciliated cells * Goblet cells * Fewer seromucous glands than trachea (none seen here) * Incomplete layer of smooth muscle in mucosa * Plates & islands of cartilage – support patency while enabling branching ## Bronchiole * Simple columnar à cuboidal epithelium * Ciliated cells * Clara cells * No glands * Continuous layer/ring of smooth muscle in mucosa * No cartilage – airway patency maintained by alveolar pressure ## Clara Cells * Dome-shaped cuboidal cells, only in bronchioles * Produce a surfactant–like secretion that prevents luminal adhesion in passive respiration * Secrete protein CC16 that protects against inflammation & oxidative stress ## Trends in Wall Structure ## Trends: Epithelium, Goblet Cells & Cilia | | Upper | Lower | | :-------------------- | :------------------------------------------------------------------------------------------------------------------- | | Respiratory epithelium | Epithelium changes from pseudostratified to simple | | | Cell height decreases; columnar to cuboidal to squamous | | | Goblet cells reduce in number & disappear at tertiary (smallest) bronchi | | | Cilia reduce & disappear at respiratory bronchioles | | Respiratory membrane | Simple squamous, types I & II alveolar cells | ## Trends: Glands & Cartilage **Cartilage:** * C-shaped rings in trachea * Plates & islands in bronchi * Absent from bronchioles downward **Glands:** * Abundant seromucous glands in mucosa of nose & trachea * Fewer glands in bronchi * Absent from bronchioles downward ## Trends: Smooth Muscle & Elastic Fibres **Smooth Muscle:** * Little in trachea (except posterior trachealis muscle) * Incomplete layer in bronchi * Complete layer in bronchioles * Less in alveolar ducts & alveoli **Elastic Fibres** * Insignificant in nose & trachea * Abundant in bronchi & bronchioles * Most abundant (proportional to other tissue) in alveolar ducts & alveoli ## Variation in Airway Diameter * Cartilage * Semi-rigid support prevents occlusion; small reductions in tracheal diameter due to contraction of trachealis muscle * Cough reflex: decreased diameter à increased pressure à ejection * Smooth muscle * Responds to neural signals * Contraction is more effective where there is no cartilage but a complete layer of smooth muscle, e.g. bronchioles * Allows control of resistance to airflow & distribution of air in lungs * Dilation of muscle via sympathetic stimulation * Elastic fibres * Allow passive stretching & then recoil; recoil a major contributor to quiet (passive) expiration ## Summary of Key Points * All large airways have a basic wall plan consisting of a mucosa, submucosa and adventitia. * Main wall components are: * Epithelium * Smooth muscle & seromucous glands of mucosa * Hyaline cartilage * With increasing proximity to the respiratory exchange surface, the airway walls reduce in thickness and complexity. ## Learning Summary * Correlated the histological structure of the trachea & bronchial tree with their role in movement of air to & from the lungs * Differentiated between the trachea, bronchi & bronchioles according to their structural & functional characteristics