Airways & Alveoli

Upper Airway Terminology

  • Physiologic or Physiological:

    • Relating to the normal functions of the body

    • Indicates processes that occur in a healthy state.

  • Pathophysiological:

    • Refers to disease processes or abnormal functions in the body.

  • Dyspnea:

    • Medical term for shortness of breath.

    • Considered subjective, as it reflects patient experiences:

    • Patient may express difficulty in breathing, e.g., "I can't breathe".

    • Signs of respiratory distress may include:

    • Increased work of breathing; accessory muscle use; retractions; increased respiratory rate.

    • Borg Dyspnea Scale:

    • Tool for assessing dyspnea severity on a scale from 0 (not breathless) to 10 (extremely severe shortness of breath).

Introduction

  • The conducting airways connect atmospheric air with the gas-exchanging regions of the lungs.

    • Conducting airways: No gas exchange occurs; they only serve as pathways for air to reach the gas-exchanging surfaces of the lungs.

    • As air travels, it is warmed, humidified, and filtered by the upper conducting airways.

Upper Airways Anatomy

  • Components of the Upper Airways:

    • Nose

    • Oral Cavity

    • Pharynx

    • Larynx

    • The larynx signifies the transition from upper to lower airways.

  • The Pharynx (throat):

    • Divided into three sections:

    1. Nasopharynx

    2. Oropharynx

    3. Laryngopharynx

Respiratory Therapists and Upper Airway Knowledge

  • Responsibilities of Respiratory Therapists (RTs):

    • Differentiate between upper and lower airway disorders.

    • Perform endotracheal intubation (inserting an artificial airway into the trachea).

    • Replace functions when the upper airway is bypassed by an artificial airway.

    • Determine when to utilize sterile procedures when entering lower airways.

Main Functions of the Upper Airway

  1. Protection from Aspiration:

    • Includes pharyngeal/gag reflex and laryngeal reflex (vocal cord adduction).

  2. Conditioning the Air:

    • Involves warming, humidifying, and filtering inspired air.

  3. Pathway for Conducting Airflow:

    • Provides a route for air to flow from the atmosphere to the lungs.

Differences Between Respiration and Ventilation

  • Respiration:

    • Process of gas exchange at the cellular level.

    • External Respiration: Exchange of oxygen and carbon dioxide between the alveoli and pulmonary capillary blood.

    • Internal Respiration: Exchange of gases where oxygen is made available to the body's cells, and carbon dioxide is removed.

  • Ventilation:

    • Mechanical movement of air into and out of the body.

    • Inhalation: Air enters, bringing oxygen into the body.

    • Exhalation: Air exits, removing carbon dioxide (a metabolic byproduct).

    • Measurement of Ventilation:

    • Evaluated by assessing carbon dioxide (CO2) levels in the blood or measuring exhaled CO2.

    • Hypoventilation: Insufficient ventilation leads to elevated CO2.

    • Hyperventilation: Excessive ventilation results in low CO2 levels.

      • Summary:

      • Increased ventilation = Decreased CO2 in the blood

      • Decreased ventilation = Increased CO2 in the blood

Anatomy of the Upper Airway: The Nose & Nasal Cavity

  • Functions of the Nose & Nasal Cavity:

    • Warm, humidify, and filter inspired air.

    • Air Entry Point:

    • Through nostrils (nasal vestibules).

    • Naress:

    • Two openings at the end of the exterior nose, separated by the nasal septum (composed of bone & cartilage).

    • Vibrissae:

    • Hairlike structures inside the nares that filter large particles from the inspired air.

    • Effectiveness: Filters out particles larger than 6 μm in diameter.

Nasal Conchae (Nasal Turbinates)

  • Nasal Conchae:

    • Three irregular layers that protrude into the nasal cavity.

    • Functions:

    • Separate inspired air into multiple airstreams, increasing contact surface area with warm, moist nasal mucosa.

    • Nasal Mucosa:

    • Rich in blood vessels for warming air and nerve endings that trigger protective responses like sneezing.

The Sinuses

  • Sinus: Air-filled cavity within a bone or part of the body.

  • Paranasal Sinuses:

    • Two groups located on each side of the face and skull draining into the nasal cavity.

    • Functions:

    • Act as resonance spaces for speech

    • Produce mucus, lined with pseudostratified ciliated columnar epithelium.

    • Types of sinuses:

    • Frontal sinuses

    • Sphenoid sinuses

    • Ethmoid sinuses

    • Maxillary sinuses

Conditions of the Nose

  • Nasal Congestion:

    • Inflammatory response leading to swelling in nasal cavity (edema) and excess mucus production, often from upper respiratory infections.

  • Epistaxis:

    • Nosebleed.

  • Nasal Flaring:

    • Occurs when nostrils widen during inhalation, indicative of respiratory distress, commonly assessed in newborns.

  • Nasal Polyps:

    • Noncancerous growths in the nasal cavity or sinuses, potentially related to chronic inflammation, asthma, and allergies.

  • Rhinitis (Runny Nose):

    • Inflamed mucous membranes in the nasal cavity, which may be allergic, nonallergic, or infection-related.

  • Sinusitis:

    • Inflammation of sinus cavities, which may be triggered by bacterial, fungal, or viral infections.

The Oral Cavity

  • Components of the Oral Cavity:

    • Includes vestibule, teeth, tongue, hard palate, soft palate, palatoglossal arch, palatine tonsil, palatopharyngeal arch, uvula, and tongue.

    • Vestibule: Space between lips, cheeks, gums, and teeth.

    • Dentistry:

    • Children typically have 20 deciduous teeth replaced by 32 permanent teeth in adults, comprising:

      • 4 incisors, 2 canines, 4 premolars, and 6 molars in both upper and lower jaws.

Conditions of the Oral Cavity

  • Oral Candidiasis (Thrush):

    • Infection caused by Candida albicans (fungus).

    • Increased risk associated with inhaled steroids; mouth rinsing advised after steroid use to mitigate risk.

The Pharynx (Throat)

  • The pharynx extends from the back of the nasal cavity down to the larynx.

  • Divided into three sections:

    1. Nasopharynx: Back of nasal cavity to the soft palate.

    1. Oropharynx: From soft palate to superior border of the epiglottis.

    • Contains vallecula: A small indentation between tongue and epiglottis, significant during intubation to visualize the vocal cords.

    1. Laryngopharynx: Base of the tongue to the vocal cords.

  • Pharyngeal Reflex (Gag Reflex): A contraction triggered by an object touching the hard or soft palate or oropharynx, to protect against choking/aspiration.

The Larynx

  • Essential for preventing aspiration of foreign materials into the trachea.

  • Composed of:

    • Single Cartilages: Thyroid, cricoid, and epiglottic.

    • Paired Cartilages: Arytenoid, corniculate, and cuneiform.

    • Cricoid Cartilage: Only complete ring and lies below thyroid.

      • -

    • Thyroid Cartilage: Largest; composed of two rectangles at the midline.

    • The glottic opening: Narrowest part of the adult airway.

    • The epiglottis: A cartilage piece above the larynx, used during intubation.

Functions of the Larynx Muscles

  • Posterior Cricoarytenoid Muscles:

    • Open the rima glottidis (glottis) by rotating arytenoid cartilages laterally.

  • Transverse Arytenoid Muscle:

    • Closes (adducts) the glottis by pulling arytenoid cartilages together.

Conditions of the Larynx & Pharynx

  • Airway Edema:

    • Caused by allergic responses, trauma, infection, or inhalation of noxious gases, deemed potentially life-threatening.

  • Stridor:

    • A high-pitched sound indicating upper airway edema.

  • Croup (Laryngotracheobronchitis, LTB):

    • Characterized by inspiratory stridor and 'seal bark' cough caused by subglottic edema, typically viral in nature.

  • Epiglottitis:

    • Inflammation of the epiglottis, can be life-threatening due to potential airway obstruction.

    • Symptoms: Present with the four D’s: Dysphagia, Dysphonia, Drooling, Distress.

  • Grunting:

    • A sign of respiratory distress in neonates, noted during exhalation to increase lung volume.

Histology of the Respiratory System

  • Epithelium: Lining of cavities classified by layers and shape:

    • Cuboidal Epithelium: Square cell appearance.

    • Columnar Epithelium: Taller than wide.

    • Pseudostratified Columnar Epithelium: Appears layered due to nuclei positioning but is a single layer.

    • Found in posterior 2/3 of the nasal cavity and tracheobronchial tree.

    • Simple Squamous Epithelium: Flattened cells lining alveoli.

    • Stratified Squamous Epithelium: Layered cells, keratinized or not.

Mucociliary Clearance Mechanism

  • Each ciliated airway cell has ceils beating at around 1300 times per minute, moving a mucus sheet toward the pharynx at a rate of 2 cm/min.

  • Gel Layer: Traps microbes and particles; cilia quickly propel mucus forward.

Mucociliary Function and Composition

  • Normal function is critical for lung clearance (mucociliary escalator).

  • Conditions affecting mucociliary function include:

    • Dehydration: Mucous becomes thick and immobile.

    • Overhydration: Results in thin, watery mucus, impairing ciliary transport.

    • Mucus Production: Normal production is approximately 100 mL/day, which may increase with inflammation.

    • Conditions: Includes chronic bronchitis, asthma, pneumonitis, and cystic fibrosis leading to sticky mucus and impaired clearance.

  • Immotile Cilia Syndrome: Genetic disorder leading to lack of normal ciliary function, causing recurrent infections and potentially bronchiectasis.

Optimal Conditions for Mucociliary Clearance

  • Gas should be warmed to body temperature (37℃) and humidified to 100% relative humidity.

    • Relative Humidity (RH): The amount of water vapor according to gas capacity at a given temp.

    • At 37℃, gas capacity is 44 mg/L for water vapor.

Endotracheal Intubation and Tracheostomy

  • Endotracheal Intubation:

    • Insertion of an artificial airway through the nose or mouth, bypassing upper airway functions.

  • Tracheostomy:

    • Insertion through a surgical incision into the trachea, creating a stoma and also bypassing the upper airway.

The Lower Airways: Introduction

  • Airways below the level of the larynx referred to as lower airways.

  • These structures branch in an inverted treelike fashion toward the alveoli, termed the tracheobronchial tree.

The Trachea

  • Trachea: Largest airway beneath the larynx, extending from C6 to T5, approximately 11-13 cm in length.

    • Contains 16-20 C-shaped cartilage rings with the open part facing posteriorly, completed by a membrane with the trachealis muscle.

    • Lining: Pseudostratified ciliated columnar epithelium with goblet cells.

Cartilaginous vs. Noncartilaginous Airways

  • Cartilaginous Airways: Includes trachea, main stem bronchi, lobar, segmental, and subsegmental bronchi.

  • Noncartilaginous Airways: Includes bronchioles and terminal bronchioles, which lack cartilage and are surrounded by smooth muscle, susceptible to bronchospasm.

Conditions of the Trachea

  • Tracheomalacia: Weakness in walls leading to collapse; congenital or acquired.

  • Tracheal Stenosis: Narrowing of the airway; may also be congenital or acquired.

Mainstem Bronchi

  • Carina: Point where the trachea bifurcates into left and right main stem bronchi at approximately T5.

    • Right bronchus: Wider and shorter; 20° to 30° angle.

    • Left bronchus: Smaller diameter, longer; 45° to 55° angle; higher aspiration risk with endotracheal displacement.

Endotracheal Tube Placement

  • Placement Guidelines: The endotracheal tube tip should be 4 to 6 cm above the carina, as viewed on chest radiographs.

Lobar, Segmental, and Subsegmental Bronchi

  • The division of main stem bronchi into lobar bronchi begins airway generations, differentiating right (3 lobar) and left (2 lobar) branches.

  • Isothermic Saturation Boundary (ISB): Point at which inspired air reaches body temperature (37°C) and optimal humidity.

    • Affected by mouth breathing, cold air inhalation, and the need for gas humidification after bypassing upper airway.

The Bronchioles and Terminal Bronchioles

  • Bronchioles: Less than 1 mm in diameter, lack cartilage, surrounded by elastic fibers aiding airway stability.

  • Terminal Bronchioles: Final generation in the conducting zone, about 0.5 mm in diameter.

    • Changes: Cilia and mucus-producing cells disappear, epithelium becomes cuboidal.

    • Canals of Lambert: Provide collateral ventilation.

The Alveoli: The Respiratory Zone (Acini)

  • The respiratory zone consists of structures responsible for gas exchange:

    1. Respiratory bronchioles

    2. Alveolar ducts

    3. Alveolar sacs

  • Alveolar Count: Adults have over 300 million alveoli, providing significant surface area for gas exchange equivalent to half a tennis court.

Alveolar Cells (Pneumocytes)

  • Type I cells: Squamous pneumocytes; primary site for gas exchange.

  • Type II cells: Granular pneumocytes; produce surfactant (reduces surface tension); principal phospholipid is DPPC (dipalmitoylphosphatidylcholine).

  • Alveolar Macrophages (Type III): Scavengers; phagocytose pathogens.

  • Fibroblast Cells: Involved in lung repair/fibrosis processes.

The Alveolar-Capillary Membrane

  • Site for external respiration where gas exchange occurs:

    • O2 diffuses into blood, CO2 diffuses into alveoli for exhalation.

    • AC membrane thickness is ~0.5 μm, with permeability being crucial for adequate gas diffusion.

  • Any loss of alveolar area or thickening of this membrane decreases O2 diffusion, leading to hypoxemia.