Respiratory System Introduction and Mechanics

Anatomical Sections and Lobes of the Lungs

• Lobular Visualization: The lungs are divided into specific lobes that can be viewed through different anatomical planes:

  • Left Upper Lobe: Visible in both coronal (frontal) and sagittal (side) sections.

  • Right Middle Lobe: One of the distinct segments of the right lung.

  • Left Lower Lobe: Pictured in both coronal and sagittal sections.

• Major Fissure: This anatomical landmark serves as the primary separator between the left upper lobe and the left lower lobe.

Microscopic anatomy of the Alveoli and Respiratory Tubes

• Pathway of Airflow: The respiratory system branches from bronchioles into smaller segments within the lungs, leading to:

  • Alveolar Ducts: Passageways connecting bronchioles to sacs.

  • Alveolar Sacs: Clusters of individual alveoli.

  • Individual Alveoli: The terminal ends of the respiratory tree where gas exchange occurs.

• Alveolar Characteristics:

  • Quantity: There are approximately $300,000,000$ alveoli in the human lungs.

  • Surface Area: This vast number of alveoli significantly increases the surface area available for gas exchange.

  • Capillary Network: Each individual alveolus is enveloped or enclosed by a network of blood capillaries.

  • Diffusion Barrier: The separation between the airspace and the bloodstream consists of only one layer of epithelium, specifically simple squamous epithelium, which facilitates easy diffusion.

• Surfactant and Lung Patency:

  • Definition: Surfactant is a sticky substance that acts as a film lining the alveoli.

  • Function: It creates the opportunity for alveoli to remain open during exhalation, preventing the lungs from completely deflating.

  • The Balloon Metaphor: If you blow up a balloon and let it go, it deflates completely and flies across the room. However, if you were to put a "sticky syrupy film" (like maple syrup) inside the balloon and shake it, letting go would not result in total deflation. The sticky film would keep part of the balloon inflated; this represents the role of surfactant in the lungs.

Cellular Composition of the Alveolar Space

• Type I Pneumocytes:

  • Tissue Type: Simple squamous epithelium.

  • Location: These cells create the perimeter of the alveoli.

  • Function: Responsible for the primary work of diffusion and gas exchange.

• Type II Pneumocytes:

  • Tissue Type: Simple cuboidal epithelium (Note: The transcript clarifies that while some sources may say simple columnar, they are actually simple cuboidal).

  • Location: Found at the corners of each of the alveoli.

  • Function: Act as surfactant cells, secreting the sticky substance (surfactant) and helping maintain the openings of the airways.

• Macrophages:

  • Location: Within the alveolar space.

  • Function: They serve as a defense mechanism against particulates that penetrate physical and chemical barriers (the first and second lines of defense) by phagocytizing pathogens or foreign matter.

• Practical Guide Labeling (Identification):

  • RV: Representing Respiratory Bronchioles.

  • AD: Representing Alveolar Ducts.

  • A and S: Representing Alveolar Sacs.

  • A: Representing individual Alveoli.

  • v (in red): Representing a vein or venule. In the lungs, these are colored red because they carry oxygen-rich blood.

Histological Overview of the Respiratory System

• Nose and Nasal Cavity: Comprised of pseudostratified columnar epithelium.

• Nasopharynx: Comprised of pseudostratified columnar epithelium.

• Mouth and Oropharynx: Comprised primarily of stratified squamous epithelium.

• Laryngopharynx: Comprised of stratified squamous epithelium.

• Trachea:

  • Tissue: Pseudostratified columnar epithelium with cilia.

  • Structures: Contains C-shaped rings of cartilage and smooth muscle.

• Bronchi:

  • Tissue: Pseudostratified columnar epithelium with cilia.

  • Structures: Contains O-shaped rings of cartilage and smooth muscle. This is the last location where cartilage is found in the respiratory tree.

• Bronchioles:

  • Tissue: Simple columnar epithelium.

  • Structures: Contains some smooth muscle but lacks any cartilage.

• Alveoli:

  • Composition: Simple squamous epithelium (Type I), simple cuboidal epithelium (Type II), and macrophages.

Pulmonary and Bronchial Circulatory Supply

• Pulmonary Circuit (For Gas Exchange):

  1. Pulmonary Arteries: Carry deoxygenated (oxygen-poor) blood from the right ventricle to the lungs.

  2. Pulmonary Arterioles: Branch off the arteries toward the alveoli.

  3. Pulmonary Capillaries: Surrounding the alveoli; the site where $CO_2$ is released and $O_2$ is acquired/bound to red blood cells.

  4. Pulmonary Venules: Pick up oxygen-rich blood from the capillaries.

  5. Pulmonary Veins: Return oxygen-rich blood to the left atrium of the heart.

• Bronchial Arteries (Tissue Supply): These are the main blood supply to the actual tissues of the lung. They enter through the hilum (the centermost portion of each lung).

Mechanics of Ventilation: Muscles and Phases

• Phases of Breathing:

  • Inspiration (Inhalation): An active process that brings air into the lungs.

    • The diaphragm and intercostal muscles contract.

    • The diaphragm flattens, and the rib cage moves upward and outward.

    • This increases thoracic volume, causing lung volume to increase and air pressure within the lungs to decrease, allowing air to flow in.

  • Expiration (Exhalation): Usually a passive process that expels air.

    • The diaphragm and intercostal muscles relax.

    • The diaphragm moves upward into a dome shape; the rib cage moves downward and inward.

    • Thoracic and lung volume decrease, increasing internal pressure and forcing air out.

• Muscles of Respiration:

  • Inspiration Muscles: External intercostals and the diaphragm.

  • Expiration Muscles (especially forced): Internal intercostals, external abdominal obliques, internal obliques, transverse abdominis, and rectus abdominis.

  • Core Stability: Athletes like runners and swimmers often have highly defined core muscles due to the frequent use of these muscles for forced inspiration and expiration during aerobic exercise.

Respiratory Volumes and Spirometry

• Tidal Volume: The small amount of air moving in and out with each normal, resting breath.

• Inspiratory Reserve Volume (IRV): The increased volume of air that can be inhaled after a normal inhalation; requires increased effort to fill the lungs completely.

• Expiratory Reserve Volume (ERV): The increased volume of air that can be forced out after a normal exhalation; requires significant effort.

• Residual Volume: Air that remains in the lungs even after maximum exhalation. This is maintained by surfactant in the alveoli.

• Vital Capacity (VC): The maximum amount of air that can be moved in plus the maximum amount that can be moved out during one breath.

  • $VC = \text{Inspiratory Reserve} + \text{Expiratory Reserve} + \text{Tidal Volume}$

• Total Lung Capacity: The sum of all volumes, including residual volume.

  • $\text{Total Lung Capacity} = VC + \text{Residual Volume}$

• Spirometer: A device used to assess forced vital capacity.

  • Procedure: The patient takes a full breath and forces air into the tube for $6$ seconds.

  • Clinical Use: Used to diagnose or monitor COPD, chronic bronchitis, and asthma.

  • Therapeutic Assessment: Doctors may administer Albuterol and re-test to see if vital capacity or forced expiratory reserve improves, indicating a desirable therapeutic result.

Neurological and Chemical Control of Breathing

• Nervous System Control:

  • Respiratory Control Center: Located in the Medulla Oblongata and several regions within the Pons.

  • Phrenic Nerve: The Medulla Oblongata sends nerve impulses along the phrenic nerve to innervate the diaphragm, causing it to contract for inspiration.

• Sudden Infant Death Syndrome (SIDS):

  • Theory: Thought to occurs when the center in the Medulla Oblongata stops sending nerve signals.

  • Risk Factors: High prevalence in premature infants (born as early as $2$, $3$, $10$, or $15$ weeks premature).

  • Prevention: Advocacy for "Safe Sleep" — infants should sleep alone, on their backs, in a crib with no other items.

• Chemical Control:

  • Chemoreceptors: Two sets of sensors located in the brain and the circulatory system.

  • Trigger: They sense a drop in pH caused by levels of carbon dioxide ($CO_2$) resulting from metabolism.