Lecture Monday 12.1-12.3

Anatomical and Functional Divisions of the Respiratory System

The respiratory system is structured with a significant design overlap where the airway and the digestive opening share local anatomy (the pharynx). The system is categorized through two primary lens: anatomical location and physiological function.

• Anatomical Classification:

  • Upper Respiratory Tract: Includes the nose, nasal cavity, pharynx (the throat), and the larynx (voice box). The larynx serves as the anatomical cutoff between the upper and lower systems.
  • Lower Respiratory Tract: Extends from the trachea (windpipe) down to the bronchial trees, which further branch into bronchioles and conclude in the microscopic alveolar sacs.

• Functional Classification:

  • Conducting Zone: Comprised of organs that serve as conduits to move air in and out of the lungs. This zone includes all structures from the nose down to the terminal bronchioles.
  • Respiratory Zone: The specific site where gas exchange occurs between the blood and the air. This begins at the respiratory bronchioles, which have walls thin enough ($simple\,squamous\,epithelium$) for minor gas exchange, and includes the alveolar sacs.

The Four Primary Processes of Respiration

Respiration involves a coordinated sequence of four distinct processes to manage oxygenation and carbon dioxide levels:

1. Pulmonary Ventilation: The physical movement of air in ($Inhalation/Inspiration$) and out ($Exhalation/Expiration$) of the lungs.
2. Alveolar Gas Exchange: The movement of gases between the air in the alveoli and the blood within the pulmonary capillaries. Deoxygenated blood picks up $O_2$ and releases $CO_2$.
3. Gas Transport: The movement of oxygenated and deoxygenated gases through the blood via the systemic and pulmonary circuits.
4. Internal Gas Exchange: The exchange of gases between the systemic capillaries and the body's tissues (e.g., brain, myocardium, digestive tract). The blood drops off $O_2$ and picks up waste $CO_2$.

Auxiliary Functions of the Respiratory System

Beyond gas exchange, the system performs several vital non-respiratory tasks:

• Vocalization and Phonation: Coordinating the diaphragm, vocal folds, tongue, and lips to produce speech.
• Olfaction: Detection of odors via the olfactory membrane located in the upper nasal cavity.
• Abdominal Pressure (Valsalva Maneuver): Helping to expel contents from the abdominopelvic cavity (e.g., during defecation, urination, or childbirth) by "bearing down." Overuse can affect the "respiratory pump" and heart preload, potentially causing fainting.
• Venous Return: Facilitating the return of blood to the heart through the respiratory pump mechanism.
• Acid-Base Homeostasis: Maintaining blood $pH$; for example, holding one's breath causes the blood to become acidic ($lowers\,pH$) due to $CO_2$ buildup.
• Blood Pressure Maintenance: Assisting in the production of Angiotensin II, a hormone that acts as a powerful vasoconstrictor and increases heart rate to maintain blood pressure during extreme dehydration.

Detailed Anatomy: Nose, Pharynx, and Larynx

• Nasal Cavity and Conchae:

  • The Nasal Conchae (also known as Turbinates) include the inferior nasal concha and protrusions of the ethmoid bone.
  • Function: They cause inhaled air to spin through the mucous membrane, warming it and cleaning it via hair and mucus. Warming is critical because cold air can cause pulmonary blood vessels to vasoconstrict, hindering blood flow to the lungs.

• Pharynx (The Throat):

  • A vertical tube divided into three regions: Nasopharynx, Oropharynx, and Laryngopharynx.
  • The Nasopharynx is strictly respiratory. The Oropharynx and Laryngopharynx serve both respiratory and digestive functions.
  • Histology: Most of the airway is lined with pseudostratified ciliated columnar epithelium to trap debris. However, the pharynx is lined with stratified squamous epithelium to protect against mechanical stress from swallowing (e.g., the thermal burn from "flaming hot Cheetos").

• Larynx (The Voice Box):

  • It sits at the opening of the trachea and is composed of solid cartilage.
  • Epiglottis: A flap of elastic cartilage that closes the airway during swallowing to divert food into the posterior esophagus.
  • Hyoid Bone: The only bone not attached to another bone; it is tethered to the muscles of the tongue and the cartilage of the larynx.
  • Glottis: The opening between the vocal folds. Sound pitch is determined by the tension of the muscles; high tension creates high pitch, while relaxation creates low pitch. Volume is controlled by the force of air expelled.

The Trachea and Bronchial Tree

• Trachea: Kept open (patent) by $C-shaped$ cartilaginous rings (incomplete rings of hyaline cartilage). The posterior opening allows the esophagus to expand during the passage of a bolus.
• Carina: The specialized cartilage ridge at the base of the trachea where it splits into the right and left bronchi. It is highly sensitive; stimulation triggers a deep, forceful cough.
• Bronchial Branching:
  • Primary Bronchi: One for each lung (Left and Right).
  • Secondary (Segmental) Bronchi: The right lung has $3$ lobes and $3$ secondary bronchi. The left lung has $2$ lobes and $2$ secondary bronchi (to accommodate the heart).
  • Tertiary Bronchi: Supply specific segments of the lung. Each segment has its own dedicated artery and vein, making it possible to surgically remove a single diseased segment without disturbing surrounding tissue.

Bronchioles and Alveolar Cell Types

• Bronchioles: As airways get smaller, the cartilage disappears and the amount of smooth muscle increases. This allows for bronchoconstriction (parasympathetic) and bronchodilation (sympathetic).

  • Terminal Bronchioles: The end of the conducting zone.
  • Respiratory Bronchioles: The start of the respiratory zone.

• Alveoli: Microscopic air bubbles where gas exchange occurs. They contain three cell types:

  1. Alveolar Type 1 Cells: Simple squamous epithelial cells that are extremely flat and provide a thin surface for rapid diffusion ($nonpolar\,gases$).
  2. Alveolar Type 2 Cells (Surfactant Cells): Make up roughly $10\%$ of cells. They secrete Surfactant, a lipid-based molecule that breaks the surface tension of water in the alveoli. Without surfactant, the water molecules would stick together and cause the tiny sacs to collapse. Surfactant has a distinct metallic taste, similar to "pennies."
  3. Alveolar Macrophages: Monocyte-derived immune cells that consume debris, bacteria, and viruses that bypass the upper filters.

The Respiratory Membrane and Pleura

• Respiratory Membrane: The physical barrier gases must cross. It consists of the Alveolar Type 1 cell, a thin basal lamina, and the capillary endothelial cell. Scarring or thickening (due to tar or disease) impairs oxygenation and $CO_2$ expulsion.
• The Pleura: A serous membrane with a visceral layer (on the lung) and a parietal layer (on the thoracic wall). The Pleural Cavity contains a thin layer of serous fluid.
• Pleuritis: Inflammation of this membrane can lead to fluid buildup and lung collapse.

Boyle’s Law and the Mechanics of Ventilation

Pulmonary ventilation is governed by Boyle’s Law, which states that at a constant temperature and constant number of gas molecules, the pressure and volume of a gas are inversely related: $P \propto \frac{1}{V}$.

• Inspiration:

  1. The volume of the thoracic cavity increases.
  2. Pressure inside the lungs (intrapulmonary pressure) decreases below atmospheric pressure ($760\,\text{mmHg}$).
  3. Air flows down its pressure gradient into the lungs.

• Expiration:

  1. The volume of the thoracic cavity decreases.
  2. Intrapulmonary pressure increases above atmospheric pressure.
  3. Air is pushed out of the lungs.

• Pressure Dynamics:

  • Intrapleural Pressure: The pressure within the pleural cavity. It must stay approximately $4\,\text{mmHg}$ lower than the intrapulmonary pressure to act like a vacuum and keep the lungs inflated.
  • Pneumothorax: If the pleural cavity is breached (e.g., a stab wound), air enters the space, the intrapleural pressure equilibrates with atmospheric pressure, and the lung collapses.

Muscles of Respiration

• Diaphragm: The primary muscle of breathing. It is dome-shaped.
  • Contraction: It flattens and moves down, increasing thoracic volume (Inspiration).
  • Relaxation: It returns to its dome shape, pushed by abdominal organs (Passive Expiration).
• Inspiratory Muscles: Diaphragm and External Intercostals (which elevate the ribs). Accessory muscles include the Sternocleidomastoid and Serratus Anterior.
• Expiratory Muscles: Normal expiration is passive (relaxation). Forced expiration uses the Internal Intercostals and abdominal muscles to squeeze the rib cage inward.

Non-Respiratory Air Movements

Reflexes in the brain (specifically the Pons) control specific air movements:

• Sighing: A slow, deep inspiration triggered when the lungs are shallow breathing for too long to prevent ischemia in closed-off bronchioles.
• Yawning: A larger sigh triggered by more extensive lung ischemia or fatigue.
• Sneezing: Triggered by irritants in the nasal cavity to expel air through the nose.
• Coughing: Triggered by irritants in the larynx, trachea, or carina to expel air through the mouth.

Questions & Discussion

• Tuberculosis (TB) Case Study: Discussion centered on a clinical case (Vanessa) where a patient was misdiagnosed at Centennial Hills Hospital after exposure in Mexico. TB is a bacterium that is often asymptomatic but can move to other organs like the kidneys. It is unique because it thrives when consumed by macrophages. It can cross the placenta, which was fatal in the discussed case. High-risk factors include proximity to infected individuals and suppressed immune systems (e.g., during pregnancy).