Respiratory System: Mechanism, Pathway, and Control

Mechanism of Ventilation

• Boyle's Law Application: The relationship between volume and pressure is inversely proportional $(V imes P = ext{constant})$. This principle is fundamental to breathing.
• Inhalation (Breathing In):
  • Diaphragm Contraction: The diaphragm flattens, and the intercostal muscles contract, causing the ribs to extend outwards.
  • Volume Increase: This action significantly increases the volume of the thoracic cavity.
  • Pressure Decrease: According to Boyle's Law, an increase in thoracic volume leads to a decrease in pressure within the lungs.
  • Air Movement (Law of Diffusion): Due to the resulting pressure differential, air moves from an area of higher pressure (outside the body) to an area of lower pressure (inside the lungs). Oxygen enters the lungs because its partial pressure is higher externally, while internal tissues have consumed much of the oxygen, creating a lower internal oxygen partial pressure.
• Exhalation (Breathing Out):
  • Muscle Relaxation: The diaphragm relaxes and moves upwards, and the intercostal muscles relax.
  • Lung Recoil: The lungs naturally recoil into their original size.
  • Volume Decrease: This process decreases the volume of the lungs and thoracic cavity.
  • Pressure Increase: Consequently, the pressure within the lungs increases.
  • Air Movement (Law of Diffusion): Air, primarily carrying carbon dioxide, moves from the higher pressure area inside the lungs to the lower pressure environment outside. Carbon dioxide leaves the body because its partial pressure is relatively high internally (due to cellular respiration) compared to the external environment.

Airway Pathway and Protection

• Pathway: Air enters through the nares (nose) or mouth, then proceeds into the pharynx (a shared area for both food and air), followed by the larynx, then the trachea, which branches into the bronchi, leading eventually to the alveoli.
• Protective Mechanisms in the Nares (Nose): The nasal cavity is equipped with several structures to humidify/dehumidify, filter air, and trap pathogens or foreign particles.
  • Nose Hairs: Act as initial filters for larger particles.
  • Adenoids: Lymphoid tissue that helps neutralize pathogens.
  • Mucus: Traps pathogens and particulate matter.
  • Nasal Conchae (Turbinates): Bony structures that increase surface area, aiding in filtering, humidifying (or dehumidifying) the air to body temperature, and preventing irritation or injury to the lower airways.
• Mucociliary Escalator: A crucial defense mechanism throughout the entire airway (from nasal cavity down to the bronchioles). Mucus continuously traps foreign particles and pathogens, and cilia (tiny hair-like projections) rhythmically beat to move this mucus upwards and out of the lungs. This process contributes to coughing and sneezing, which help expel trapped substances.

The Larynx and Airway Management

• Larynx (Voice Box): Serves as the entrance to the trachea and houses the vocal cords, which produce voice.
  • Glottis: The opening between the vocal cords; plays a vital role in preventing food from entering the trachea.
  • Vocal Cords: Are typically the entry point of the larynx and can be visualized using a laryngoscope.
• Laryngoscope: A medical tool used to visualize the airway, specifically the vocal cords, during procedures like endotracheal intubation, where an endotracheal tube is inserted through the vocal cords into the trachea.
• PCP (Primary Care Paramedic) Scope of Practice for Airway Management:
  • Infraglottic vs. Supraglottic: PCPs generally do not perform infraglottic intubation (inserting tubes past the vocal cords into the glottis) due to the increased risk of infection and damage to the patient.
  • PCP Airway Devices (Supraglottic and Adjuncts): PCPs are trained to use supraglottic devices and airway adjuncts, which are less invasive:
    • I-gels: Supraglottic airway devices that sit over the laryngeal opening.
    • OPAs (Oropharyngeal Airways): Maintain an open airway by displacing the tongue.
    • NPAs (Nasopharyngeal Airways): Inserted through the nostril to maintain an open airway.
  • Historical Device (Plumby Tube): An older device, now largely out of scope for PCPs due to causing more harm than good. It was a blindly inserted tube with two ports; if inflating one port caused the stomach to inflate, it indicated esophageal placement, and the other port would then be used. Despite the trachea being generally straighter, leading to a high success rate for tracheal placement, the risks outweighed the benefits.

Lower Airways: Trachea, Bronchi, and Alveoli

• Trachea (Windpipe):
  • A cartilaginous tube that extends from the larynx, carrying air downwards.
  • Lined with ciliary escalators and mucus to continue trapping pathogens and debris.
• Bronchi:
  • The trachea divides into two main stem bronchi (one for each lung), which then branch extensively into smaller and smaller airways, resembling an inverted tree structure.
• Alveoli:
  • Tiny, grape-like air sacs located at the terminal ends of the bronchioles.
  • Primary Site of Gas Exchange: Their thin walls and rich capillary network facilitate the exchange of oxygen and carbon dioxide between the air and blood.
  • Structure: Covered with a dense network of capillaries for gas exchange and surrounded by elastin fibers that allow them to expand and contract during breathing.
  • Surfactant: A lipoprotein substance lining the inner surface of the alveoli. Its crucial role is to reduce surface tension within the alveoli, preventing them from collapsing (atelectasis) or rupturing due to friction during expansion and contraction.

Neurological Control of Breathing

• Primary Stimulus: Contrary to common belief, the primary stimulus for breathing is not a decrease in oxygen, but rather an increase in the relative levels of carbon dioxide ($ext{CO}_2$) in the body.
• Chemoreceptors: Specialized sensory receptors that detect changes in chemical concentrations.
  • Central Chemoreceptors: Located in the Cerebrospinal Fluid (CSF) of the brain; highly sensitive to changes in $ext{CO}_2$ levels (specifically, the resultant pH change from carbonic acid).
  • Peripheral Chemoreceptors: Located in the aortic arch (and to a lesser extent, the carotid bodies and vertebral bodies); also sense changes in $ext{CO}_2$ levels (and to a lesser degree, oxygen and pH).

Normal Gas Levels and Chemoreceptors

• Normal $ext{PCO}_2$ (Partial Pressure of Carbon Dioxide): Between $35 ext{ to } 45 ext{ mmHg}$.
• Normal $ext{PO}_2$ (Partial Pressure of Oxygen): Between $80 ext{ to } 100 ext{ mmHg}$.
• Trigger for Breathing: An elevation in $ext{CO}<em>2$ above normal levels signals the brainstem (medulla oblongata and pons) to initiate or adjust breathing to expel excess $ext{CO}</em>2$.

Roles of Medulla Oblongata and Pons

• The medulla oblongata and pons in the brainstem work synergistically to control breathing.
• Medulla Oblongata: Primarily responsible for the actual act of breathing and regulating the rate of respiration. Damage to the medulla can lead to irregular breathing patterns.
• Pons: Modulates the breathing patterns, influencing the rhythm, depth, and quality of respiration. Damage to the pons can affect the breathing rhythm and pattern.
  • Pneumotaxic Center: (Located in the pons) Limits inspiration, leading to a faster breathing rate.
  • Apneustic Center: (Located in the pons) Prolongs inspiration, leading to a slower breathing rate.
• Summary: While the medulla focuses on the rate and initiation of breathing, the pons refines the rhythm and depth, ensuring coordinated and appropriate respiratory responses. Both are essential for maintaining effective ventilation.