Respiratory System 6: Respiratory System Defences

Respiratory Defense Mechanisms

Introduction

The respiratory system faces constant exposure to potentially damaging agents such as:

• Irritants

• Particulates

• Pathogens

The primary goal is to protect the delicate tissues of the alveoli where gas exchange occurs. Defenses are crucial at the entry points (mouth and nose) to prevent damage to these sensitive tissues.

Nasal Passage Defenses

The nasal passage employs several defense mechanisms:

• Turbinates (baffles): These structures disrupt airflow, aiding in:

  • Trapping particles

  • Warming inhaled air

  • Humidifying inhaled air

• Lymphoid tissues: Located within the nasal passage, these tissues contain white blood cells that can:

  • Engage in phagocytosis

  • Activate immune responses

  • Deal with pathogens quickly

Trachea and Bronchi

Air travels down the trachea, through the bronchi, and past additional lymph nodes before reaching the alveoli.

• Alveolar Macrophages: These are present in the alveoli and act as a final line of defense.

• Trachea and Throat Tissues: The tissues lining the trachea and the back of the throat are composed of cells containing goblet cells (mucus-secreting cells).

Goblet Cells and Cilia

• Goblet Cells: These are mucus-secreting cells.

• Epithelial Tissue Type: Primarily pseudostratified and, in some regions, columnar.

• Cilia: Hair-like protrusions on epithelial cells that form the "brush border."

Mucus and Cilia Interaction

• Mucus Layer: Goblet cells secrete mucus that sits atop the cilia.

• Cilia Movement: Cilia rhythmically push the mucus towards the mouth and nose, driven by internal microskeletons.

• Particle Trapping: Inhaled particles and pathogens become trapped in the sticky mucus.

Mucus Consistency

The consistency of mucus is critical for effective defence:

• Ideal Consistency: The mucus needs to be:

  • Sticky enough to trap particles

  • Mobile enough for cilia to move it effectively

  • Not too runny, to prevent dripping into the alveoli

  • Not too thick, to prevent overwhelming the ciliary transport

Consequences of Improper Mucus Production

• Excessive Mucus: While trapping more effectively, it can:

  • Overwhelm the cilia

  • Fall into the alveoli

  • Disrupt gas exchange

• Gas Exchange Disruption: Mucus accumulation in the alveoli creates a barrier, hindering effective oxygen and carbon dioxide exchange.

Upper vs. Lower Respiratory Tract Defences

• Upper Respiratory Tract:

  • Nasal hairs trap particulates.

  • Turbinates warm, humidify, and trap inhaled particles.

  • Breathing through the nose provides enhanced defense compared to mouth breathing.

• Lower Respiratory Tract:

  • Ciliated cells and mucus-secreting goblet cells trap and remove particles.

• Alveoli:

  • Resident macrophages perform phagocytosis.

  • Macrophages clean up debris to maintain respiratory membrane function.