Respiratory Defense Mechanism

• The lungs remain protected from various inhaled particles, microorganisms and toxic gases by the respiratory defense mechanism.
• There are various ways by which this mechanism works such as filtration of the air, the muco-ciliary clearance system, the cough reflex, reflex broncho-constriction and alveolar macrophages.

A) Filtration of air

• Presence of nasal hairs keeps the inspired air filtered through out the life.
• In addition, the increase air turbulence due to the abrupt changes in direction of air flow that occur as the air moves through the naso-pharynx and larynx also results in the contact of particles and bacteria with the mucosa, lining these structures.
• Particles larger than 5 microns are less dangerous as they are removed in the naso-pharynx or bronchi and are unable to reach the alveoli.
• The velocity of airflow slows greatly as it passes the larynx which facilitates the deposition of smaller particles of size 1-5 microns.
• They settle out the way sand does in a river which is called sedimentation.
• Particles that are of size less than 1 micron are too small to settle in this manner and are deposited in the alveoli.
• Coal dust can lead to pneumoconiosis which is an example of smaller size particles.

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B) Muco-ciliary Clearance System

• Muco-ciliary escalator which is also known as muco-ciliary clearance system accomplishes the movement of mucus below the larynx.
• This term relates the secretion of mucus and the ciliary activity.
• Mucus is continuously secreted by globlet cells and sub-mucosal glands at a rate of about 100 ml/day which forms a mucus blanket that contains the impacted particles and debris from distal lung areas.
• The small amount of mucus secreted is swallowed without being noticed normally.
• Bacteria and viruses are controlled by secretory immunoglobulin A that is present in the mucus.
• Cilia are present in the airways from the level of trachea to the respiratory bronchioles.
• Each ciliated cell contains around 200 cilia which beat in rhythmic way around 1000 times per minute in the large airways that helps in moving the mucus towards the mouth.
• The ciliary beat rate slower as we move down the tracheo-bronchial tree.
• The particles that penetrate more deeply into the airways are removed less rapidly due to the above reason.
• The ciliary action is impaired by dehydration, smoking, inhalation of high oxygen concentrations, infections, and ingestion of drugs such as atropine, anesthetics, alcohol and cocaine.
• Lower respiratory tract infections are repeated in the patients with COPD and cystic fibrosis.
• Cilia are often destroyed during these infections which result in impaired secretion clearance, a chronic productive cough and chronic colonization by bacteria that leads to frequent respiratory tract infections.

C) Cough Reflex

• It is one of the protective reflex-action which clears the airway by a high-pressure, high-velocity flow of air.
• It is considered to be as a backup for mucociliary clearance if this system is overwhelmed or ineffective.
• Removal of secretions above the sub-segmental level (large or main airways) is only possible by coughing.
• Secretions below this level must be moved upward by the mucociliary mechanism before they can be removed by coughing.

D) Reflex Broncho-constriction

• It is another defense mechanism where the bronchi constrict in an effort to prevent entry of the irritants like dusts and aerosols that are inhaled in large amount.
• A person with hyperactive airways like in asthma may experience broncho-constriction after inhalation of triggers such as cold air, perfume or other strong odors.

E) Alveolar Macrophages

• Alveolar macrophages are the primary defense mechanism at the alveolar level as the ciliated cells are not found below the level of respiratory bronchioles.

• The inhaled foreign particles like bacteria are rapidly phagocytized by alveolar macrophages.
• The debris after the phagocytosis is moved up-to the level of the bronchioles so that they can be removed by cilia or removed from the lungs by lymphatic system.
• The inflammatory responses get stimulated if the particles (coal, dust, silica) cannot be phagocytized which remains in the lungs for indefinite periods.
• The smoker who is employed in an occupation with heavy dust exposure is at higher risk for lung disease as the alveolar macrophages activity gets impaired by cigarette smoke.

References:

i) https://err.ersjournals.com/content/14/95/45

ii) https://link.springer.com/chapter/10.1007/978-1-4899-1063-9_1?noAccess=true

iii) https://www.atsjournals.org/doi/abs/10.1164/arrd.1977.115.3.479?journalCode=arrd

Respiratory Defense Mechanism