- 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.
Respiratory Defense Mechanism