Briefly describe the defence mechanisms of the lungs.

Outline:

·        Need for defense mechanisms

·        Inherent features of lining epithelium:

- goblet cells which secrete mucus

- cilia

·        Macrophages in the lungs

·        Reflexes: coughing and sneezing

Essay:

            Breathing is a continual process, involving the intake of air into the lungs via the respiratory passages and expiration of air. Air contains many particulate matter, dust, irritants and pathogens which come into contact with the respiratory tissues everyday. Therefore, there is a need for a defense mechanism to remove these particles to prevent them from clogging the airways and alveoli and to incapacitate any bacteria or air-borne virus before they enter the lungs or the bloodstream.

            Various mechanisms operate to prevent foreign matter from reaching the alveoli. The hairs in the nostrils strain out many particles larger than 10 mm in diameter. Most of the remaining particles of this size settle on mucous membranes of the nose and pharynx. All the respiratory passages from the nose to the terminal bronchioles are kept moist by a layer of mucus that coats the entire surface. The mucus is secreted partly by individual goblet cells in the epithelial lining of the passages and partly by small submucosal glands. The mucus traps particles out of the inspired air and keeps most of them from reaching the alveoli. Its immunoglobulins disable many inhaled microorganisms.

            The entire surface of the respiratory passages, both in the nose and in the lower passages down to the terminal bronchioles is lined with ciliated epithelium. The mucus layer is continuously moved towards the pharynx by the upward beating of the cilia, a process called the mucus escalator in which the cilia in the lungs beat upwards whereas those in the nose beat downwards. Once mucus reaches the pharynx, it is swallowed so that acid and enzymes in the stomach can destroy any remaining microorganisms.

            The pulmonary alveolar macrophages are important components of the pulmonary defense mechanisms. They are actively phagocytic and ingest inhaled bacteria and small particles. They also help process inhaled antigens for immunologic attack, and they secrete substances that attract granulocytes to the lungs as well as substances that stimulate granulocyte and monocyte formation in the bone marrow. When the macrophages ingest large amounts of the substances in cigarette smoke, they may also release lysosomal products into the extracellular space, causing inflammation.

            There are also protective reflexes originating from the lungs that respond to physical injury and to overinflation of the lungs (due to blockage by a foreign body).  The bronchi and trachea are so sensitive to light touch that excessive amount of foreign matter or other cause of irritation initiates the cough reflex. Afferent impulses pass from the respiratory passages through the vagi to the medulla. First, about 2.5 liters of air is inspired. Second , the epiglottis closes, and the vocal cords shut tightly to entrap the air within the lungs. Third, the abdominal muscles contract forcefully, pushing against the diaphragm while the internal intercostals also contract, increasing pressure in the lungs to 100 mmHg or more. Lastly, the vocal cords and epiglottis suddenly open widely, so that air under pressure in the lungs explodes outward. The rapidly moving air usually carries with it any foreign matter that is present in the bronchi.

            The sneeze reflex is very much like the cough reflex except that it is initiated in the nasal passages due to irritation. Afferent impulses pass in the fifth nerve to the medulla where the reflex is triggered. A series of reactions similar to those for the cough reflex takes place, however, the uvula is depressed so that large amounts of air pass rapidly through the nose, thus helping clear the nasal passages of foreign matter.