Protective barriers in the lungs
It is not difficult to imagine what would happen if the smallest particles of dust, aerosols, various microorganisms contained in the inhaled air fall into the lungs. Very soon, their delicate and extremely vulnerable tissue would collapse: after all, the wall thickness of the alveoli – microscopic vesicles – ranges from 0.1 to 4 micrometers. However, thanks to the system of protective mechanisms that stand along the entire path of the air stream from the moment of inhalation up to the alveoli, this does not happen, and the air enters the lungs sufficiently purified from harmful impurities and even warmed up.
The first filter through which the inhaled air passes can be seen with the naked eye, looking at yourself in the mirror: these are the hairs at the beginning of the nasal openings that trap large dust particles suspended in the air. The details of the next filter – the mucous membrane of the nasal cavity – can only be examined with a microscope. By the way, this picture is surprisingly beautiful: under a scanning electron microscope, the mucous membrane looks like a swaying field of rye or wheat. Such an impression is created by ciliated epithelial cells, covering the entire mucous membrane of the airways throughout. Each ciliated cell has up to 200 cilia with a length of 3 to 7 micrometers, producing 160 to 250 vibrations per minute. The movements of all cilia are strictly coordinated: they fluctuate towards the nasopharynx, constantly pushing there the nasal mucus produced by the nasal mucosa.
On the mucous membrane of the winding walls of the nasal cavity, up to 40% of various foreign particles contained in the air settle, and only relatively large particles with a size of more than 50 micrometers are retained here, while smaller ones penetrate deeper into the trachea, bronchi, where air purification continues as a result of functions of the ciliary epithelium (cilia of epithelial cells move from bottom to top, pushing the secret into the nasopharynx), and due to the presence of tracheobromchial secretion. It is produced by the mucous glands located in the wall of the trachea and bronchi, as well as the goblet cells of the epithelium covering the mucous membrane of these organs. Antibodies, immunoglobulins, proteins, carbohydrates, lipids, enzymes – this is not a complete list of the components of this secret. But with its antiviral and antibacterial properties, the tracheobronchial secret is primarily due, perhaps, to substances such as lysozyme, interferon, lactofirrin and pyrogens.
Lysozyme destroys the cell membrane of microbes, making them nonviable. This substance is a rather effective weapon against pathogens, and when its content in the tracheobronchial secretion decreases, favorable conditions are created for the development of infection.
Another important component of the secret – interferon provides nonspecific protection of the respiratory system. It acts mainly on body cells, making them inaccessible to pathogens. It is no accident that interferon preparations are widely used in medical practice as a prophylactic against influenza; Interferon is also effective in the initial stages of the development of acute respiratory viral infections.
Endogenous pyrogens, which are part of the tracheobronchial secretion, cause the development of a protective febrile reaction: an increase in body temperature by 2 – 3 degrees is detrimental to many bacteria and viruses. And no less significantly, an increase in temperature stimulates the production of interferon by the cells of the body. In this regard, I want to remind once again: do not take it immediately, as soon as the temperature has risen, aspirin, amidopyrine and other antipyretic drugs. Let the body’s natural defense factors work at full strength!
Together with ciliated epithelium and tracheobronchial secret, the phagocytic defense system is also on guard of the purity of air entering the lungs. It turns on when pathogens and viruses, bypassing all previous barriers, reach the alveoli. Here they are waiting for alveolar macrophages and neutrophils – orderlies. They actively move and pounce on foreign microorganisms, absorb and digest them.
In the lungs, there is also an immunological defense system, which, as in other organs, is provided by lymphoid cells. Throughout the airways in the mucous and submucous membranes are accumulations of lymphoid cells – lymph – epithelial nodules: small bronchi are especially rich in them. Nodule cells in response to the invasion of bacteria and viruses produce antibodies that neutralize pathogens. When there are few aggressors, lympho – epithelial nodules cope on their own. In case of mass infection, the immune system – regional lymph nodes – is included in the protective reactions. thymus, red bone marrow, spleen.
Finally, the most recent protective barrier is a surfactant.