The respiratory tract has multiple self-protection mechanisms

Babies begin to cry when they are born. Crying expands their respiratory tract and their lungs suddenly swell up like balloons. After that, they will continue to breathe until the last moment of their lives. The process of human breathing seems simple but is closely linked, ensuring that every cell obtains the selfless gift of nature - oxygen. Oxygen supplies cell energy through biological oxidation and produces metabolites such as carbon dioxide. In extreme cases, humans may not eat for 7 days and not drink water for 3 days, but they cannot go without breathing for 5 minutes. If the brain is deprived of blood and oxygen for 5 minutes, irreversible necrosis will occur.

In a quiet state, an average adult breathes 16 times per minute, inhaling 500 ml of air (containing about 21% oxygen) each time. The whole process of breathing includes three links: first, external respiration, through breathing movements, the outside air enters the lungs, and oxygen passes through the alveolar wall into the pulmonary capillaries; second, the gas is transported in the blood, and oxygen is transported to tissues throughout the body through blood circulation; third, internal respiration, that is, gas exchange between blood and cells and oxidative metabolism within cells. If any of these three links is blocked, it will lead to suffocation: blocking the mouth and nose will block external respiration; gas poisoning will block the transportation of gas in the blood, and oxygen will not be able to bind to hemoglobin; cyanide poisoning will block internal respiration, and cells will not be able to use oxygen.

The human respiratory system has evolved over a long period of time. Primitive organisms exchanged gases directly through cell membranes. It was not until mollusks that independent respiratory organs, gills, appeared, and it was only in amphibians that lungs appeared. The human respiratory system includes organs at all levels from the nasal cavity, pharynx, trachea, bronchi to the lungs (collectively known as the respiratory tract), as well as the pleura and mediastinum, which work together with blood circulation to complete the task of gas exchange.

The respiratory tract is directly connected to the outside world, and air is constantly flowing in and out with breathing movements, which also facilitates the spread of bacteria and viruses. Colds (infections of the nose, pharynx, and throat) are the most common respiratory diseases, in addition to tracheitis, bronchitis, or pneumonia. In recent years, the spread of SARS virus, avian influenza, and the new coronavirus has all been through the respiratory tract, and the incidence of allergic rhinitis, asthma, and lung cancer has also been increasing year by year. The spread of these diseases warns us that if we do not properly protect the environment, reduce pollution, and maintain the ecological balance of nature, the freedom of human breathing will be affected.

There are dust and microorganisms floating in the air, but we don't have to be afraid of the air like a frightened bird. The respiratory tract has multiple protection mechanisms, and it is not easy to get sick under normal circumstances. These protection mechanisms include: the respiratory tract has many branches, bacteria and viruses cannot enter directly, and they are easily dispersed and surrounded by the immune system; when encountering irritating gases, the smooth muscles of the respiratory tract can contract, which can cause coughing and sneezing, reducing the entry of toxic gases; the respiratory tract can secrete mucus to adhere to foreign matter, forming sputum to be discharged, so that it does not enter the alveoli; the respiratory tract below the trachea is covered with cilia, and the swing of cilia can expel dust and bacteria. There are also macrophages below the 17th level bronchioles, which can engulf pathogens. With these protection mechanisms, the respiratory tract is generally not invaded by bacteria and viruses. The respiratory tract can only be infected when the protection mechanism decreases, or when the number of bacteria and viruses is large and the toxicity is strong.

To protect the respiratory tract, you need to maintain good work and rest habits, exercise appropriately, pay attention to the intake of foods containing vitamin A or carotene, protect the respiratory mucosal epithelium, maintain the integrity of the cilia, drink more water in autumn and winter, and keep the respiratory tract moist.

Air quality is closely related to health. Although it is impossible to buy canned air from the Hulunbuir grassland all year round, you can try to reduce going out in foggy weather, not smoke, reduce secondhand smoke, open windows frequently to ventilate the room, use environmentally friendly home decoration materials, and use an air purifier if conditions permit.

(The author is an associate professor at Central China Normal University and a director of the Hubei Physiological Society)