What Is Pneumonia: Causes, Symptoms and Tips

Pneumonia is a lung infection that can causes the air sacs or alveoli of the lungs to fill up with fluid or pus. It is a common disease which can be caused by many factors like bacteria, air pollution, virus, etc.

To know more about pneumonia, we need to make clear of our physical scructure first.

Respiratory System Structure

The lungs are connected with the mouth and nose, which collectively constitutes the respiratory system. Among them, according to its structural characteristics, the respiratory tract is divided into upper respiratory tract and lower respiratory tract.

Source: training.seer.cancer.gov

The upper respiratory tract consists of the nose, pharynx and larynx, while the lower respiratory tract consists of the trachea, bronchus and lungs.

The medical professional name of "cold" we often say is "acute upper respiratory tract infection", which is often caused by a series of inflammatory reactions caused by viruses, bacteria and other pathogens that first attach to the nasopharynx after invading the respiratory tract.

The respiratory tract is surrounded by respiratory epithelium and goblet cells. The epithelial cells will extend long cilia into the respiratory tract, and the goblet cells will secrete mucus.

Source: wikipedia.org

After infection of respiratory epithelium and other tissues caused by viruses and bacteria, goblet cells will secrete a large amount of mucus to wrap pathogens, and then regularly swing to the mouth and nose through the cilia of epithelial cells, which can eventually let the "sputum" rich of pathogen be discharged from the body.

Generally speaking, pathogens such as viruses rarely enter the lower respiratory tract directly.

However, there are always some special conditions that can eventually lead to the formation of bronchitis and pneumonia.

Physiological Function of Lung

The lung is composed of left and right lungs, of which the left lung has two lobes and the right lung has three lobes.

When oxygen rich air is inhaled from the mouth and nose, it reaches the five lobes of the lung along the trachea and bronchus.

The lung has extremely rich and developed capillaries, and oxygen rich air can finally reach alveoli through air tubes and bronchi.

One of the components of the alveoli is the extremely developed capillaries.

Source: verywellhealth.com

The alveoli can contract and relax rhythmically. When the alveoli dilate and expand, the fresh air rich in oxygen will rapidly flow in; When the alveoli shrink, the exhaust gas rich in carbon dioxide will be discharged.

When the oxygen rich fresh air enters the alveoli, the oxygen content in the alveoli is higher than that in the capillary blood, so the partial pressure of oxygen in the alveoli is higher than that in the blood.

Thus, oxygen will continuously enter the blood from the alveoli, and eventually be carried by hemoglobin and run to all parts of the body. 

Source: medicalnewstoday.com

When cells in all parts of the body metabolize, they inevitably consume oxygen and produce a large amount of carbon dioxide.

Thus, when the blood flows through the alveoli, the concentration of carbon dioxide in the blood is naturally much higher than that in the alveoli, that is, the partial pressure of carbon dioxide in the capillary blood is higher than that in the alveoli.

Therefore, the carbon dioxide is discharged from the blood to the alveoli, and finally discharged from the body through the layers of respiratory tract.

Formation of Pneumonia

There are abundant cilia on the surface of the parietal cell of the respiratory tract, which will swing towards the mouth and nose.

When viruses, bacteria and other pathogens invade the respiratory tract, goblet cells will secrete mucus to stick the viruses and bacteria, and finally, with the help of cilia, they will be discharged from the body.

However, if the number of viruses and bacteria is huge and exceeds the self purification capacity of the human body, some pathogens can enter the alveoli.

For viruses and other pathogens that come to the alveoli, the immune cells of the body will fight against them.

Macrophages will swallow foreign bodies such as viruses and bacteria, and finally eliminate them with the help of the powerful "protein degradation system". 

In addition, in order to accelerate the response to external threats, T cells and B cells will also be mobilized.

The most important role of T cells is to eliminate pathogens, such as viruses, that may survive in tissue cells.

Although immune cells such as macrophages can directly kill pathogens, they can not distinguish the pathogens hidden in the cells. While T cells can distinguish normal cells from infected cells by specific mediators.

Then they will adsorb on the surface of the infected cells, and finally start the self-apoptosis program of the cells. 

In this way, the cell will break down by itself, exposing the virus and other pathogens in it, and then macrophages can eliminate them.

Although the immune system is strong, it does not always win.

When people with weak immune system, such as children and the elderly, or people are tired, viruses and other pathogens may "win". Then they will further multiply in tissues such as alveoli.

In order to fight them, the body starts to have a fever and inhibit the propagation of pathogens. A large amount of energy is used for the proliferation of cells of the immune system to resist the further invasion of pathogens.

And the inflammatory reaction will become more and more serious. Finally, a large amount of liquid will be produced in the alveoli, occupying the original alveolar cavity.

As a result, the body began to lack adequate oxygen supply.

Finally, it may cause dyspnea, chest pain, headache and coma, and even suffocation death in severe cases.


Now, the COVID-19 is raging. The reason why the coronavirus is frightening is that it can cause severe inflammatory reaction (pneumonia) in the lungs, and then lead to critical illness such as dyspnea and hypoxia.

In addition, this is a new virus, and our immune system has never been exposed to it. Therefore, when it enters our body, the immune system cannot respond quickly like other common viruses. This may also be the reason why it can directly reach the lungs through the upper respiratory tract. 

Objectively speaking, it is very dangerous for any pathogen to enter the alveoli and cause pneumonia, such as the tuberculosis pneumonia caused by Mycobacterium tuberculosis.

Therefore, clean and fresh air is especially important for us, and an useful air purifier is necessary if possible, because it can greatly reduce the possibility of pathogens entering our body.

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