The lungs are an important part of our respiratory system and also play a vital role in detoxifying the body. The respiratory tract is divided into two parts - the upper air passage, which comprises the nose, the hypo-pharynx, pharynx and the larynx that accommodates the vocal cords, while second part, the lower air passage extends from the vocal cords to all the way down into the lungs via the trachea.
When we inhale through our nose, the air goes into the upper air passage and passes to the trachea, which is the most contracted section of the respiratory tract. The trachea is branched and comprises the right main stem bronchi and the left main stem bronchi (which are also known as bronchial tubes) that are located at the back of the ribcage. One bronchus (singular of bronchi) goes into each of the two lungs and is subsequently branched into two or three more stems, which are known as bronchioles. The bronchioles end in the air sacs, which are also known as alveoli or alveolar sacs. It is estimated that the entire surface area of the respiratory tract is about 70 square meters.
Inside the lungs, oxygen is removed from the air inhaled by us and supplied to the several million alveolar sacs, which subsequently submit it to the numerous capillaries. Eventually, this oxygen mixes with hemoglobin present in the red blood cells and is transported to the different parts of the body.
While exhaling, the carbon dioxide molecules extracted from the capillaries in the lungs are disseminated into the alveoli or alveolar sacs and eventually expelled from the body via the bronchi, trachea plus the upper air passage.
Commonly three diseases - asthma, bronchitis plus emphysema - assault the bronchial tube system. Asthma can be distinguished by the bouts of breathing problems caused by the disease, while bronchitis occurs when the bronchial tubes are inflamed. In fact, both these conditions can be triggered by toxic substances. However, both asthma as well as bronchitis are reversible, provided they are diagnosed and treated during the initial stages. You should be aware that the toxic substances present in cigarette smoke can result in chronic bronchitis as well as emphysema. It is worth mentioning here that emphysema damages the elasticity of the lungs, as it causes injury to the walls that separate the alveoli from each other, resulting in the development of minute craters. In such situations, the alveoli in the affected areas are distended permanently. For this reason, emphysema is said to be an irreversible condition.
Apart from the skin, the lungs are organs that are most exposed to the toxins present in the environment. This is primarily because the air we inhale is not pure, but contains several harmful microbes, dust, chemicals and pollution. In fact, there are three ways by which tiny solid particles as well as liquid aerosols can go into the lungs without much difficulty and remain there. These three ways are known as impaction, sedimentation plus diffusion. On the other hand, the cells that form the lining of the respiratory system directly absorb the gases.
Although the lungs are always exposed to various environmental pollutants, they can also defend themselves from such contaminants by employing filters, enzyme systems, epithelial barriers as well as immune responses. Mucus and cilia work as filters for the lungs.
The glands situated below the epithelium produce mucus, while specific cells enclose cilia. Precisely speaking, cilia are projections akin to hair that have the ability to beat in a synchronized manner at a pace of roughly 1,000 times every minute. Mucus and cilia work in tandem to protect us from environmental pollutants. While mucus locks in foreign particles that enter the lungs, cilia work to eliminate them from the lungs. Generally, these irritants are eliminated when an individual coughs or sneezes.
Cilia, however, are unable to move the particles outside the lungs when mucus is inadequate. At the same time, cilia may be rendered ineffective by the virus that causes influenza, often resulting in secondary infections by bacteria. A number of people suffer from a health condition that is called immobile cilia syndrome - denoting that cilia in the cells of their body are unable to move and also the fact that they are inclined to develop respiratory infections and sinus problems.
A particular type of cells present in the epithelium works as the epithelial barrier. A particular form of white cells known as alveolar macrophages absorbs these foreign particles as well as eradicates the viruses and bacteria, which are subsequently moved to another form of white cells called lymphocytes. On its part lymphocytes destroy these harmful particles that manage to enter the lungs. In addition, alveolar macrophages enclose a special type of enzyme called aryl hydrocarbon hydroxylase, which decontaminates the chemicals.
Besides these, there is an enzyme system that also works to protect the lungs from invasion by foreign particles and chemicals. When we inhale such particles, they trigger the secretion of inflammatory enzymes, which are called proteases, in the lungs. The proteases are harmful as they not only have the potential to damage the cells of the lungs, but also injure the connective tissues. In such circumstances, the alveoli or alveolar sacs are protected by a particular type of proteins called anti-protease. These proteins mix with protease and thereby render them ineffective. However, cigarette smoke has the potential to devastate the normal equilibrium between proteases and anti-proteases. Alpha-I-anti-trypsin is the most widespread anti-protease in the lungs. It has been seen that individuals suffering from anti-protease deficiency are more susceptible to develop emphysema.
A number of enzymes belonging to the mixed function oxidase family are found in the lungs. These enzymes enable the lungs to break down drugs as well as xenobiotics into chemicals that are soluble in water. Eventually, the kidneys can excrete these simpler chemicals from the body.
In addition to the mixed function oxidase enzymes, the lungs also contain antioxidant enzymes such as, catalase, glutathione enzymes and superoxide dismutase, which neutralize the free radicals. The fluid that covers the alveolar sacs and contains ceruloplasmin, glutathione and transferrin also helps to shield the lungs from undergoing oxidant stress. Moreover, vitamin E, a potent antioxidant is present in the cell membranes and it saves the lungs from harm caused by toxic lipid peroxides. It has been seen that patients smoking cigarettes may suffer from vitamin E deficiency in the fluid that forms the lining of their alveolar sacs.
Before we conclude, it is worth mentioning that lungs also have their own immune response, which works to shield them from harmful organisms inhaled by us. The lungs enclose lymphocytes, which produce antibodies called immunoglobulins. Some other immunoglobulins are also passed on to the lungs from the blood.
It has been found that various types of immunoglobulins, including IgA, IgE and IgG, are present in the respiratory tract. IgA is effective in countering several types of viruses and it appears that it also works to put off the absorption of antigen throughout the lungs. Similarly, a type of white blood cells called T-lymphocytes, which aid in combating infection, also facilitate the lungs' efforts to protect them from pathogenic microbes and tumour cells. In addition, T-lymphocytes release lymphokines - molecules that trigger as well as invigorate macrophages, which are basically a type of white blood cells that destroy all foreign substances that are harmful for the body.