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Lower respiratory tract

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Lower respiratory tract

Lower respiratory tract
Conducting passages.

The term lower respiratory tract refers to the portions of the respiratory system from the trachea to the lungs.

Lower respiratory tract infections can be the cause of several serious illnesses, including pneumonia.

Main parts of the lower respiratory tract

The lower respiratory tract consists of:

Some sources include the larynx as part of the lower respiratory tract, whereas others include it in the upper respiratory tract (which also comprises the nasal cavity (nose) and the pharynx).

Between them, the upper and lower respiratory tracts make up the whole respiratory system.

Function of the lower respiratory tract

The larynx is not just our voice box; it also serves as protection for our trachea. The larynx has a flap called the epiglottis which prevents food and water from entering our lungs when we breathe. If the epiglottis does not close when swallowing food or water, there is an involuntary reaction to cough. The larynx also has cilia. The trachea consists of cartilage, and is the largest tube in the respiratory system. At the bottom of the tube, it branches off into two individual tubes, called the bronchial tubes, or bronchi. The bronchial tubes then branch off into smaller sections inside the lungs, called bronchioles. These bronchioles hold the air sacs in the lungs called the alveoli.[2] The lungs are the biggest organ in the lower respiratory tract. The diaphragm is the main muscle involved for the lungs to breathe and is the largest muscle. There are smaller muscles in between our rib cage that help the lungs expand and contract.

When a human being inhales, air travels down the trachea, through the bronchial tubes, and into the lungs. The entire tract is protected by the rib cage, spinal cord, and sternum bone. In the lungs, oxygen from the inhaled air is transferred into the blood and circulated throughout the body. Carbon dioxide (CO2) is transferred from returning blood back into gaseous form in the lungs and exhaled through the lower respiratory tract and then the upper, to complete the process of breathing.

Vital System

The lower respiratory tract is a vital system. This tract is responsible for receiving oxygen (O2) and distributing it throughout our whole body. All of our organs are dependent on this system, such as the brain. This is a key organ that is dependent on oxygen. If the lower respiratory tract shuts down, the body can't receive this vital oxygen, and organs that are deprived of oxygen shut down like the brain, which leads to severe brain damage

The Cell Structure of the Lower Respiratory Tract and the Inside of the Lungs

Our lungs are made up of thirteen different kinds of cells, eleven are called epithelial and the other two are mesenchymal cells. (6)[3] The epithelial cells make the lining of our tracheal, and bronchial tubes, while the mesenchymal cells line the lungs. At the end of the trachea, the two bronchial tubes split off into a left and right branch. In these two branches, there are bronchi and bronchioles. These tubes help take air into the lungs and help us breathe. Inside the bronchi, there are tiny tree like branches called Bronchioles. The bronchioles fill most of the lungs, and at the end of each bronchiole are the alveoli.(8) The alveoli are tiny little air sacs in the lungs where gas exchange takes place. There are “about 150 million per lung”. (1) When the diaphragm contracts, a negative pressure is generated in the thorax and air rushes in to fill the cavity. when that happens These sacs fill with air making the lung expand. The alveoli are rich with capillaries, called alveolor capillaries. Here the red blood cells absorb oxygen from the air and then carry it back in the form of oxyhaemaglobin, to nourish the cells. The red blood cells also carry carbon dioxide (CO2) away from the cells in the form of carboxyhaemaglobin and releases it into the alveoli through the alveolor capillaries. When the diaphragm relaxes, a positive pressure is generated in the thorax and air rushes out of the alveoli expelling the carbon dioxide (CO2).

Lobes of the Lungs

The lungs are connected by the pleural cavity. This cavity is attached to the chest. The pleura is a small membrane, singled celled, that surrounds the lungs. This membrane helps the lungs expand and contract, while breathing. This membrane can be inflated by a viral or bacterial infection to the lungs. The lungs are divided into different lobes. The right lung is larger in size than the left, so the right lung has three lobes, and the left two. The three lobes are called the costal surface, diaphragmatic, and mediastinal. The part of the lung that is closest to the rib cage is the costal area. The lobes have three different types: anterior, posterior and inferior borders. These borders have different locations in the lungs. The left lung is smaller, than the right because it is pushed out farther by the heart. The Roots of the left and right lung “Root of the right lung bronchi lie posterior, pulmonary arteries are superior, pulmonary veins are inferior and anterior”

“Root of the left lung bronchus lies posterior, pulmonary artery is superior, pulmonary vein is inferior and anterior”(6)

Infections of the Lower Respiratory Tract

Our trachea (windpipe) has a membrane lining that produces a layer of mucus that helps filter waste that an organism breathes in through the air. There is also a small lining of tiny hairs in our lungs called cilia. These tiny hairs act as a filter in our lungs and control the amount of mucus that enters our lungs. The reason why we cough is because the cilia push up the mucus, so not too much enters our lungs. If these hairs are not functioning properly, an organism is at risk of a lower respiratory tract infection.(7)

Our respiratory system is very prone to developing infections in the lungs. Infants and older adults are more likely to develop infections in their lungs, because their lungs are not as strong in fighting off these infections. Most of these infections used to be fatal, but with new research and medicine, they are now treatable. With bacterial infections, antibiotics are prescribed, while viral infections are harder to treat, but still curable. Some of these infections have environmental factors such as smoking. When you inhale a tobacco product, the smoke paralyzes the cilia, causing mucus to enter the lungs. If you smoke frequently, over time these cilia hairs die, and can no longer filter mucus. Tar from the smoke inhaled enters your lungs, turning the pink-coloured lungs black. The accumulation of this tar could eventually lead to lung cancer, emphysema or chronic obstructive pulmonary disease (C.O.P.D.).(5)


This is a common lower respiratory disease that can be caused by exposure to harmful chemicals, or prolonged use of tobacco. This disease is chronic and progressive, the damage to your lungs is irreversible and eventually fatal. This disease destroys the alveoli, and lung tissue. Damage to these air sacs, and tissue makes breathing very difficult, causing shortness in breath, hyperventilation, and raised chest. The decreased amount of alveoli causes loss of oxygen (O2) to the lungs, and more accumulation of carbon monoxide (CO). There are two types of emphysema: primary and secondary. Primary emphysema can be found in younger adults. This type of emphysema deteriorates the air sacs, and lung mass. Secondary emphysema can be found in older adults who smoke/have smoked and have a history of chronic bronchitis.(5)


The common cold/flu is the most common cause for the upper respiratory tract infection, which can cause more serious illness that can develop in the lower respiratory tract. Pneumonia is the most common, and frequent lower respiratory tract infection. This can be either viral, bacterial, or fungal. This infection is very common, because pneumonia can be airborne, and when you inhale this infection in the air, the particles enter the lungs and move into the air sacs. This infection quickly develops in the lower part of the lung, and fills the lung with fluid, and excess mucus. This causes difficulty in breathing, and coughing as the lower respiratory tract tries to get rid of the fluid in the lungs. You can be more prone to developing this infection if you have asthma, flu, heart disease, or cancer(7)


Bronchitis is another common infection that takes place in the lower respiratory tract. It is an inflammation of the bronchial tubes. There are two forms of this infection: acute bronchitis, which is treatable and can go away without treatment, or chronic bronchitis, which comes and goes, but will always affect ones lungs. Bronchitis increases the amount of mucus that is natural in your respiratory tract. Chronic bronchitis is common in smokers, because the tar from smoking accumulates over time, causing the lungs to work harder to repair themselves. (7)


Tuberculosis is one of many other infections that occurs in the lower respiratory tract. You can contract this infection from airborne droplets, and if inhaled you are at risk of this disease. This is a bacterial infection which deteriorates the lung tissue resulting in coughing up blood. (7) This infection is deadly if not treated.

Bronchial Tubes associated with Asthma

Our bronchial tubes are the main passages to our right and left lungs. These tubes carry the oxygen (O2) to the bronchioles inside the lungs. If these tubes swell up, this is the result of asthma which could lead to an asthma attack. This results in wheezing, tightness of the chest and severe difficulty in breathing. There are different types of asthma that affects the functions of the bronchial tubes. Allergies can also set off an allergic reaction causing swelling to the bronchial tubes, and as a result the air passage will swell up, or close up completely. (7)


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