The conducting zone is the part of the lung that air flows through without any gas exchange, because the tubes are too thick for the gases to move across.
The conducting zone is lined with hair-like structures called cilia that are covered in mucus , which helps trap potentially dangerous materials. Cilia are mobile, tiny, finger-like projections on the surface of airway cells. Cilia line the airways and help move mucus up and out of the lungs [ 5 ].
Cilia are about 6—7 micrometers tall, or roughly a tenth the width of a human hair [ 3 , 5 ]. There are many types of particles that could enter the lungs and cause damage. Some of these particles are pollutants, such as emissions from gas-powered vehicles, carbon monoxide from fireplaces, toxins from vaping or smoking, and aerosols like hairspray.
Particles can get trapped in various areas of the lung. Short hairs, like the kind on top of your head, line the insides of the nostrils and are covered with mucus. These mucus-covered hairs help trap larger particles as they enter the nose. Particles that enter the conducting zone can hurt the airway cells, which may reduce the movement of their cilia and lead to a buildup of mucus that cannot be cleared from the airways [ 2 ].
Mucus-clogged airways may not allow air to travel as effectively, like the way leaves in a gutter interfere with water flow. Smaller particles can sometimes get all the way to the alveoli. Damage to the alveoli makes breathing considerably more difficult, because oxygen will not diffuse as well into the blood. This reduced efficiency of gas exchange may cause the rest of the body to be hypoxic, which means low in oxygen. Only the conducting zone, from the trachea to the bronchioles, has the ability to move mucus using cilia, so alveoli cannot rely on this mechanism to get rid of these small particulates [ 3 ].
The mucociliary escalator is inside of the conducting airways and is made up of mucus and cilia, which moves the mucus up and out of the lungs where it can be expelled by coughing or swallowing Figures 1 , 2 [ 5 ]. As the first line of defense, airway mucus is made up of different components that help it trap particles and germs [ 2 , 4 ]. Mucus is a physical barrier to protect the lung and has properties that help get rid of potentially infectious bacteria, fungi, and viruses [ 4 , 5 ].
Some of the important components of airway mucus are mucins sticky, sugar-coated proteins , defense proteins, salt, and water. Together, these components form a gel that traps particles that enter the airway [ 2 ]. Mucus is mostly produced by cells that line the inside of the airways. These are called goblet cells. Goblet cells are shaped like medieval chalices, hence the name goblet. Unlike other cells in the airway that are ciliated and shaped like columns, goblet cells do not have cilia on top.
Goblet cells are not present beyond the conducting zone, to prevent mucus from interfering with gas exchange. Mucus is also involved in the hydration of airways, which is necessary for proper function of the cilia [ 3 , 5 ].
The movement of cilia is similar to the way your arms move while swimming the breaststroke [ 3 ]. The mucociliary escalator is one of the major defence mechanisms which protect the lung. If the mucociliary escalator fails, it increases the risk that the normal and potentially pathogenic, inhabitants of the nose, throat and the back of the mouth nasaophaynx , will descend into the lung, resulting in a potentially life-threatening pneumonia.
What is the mucociliary escalator? Electron microscope view of the mucociliary escalatory. From the nose to the end of the bronchi inside the lung , the surface is lined with a series of barrier cells.
These cells assist with the collection of material by producing a sticky mucoid substance - the first half of the muco ciliary escalator. This mucus contains chemicals, antibodies and immune cells to destroy any bacteria and viruses that become trapped. It protects and coats the underlying cells and its sticky nature helps the lungs to remove any dust particles and foreign chemicals in the air.
The hairs beat in a well organised manor, to move the mucus containing trapped particles — in the nose down to the throat; in the bronchi and trachea windpipe , up to the throat.
Hence the term escalator. As the mucociliary escalator moves particles over the tonsils, the immune system is stimulated to further protect the pig. Only very small particles about the size of individual bacteria can actually enter the lung tissue itself. Swallowing large amounts of secretions may cause nausea so it is generally recommended to cough into a tissue. This also enables the patient to monitor the colour and quantity of their sputum and to be aware of the presence of any haemoptysis.
The mucociliary escalator does not extend to the alveoli. Particles deposited in this region are engulfed by macrophages on the surface of the alveoli.
The foreign particles engulfed by these macrophages either move up to join the mucociliary escalator or escape via the lymphatic or venous system. Airway Clearance in the Normal Lung. It is composed of two basic parts: Mucous-producing goblet cells : The normal mucous film has two layers — the sol layer and the gel layer Fig. Factors inhibiting the mucociliary escalator include: congenital defects of cilia such as Primary Cilial Dyskinesia PCD cigarette smoke causes paralysis of the cilia dust and pollution inhalation of toxic gases such as sulphur dioxide causes paralysis of the cilia chronic respiratory diseases such as bronchiectasis where there is damage to the epithelium and the cilia are destroyed changes in the character of mucus, caused by disease or dehydration volume of secretions so great that the system is overloaded bronchiectasis, cystic fibrosis low levels of humidity such as aircraft Cough Cough, which may be initiated either voluntarily or reflexively, is an explosive expiration that provides a normal effective protective mechanism for clearing the tracheobronchial tree of secretions and foreign material, noxious substances and infections from the larynx, trachea and larger bronchi.
To produce a cough, the following sequence occurs: inspiration the glottis closes and the vocal cords contract to shut the larynx and trap air within the lungs the abdominal and internal intercostal muscles contract which significantly raises the intra thoracic pressure the vocal cords relax and the glottis opens, releasing air at over kph the rapid air flow clears out any irritants attached to the respiratory lining Swallowing large amounts of secretions may cause nausea so it is generally recommended to cough into a tissue.
Alveolar macrophages The mucociliary escalator does not extend to the alveoli.
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