Mucociliary clearance is a very important component of the protection mechanism of our respiratory organs. This mucus transport system is able to clear our airways of foreign microorganisms and bacteria. A textbook by A. A. Krishtafovich and B. M. Ariel โX-ray functional characteristics of mucociliary clearanceโ was even published on this subject.
In this article we will consider what the named process is, what it depends on and how it is studied. But first you need to figure out how expelled mucus enters the human respiratory system.
What is the essence of this phenomenon?
Every day, more than 15 thousand liters of air enter our lungs (this volume is enough to fill about 1,600 balloons). And even in the cleanest environment, untouched by humans, we still inhale about a hundred bacteria every minute, and this is more than 150 thousand pollutants per day. If you give them freedom, they are able to infect and clog our entire respiratory system.
But these foreign particles of viruses and bacteria enter the extremely sticky mucous layer of the respiratory tract. Which carries caught unfavorable material to the larynx. This process is also known as mucociliary clearance. Until now, scientists have not yet fully understood his physiology, so research is ongoing. Let's take a closer look at this process.
So, mucociliary clearance - what is it?
How does the airway clearance process take place?
The process of transferring mucus to clean the airways of foreign particles is controlled by the ciliary apparatus of the bronchi. Cilia are small tentacle-like structures with a diameter of about 1,000 times smaller than human hair. They wriggle in an asymmetric rhythm.
By scanning images of an electron microscope, it was found that these structures protrude from most of the epithelial cells that densely line the airways. They bathe in an aqueous liquid - the periciliary layer.
During the impact, the cilia straighten and sink with their tops into the mucus, after which they push it along with the foreign particles adhered to it. The named structures, as a rule, form a unidirectional movement of mucus by a coordinated movement.
The cilia of the ciliated cell have a two-phase movement: first, a quick effective hit occurs, and then a slow return movement follows. The exact mechanism by which the movement of the mucus is ensured remains unclear until the end, and today it is the subject of intensive research.
What determines the direction of mucus movement?
The direction of movement of the cilia of the mucous layer is excellent in different parts of the respiratory tract:
- if the process occurs at the front ends of the inferior nasal concha, then the mucus moves towards the entrance to the nose;
- if it occurs at the posterior ends of the nasal concha, then the mucus moves towards the oropharynx;
- from the trochea and bronchi, the mucous layer also moves towards the oropharynx.
What is the epithelium of the respiratory tract?
The tissue covering the airways is a multirow ciliated epithelium. It consists of ciliary (80%), goblet, producing mucus, as well as undifferentiated cells. As a rule, all these cells should be updated every month.
Each ciliated cell on its surface contains about 200 cilia of very small sizes (0.2 ฮผs in thickness and 5-7 ฮผm in length). But despite such a small size, the cilia are able to move the mucous layer at a speed of up to 0.5 mm / sec.
For the first time, the structure of cilia was described by Fossett and Porter in 1954 due to observations with an electron microscope. As it turned out, these formations are outgrowths of the cell. In their central part is the axoneme, which consists of 9 doublets of microtubules. And in its center there are two additional microtubules (9 + 2). Along the entire length of the microtubules, there are internal and external dynein pens necessary for converting ATP into mechanical energy.
Key role in clearance
A key role in mucociliary clearance is not only the coordinated work of the cilia, but also their beating frequency (FWH). According to some reports, it is 3-15.5 Hz in an adult, in children with FGD it is from 9 to 15 Hz.
However, some authors say that this indicator does not depend on age. Simply, the FWH in the peripheral airways is lower than, for example, in the trachea, nasal cavity and bronchi. A decrease in temperature can cause cilia to slow down. During the experiments, scientists found that the cilia moved most actively at a temperature of 37 ยฐ C.
What can lead to violations?
Violations of mucociliary clearance can occur due to damage to the mechanism of protection of the mucous membrane of the respiratory tract. These include both congenital (primary ciliary dyskinesia) and acquired disorders (due to infection). Such damage can cause a complete cessation of cilia or a decrease in FGP.
Research methods
Today, it is possible to study the state of mucociliary clearance (we have already explained what this is) by various methods. These include:
- coal test;
- saccharin test;
- radio aerosol method;
- test with colored polymer films.
Scraping from the mucous membranes also allows you to directly study the motor activity of the ciliated epithelium.
The simplest ciliated epithelium sample can be obtained from the nasal mucosa. The material can be taken with a cytological brush, but it is more convenient to make scraping with a special disposable plastic spoon. The advantage of this method is non-injuries, as well as the ability to obtain material from a particular site without anesthesia.
The state of the functions of the ciliated epithelium is evaluated by the following algorithm:
- first examine the general picture of the movement of the cilia: how many moving cells are in the field of view;
- then the average and maximum FWH are calculated;
- then assess the synchronism and amplitude of the movement of the cilia;
- after that, thanks to special programs, a more detailed analysis is carried out (the number of cilia per cell, their length, deflection angle, etc.).
Sometimes a saccharin test is performed. To do this, the food saccharin tablet must be divided into four parts and give the pieces a rounded shape. One piece of saccharin is placed on the lower nasal conch indented in cm from the front end. After this, it is necessary to note the time before the occurrence of a sweet sensation in the mouth. The norm is considered to be from 10 to 15 minutes.
Recently, a lot of attention has been paid to the ralioaerosol research method. It allows using a special gamma camera to monitor the spread and removal of a radio pharmaceutical product that is previously inhaled.
The named method allows to adequately characterize the state of clearance in various parts of the lungs. But to put it into practice is very difficult due to the lack of special laboratories, a specialized inhalation unit, aerosols and trained personnel. All this requires large financial costs. In addition, we should not forget that radiation exposure has a very adverse effect on the human body.
Clinical trial results
And what is the mucociliary clearance in children? Studies have found that in most children with bronchial asthma and allergic rhinitis, saccharin time was normal, and sometimes even accelerated. The average is 6 minutes.
The average FWH value in children with bronchial asthma was 6-7 Hz, the maximum - about 10 Hz. A comparison of indicators in children with bronchial asthma of mild or moderate severity of the disease did not reveal statistically significant differences.
Examining the mucociliary clearance (we described this phenomenon) in patients with bronchopulmonary pathology, it was found that the condition of the MCT depends on the presence of bronchial obstruction, as well as on the form of inflammation: acute or chronic.
Thus, studying the state of clearance allows us to identify the presence and severity of mucociliary insufficiency. In addition, it helps to choose an adequate treatment, and in conclusion to evaluate the improvement of mucociliary clearance by the selected therapy.