Mucociliary clearance is a very important component of the defense mechanism of our respiratory organs. This mucus transport system is able to clear our airways of foreign microorganisms and bacteria. A textbook by Krishtafovich A. A. and Ariel B. M. “X-ray functional characteristic of mucociliary clearance” was even published on this topic.
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 the expelled mucus enters the human respiratory system.
What is the essence of this phenomenon?
Every day, more than 15,000 liters of air enter our lungs (enough to fill about 1,600 balloons). And even in the cleanest, most untouched environment, we still breathe in about a hundred bacteria every minute, which is more than 150,000 pollutants per day. If left unleashed, they can infect and clog our entire respiratory system.
But these foreign particles of viruses and bacteria get into the extremely sticky mucous layerrespiratory tract. Which transfers the caught unfavorable material to the larynx. This process is also known as mucociliary clearance. Until now, scientists have not yet fully understood its physiology, so research continues. Let's take a closer look at this process.
So, what is mucociliary clearance?
How does the airway clearance process work?
The process of mucus transfer to clear the respiratory tract of foreign particles is controlled by the ciliary apparatus of the bronchi. Cilia are small, tentacle-like structures, about 1,000 times smaller in diameter than a human hair. They squirm in an asymmetrical rhythm.
By scanning electron microscope images, these structures were found to protrude from most of the epithelial cells that densely line the airways. They bathe in a watery fluid called the pericilium.
During the impact, the cilia straighten and sink their tops into the mucus, after which they push it along with the foreign particles adhering to it. The named structures, as a rule, form the unidirectional movement of mucus through coordinated movement.
The cilia of the ciliated cell have a two-phase movement: first there is a fast effective strike, and then a slow return movement follows. The exact mechanism by which mucus moves remains unclear and is currently the subject of intense research.
Fromwhat determines the direction of movement of the mucus?
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 anterior ends of the inferior turbinate, 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 mucosal layer also moves towards the oropharynx.
What is the epithelium of the respiratory tract?
The tissue covering the respiratory tract is a multi-row ciliated epithelium. It consists of ciliated (80%), goblet, mucus-producing, and 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 microns in thickness and 5-7 microns in length). But despite their small size, cilia are able to move the mucous layer at a speed of up to 0.5 mm/sec.
The structure of cilia was first characterized by Fossett and Porter in 1954 through electron microscope observations. 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 handles necessary for the conversion of ATP tomechanical energy.
Key role in clearance
The key role in mucociliary clearance is not only the coordinated work of cilia, but also their beat frequency (BFR). According to some reports, in an adult it is 3-15.5 Hz, in children, the NBR is from 9 to 15 Hz.
However, some authors say that this indicator does not depend on age. It's just that the NBR in the peripheral airways is lower than, for example, in the trachea, nasal cavity and bronchi. A decrease in temperature can lead to slowing down of the cilia. During the experiments, scientists found that the cilia moved as actively as possible at a temperature of 37 ° C.
Which may result in violations?
Impaired mucociliary clearance may result from damage to the airway mucosal defense mechanism. These include both congenital (primary ciliary dyskinesia) and acquired disorders (due to infection). Such damage can cause a complete cessation of cilia movement or a decrease in NBR.
Research methods
To date, it is possible to study the state of mucociliary clearance (what it is, we have already explained) by various methods. These include:
- charcoal test;
- saccharin test;
- radioaerosol 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 sample of ciliated epithelium can be obtained from the nasal mucosa. The material can be taken with a cytological brush, but it is more convenient to make a scraping with a special disposable plastic spoon. The advantage of this method is non-traumatic, as well as the ability to obtain material from a specific area without anesthesia.
The state of the functions of the ciliated epithelium is assessed by the following algorithm:
- first examine the overall picture of the movement of the cilia: how many mobile cells are in the field of view;
- next, the average and maximum NBR are calculated;
- then evaluate the synchronism and amplitude of cilia movement;
- after that, thanks to special programs, a more detailed analysis is carried out (the number of cilia per cell, their length, angle of deviation, etc.).
Sometimes a saccharin test is done. To do this, a tablet of food saccharin must be divided into four parts and give the pieces a rounded shape. One piece of saccharin is placed on the inferior turbinate with a cm indent from the anterior end. After that, it is necessary to detect the time before the appearance 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 radioaerosol research method. It allows using a special gamma camera to observe the spread and removal of the radiopharmaceutical, which is pre-inhaled.
The named method allows you to adequatelyto characterize the state of clearance in various parts of the lungs. But it is very difficult to put it into practice due to the lack of special laboratories, a specialized inhalation unit, aerosols and trained personnel. All this requires large financial costs. In addition, do not forget that radiation exposure has a very adverse effect on the human body.
Clinical study results
What is mucociliary clearance in children? Studies have found that most children with bronchial asthma and allergic rhinitis had a normal saccharin time, and sometimes even accelerated. The average is 6 minutes.
The average FRR in children with bronchial asthma was 6-7 Hz, the maximum was about 10 Hz. Comparison of indicators in children with bronchial asthma of mild or moderate severity of the disease did not reveal statistically significant differences.
Exploring mucociliary clearance (we described this phenomenon) in patients with bronchopulmonary pathology, it was found that the state of 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 you to identify the presence and severity of mucociliary insufficiency. In addition, it helps to choose an adequate treatment, and finally to evaluate the improvement in mucociliary clearance by the chosen therapy.