Mucociliary clearance (MCC), mucociliary transport, or the mucociliary escalator describes the self-clearing mechanism of the airways in the

respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...

. It is one of the two protective processes for the

lung The lungs are the primary Organ (biology), organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the Vertebral column, backbone on either side of the heart. Their ...

s in removing inhaled

particles In the physical sciences, a particle (or corpuscle in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

including

pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...

s before they can reach the delicate tissue of the lungs. The other clearance mechanism is provided by the

cough reflex The cough reflex occurs when stimulation of cough receptors in the respiratory tract by dust or other foreign particles produces a cough, which causes rapidly moving air which usually remove the foreign material before it reaches the lungs. This ...

. Mucociliary clearance has a major role in pulmonary hygiene. MCC effectiveness relies on the correct properties of the airway surface liquid produced, both of the periciliary sol layer and the overlying mucus gel layer, and of the number and quality of the

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

present in the lining of the airways. An important factor is the rate of

mucin Mucins () are a family of high molecular weight, heavily glycosylated proteins ( glycoconjugates) produced by epithelial tissues in most animals. Mucins' key characteristic is their ability to form gels; therefore they are a key component in ...

secretion. The

ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by Gating (electrophysiol ...

s CFTR and

ENaC The epithelial sodium channel (ENaC), (also known as amiloride-sensitive sodium channel) is a membrane-bound ion channel that is selectively permeable to sodium ions (). It is assembled as a heterotrimer composed of three homologous subunits α ...

work together to maintain the necessary hydration of the airway surface liquid. Any disturbance in the closely regulated functioning of the cilia can cause a disease. Disturbances in the structural formation of the cilia can cause a number of ciliopathies, notably

primary ciliary dyskinesia Primary ciliary dyskinesia (PCD) is a rare, autosomal recessive genetic ciliopathy, that causes defects in the action of cilia lining the upper and lower respiratory tract, sinuses, Eustachian tube, middle ear, fallopian tube, and flagella of spe ...

. Cigarette smoke exposure can cause shortening of the cilia.

Function

In the upper part of the respiratory tract, the nasal hair in the

nostril A nostril (or naris , : nares ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbinates ...

s traps large particles, and the sneeze reflex may also be triggered to expel them. The

nasal mucosa The nasal mucosa lines the nasal cavity. It is part of the respiratory mucosa, the mucous membrane lining the respiratory tract. The nasal mucosa is intimately adherent to the periosteum or perichondrium of the nasal conchae. It is continuous w ...

also traps particles preventing their entry further into the tract. In the rest of the respiratory tract, particles of different sizes become deposited along different parts of the airways. Larger particles are trapped higher up in the larger

bronchi A bronchus ( ; : bronchi, ) is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi to branch from the trachea at the carina are the right main bronchus and the left main bronchus. Thes ...

. As the airways become narrower only smaller particles can pass. The branchings of the airways cause turbulence in the airflow at all of their junctions where particles can then be deposited and they never reach the alveoli. Only very small pathogens are able to gain entry to the alveoli. Mucociliary clearance functions to remove these particulates and also to trap and remove pathogens from the airways, in order to protect the delicate lung parenchyma, and also to provide protection and moisture to the airways. Mucociliary clearance also takes part in pulmonary elimination, which with

exhalation Exhalation (or expiration) is the flow of the breathing, breath out of an organism. In animals, it is the movement of air from the lungs out of the airways, to the external environment during breathing. This happens due to elastic properties of ...

removes substances discharged from the

pulmonary capillaries A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the inn ...

into the alveolar space. Bronchiolar epithelium 3 - SEM

Components

In the

respiratory tract The respiratory tract is the subdivision of the respiratory system involved with the process of conducting air to the alveoli for the purposes of gas exchange in mammals. The respiratory tract is lined with respiratory epithelium as respirato ...

, from the

trachea The trachea (: tracheae or tracheas), also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all animals' lungs. The trachea extends from ...

to the

terminal bronchiole The bronchioles ( ) are the smaller branches of the bronchial airways in the lower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start of the respiratory zone delivering air to th ...

s, the lining is of

respiratory epithelium Respiratory epithelium, or airway epithelium, is ciliated pseudostratified columnar epithelium a type of columnar epithelium found lining most of the respiratory tract as respiratory mucosa, where it serves to moisten and protect the airways ...

that is

ciliated The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike pr ...

. The

are hair-like, microtubular-based structures on the luminal surface of the

epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

. On each epithelial cell there are around 200 cilia that beat constantly at a rate of between 10 and 20 times per second. The

are surrounded by a periciliary liquid layer (PCL), a ''sol'' layer that is overlain with the ''gel'' layer of

mucus Mucus (, ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both Serous fluid, serous and muc ...

. These two components make up the epithelial lining fluid (ELF), also known as the airway surface liquid (ASL), the composition of which is tightly regulated. The

s CFTR, and

work together to maintain the necessary hydration of the airway surface liquid. An important factor is the rate of

secretion. The mucus helps maintain epithelial moisture and traps particulate material and

s moving through the airway, and its composition determines how well mucociliary clearance works.

Mechanism

Within the thin periciliary liquid layer the cilia beat in a coordinated fashion directed to the

pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

where the transported mucus is either swallowed or coughed up. This movement towards the pharynx is either upward from the lower respiratory tract or downwards from the nasal structures clearing the mucus that is constantly produced. Each cilium is about 7 μm in length, and is fixed at its base. Its beat has two parts: the power stroke, or effector stroke, and the recovery stroke. The movement of the cilia takes place in the periciliary liquid which is a little shorter in depth than the height of an extended cilium. This allows the cilia to penetrate the mucous layer during its full extension in the effector stroke, and to propel the mucus directionally, away from the cell surface. In the recovery stroke the cilium bends from one end to the other bringing it back to the starting point for the next power stroke. The returning cilia bend to immerse completely in the PCL which has the effect of reducing a reverse movement of mucus.

The coordinated movement of the cilia on all the cells is carried out in a fashion that is not clear. This produces wave-like motions that in the trachea, move at a speed of between 6 and 20 mm per minute. The wave produced is a metachronal wave that moves the mucus. Many

mathematical model A mathematical model is an abstract and concrete, abstract description of a concrete system using mathematics, mathematical concepts and language of mathematics, language. The process of developing a mathematical model is termed ''mathematical m ...

s have been developed in order to study the mechanisms of ciliary beating. These include models to understand the generation and rhythm of the metachronal wave, and the generation of the force in the effective stroke of the cilium.

Clinical significance

Effective mucociliary clearance depends on a number of factors including the numbers of cilia, and their structure particularly their height, and the quality of the mucus produced that needs to be maintained at a correct

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...

, temperature, and

acidity An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen cation, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid. The first category of acids are the ...

. The cilia need to be able to move freely in the periciliary liquid layer and when this is impaired through damage to the cilia or by imbalances in the moisture or pH of the PCL, the mucus is unable to be cleared properly from the airways.

Cystic fibrosis Cystic fibrosis (CF) is a genetic disorder inherited in an autosomal recessive manner that impairs the normal clearance of Sputum, mucus from the lungs, which facilitates the colonization and infection of the lungs by bacteria, notably ''Staphy ...

is a consequence of imbalances in the PCL. Accumulated mucus, apart from causing varying degrees of airflow obstruction, makes a breeding ground for bacteria that cause many respiratory infections that can seriously worsen existing lung disorders.

Obstructive lung disease Obstructive lung disease is a category of respiratory disease characterized by airway obstruction. Many obstructive diseases of the lung result from narrowing (obstruction) of the smaller bronchi and larger bronchioles, often because of excessiv ...

s often result from impaired mucociliary clearance that can be associated with

mucus hypersecretion Mucus (, ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...

and these are sometimes referred to as ''mucoobstructive lung diseases''. Studies have shown that the dehydration of airway surface liquid is enough to produce mucus obstruction even when there is no evidence of mucus hypersecretion.

Humidity

High

enhances mucociliary clearance. One study in dogs found that mucus transport was lower at an absolute humidity of 9 g water/m³ than at 30 g water/m³. Two methods of supporting this, particularly in

mechanical ventilation Mechanical ventilation or assisted ventilation is the Medicine, medical term for using a ventilator, ventilator machine to fully or partially provide artificial ventilation. Mechanical ventilation helps move air into and out of the lungs, wit ...

, are provided by active and passive respiratory gas humidifiers.

References

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External links

Genetic disorders of mucociliary clearance consortium
Respiratory system Histology