An alveolus is an anatomical structure that has the form of a hollow cavity. Found in
the lung parenchyma, the pulmonary alveoli are the terminal ends of the respiratory tree,
which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas
exchange with the blood as well. Alveoli are particular to mammalian lungs. Different structures
are involved in gas exchange in other vertebrates. The alveolar membrane is the gas-exchange
surface. Carbon dioxide rich blood is pumped from the rest of the body into the alveolar
blood vessels where, through diffusion, it releases its carbon dioxide and absorbs oxygen.
Structure The alveoli are located in the respiratory
zone of the lungs, at the distal termination of the alveolar ducts and atria. These air
sacs are the forming and termination point of the respiratory tract. They provide total
surface area of about 100 m2.
A typical pair of human lungs contain about 700 million alveoli, producing 70m2 of surface
area. Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area.
An adult alveolus has an average diameter of 200 micrometres, with an increase in diameter
during inhalation. The alveoli consist of an epithelial layer
and extracellular matrix surrounded by capillaries. In some alveolar walls there are pores between
alveoli called Pores of Kohn. The alveoli contain some collagen and elastic fibres.
The elastic fibers allow the alveoli to stretch as they are filled with air during inhalation.
They then spring back during exhalation in order to expel the carbon dioxide-rich air.
Histology There are three major cell types in the alveolar
wall: Type I cells that form the structure of an
alveolar wall Type II cells that secrete pulmonary surfactant
to lower the surface tension of water and allows the membrane to separate, therefore
increasing its capability to exchange gases. Surfactant is continuously released by exocytosis.
It forms an underlying aqueous protein-containing hypophase and an overlying phospholipid film
composed primarily of dipalmitoyl phosphatidylcholine. Macrophages that destroy foreign material,
such as bacteria. Reinflation of the alveoli following exhalation
is made easier by pulmonary surfactant, which is a phospholipid and protein mixture that
reduces surface tension in the thin fluid coating within all alveoli. The fluid coating
is produced by the body in order to facilitate the transfer of gases between blood and alveolar
air. The surfactant is produced by great alveolar cells, which are the most numerous cells in
the alveoli, yet do not cover as much surface area as the squamous alveolar cells.
Great alveolar cells also repair the endotheilium of the alveolus when it becomes damaged. Insufficient
pulmonary surfactant in the alveoli can contribute to atelectasis. Without pulmonary surfactant,
atelectasis is a certainty; however, there are other causes of lung collapse such as
trauma, COPD, and pleuritis. Clinical significance
Diseases
Acute respiratory distress syndrome is a severe inflammatory disease of the lung. Usually
triggered by other pulmonary pathology, the uncontrolled inflammation leads to impaired
gas exchange, alveolar flooding and/or collapse, and systemic inflammatory response syndrome.
It usually requires mechanical ventilation in an intensive care unit setting.
Infant respiratory distress syndrome is a syndrome caused by lack of surfactant in the
lungs of premature infants. In asthma, the bronchioles, or the "bottle-necks"
into the sac are restricted, causing the amount of air flow into the lungs to be greatly reduced.
It can be triggered by irritants in the air, photochemical smog for example, as well as
substances that a person is allergic to. Emphysema is another disease of the lungs,
whereby the elastin in the walls of the alveoli is broken down by an imbalance between the
production of neutrophil elastase and alpha-1-antitrypsin. The resulting loss of elasticity in the lungs
leads to prolonged times for exhalation, which occurs through passive recoil of the expanded
lung. This leads to a smaller volume of gas exchanged per breath.
Chronic bronchitis occurs when an abundance of mucus is produced by the lungs. The production
of this substance occurs naturally when the lung tissue is exposed to irritants. In chronic
bronchitis, the air passages into the alveoli, the broncholiotes, become clogged with mucus.
This causes increased coughing in order to remove the mucus, and is often a result of
extended periods of exposure to cigarette smoke.
Cystic fibrosis is a genetic condition. A mutation of the cystic fibrosis transmembrane
conductance regulator gene causes defective CFTR proteins, which are transmembrane proteins
that function in Cl− transport in wet epithelia. Because wet epithelium is such a ubiquitous
and multipurpose tissue type, CF has myriad deleterious effects, some of the most serious
of which are severe respiratory problems. Many of the mechanisms by which CF causes
damage or inadequate function in the wet epithelia of other tissues, such as the digestive and
reproductive tracts, are well-understood. CF's mechanisms in causing lung disease, however,
remain poorly elucidated. One popular hypothesis suggests increased viscosity due to increased
salinity of the mucous secreted by glands of the pseudostratified respiratory epithelium,
causing difficulty in maintaining normal respiratory tract mucociliary clearance. The frequency
of certain specific bacterial infections has prompted two other popular categories of hypotheses:
that the high salt content may interfere with defensins and lysosome, and/or may encourage
the growth of the several bacterial species typically infecting the ordinarily-sterile
lower lungs of CF patients. Regular treatment is usually required—primarily percussive
therapy and antibiotics. Promising research into gene therapies is taking place.
Diffuse interstitial fibrosis Lung cancer is a common form of cancer causing
the uncontrolled growth of cells in the lung tissue. Due to the sensitivity of lung tissue,
such malignant growth is often hard to treat effectively.
Pneumonia is an infection of the lung parenchyma, which can be caused by both viruses and bacteria.
Cytokines and fluids are released into the alveolar cavity and/or interstitium in response
to infection, causing the effective surface area of gas exchange in the lungs to be reduced.
If this happens to such a degree that the patient cannot draw enough oxygen from his
or her environment to maintain cellular respiration, then the victim may need supplemental oxygen.
Cavitary pneumonia is a process in which the alveoli are destroyed and produce a cavity.
As the alveoli are destroyed, the surface area for gas exchange to occur becomes reduced.
Further changes in blood flow can lead to decline in lung function.
Pulmonary contusion is a bruise of the lung tissue.
References
External links Pulmonary Alveoli at the US National Library
of Medicine Medical Subject Headings