Diagram of the alveoli with both cross-section and external view
An alveolus (plural: alveoli, from Latin alveus, "little cavity"), is an anatomical structure that has
the form of a hollow cavity. Mainly found in the lung, the pulmonary alveoli are spherical outcroppings of the respiratory
bronchioles and are the primary sites of gas exchange with the blood. Alveoli are particular to mammalian lungs; different structures are involved in gas exchange in other
vertebrates.[1]
Location
The alveoli are found in the respiratory zone of the lungs.
Anatomy
The lungs contain about 300 million alveoli, representing a total surface area of 70-90 square metres, each wrapped in a fine
mesh of capillaries.
The alveoli have radii of about 0.1 mm and wall thicknesses of about 0.2 µm.
The alveoli consist of an epithelial layer and extracellular matrix surrounded by capillaries. In some alveolar walls there
are pores between alveoli.
There are three major alveolar cell types in the alveolar wall (pneumocytes):
- Type I cells that form the structure of an alveolar wall
- Type II cells that secrete surfactant to lower the surface tension of water and allows
the membrane to separate thereby increasing the capability to exchange gases.
- Type III cells that destroy foreign material, such as bacteria.
The alveoli have an innate tendency to collapse (atelectasis) because of their spherical
shape, small size, and surface tension due to water vapor. Phospholipids, which are called
surfactants, and pores help to equalize pressures and prevent collapse.
Pulmonary gas exchange
Pulmonary gas exchange is driven by passive diffusion and thus does not require energy for
exchange. Substances move down a concentration gradient. Oxygen moves from the alveoli (high
oxygen concentration) to the blood (lower oxygen concentration, due to the continuous consumption of oxygen in the body).
Conversely, carbon dioxide is produced by metabolism and has a higher concentration in the blood than in the air.
Oxygen in the lungs first diffuses through the alveolar wall and dissolves in the fluid phase of blood. The amount of oxygen
dissolved in the fluid phase is governed by Henry's Law. Oxygen dissolved in the blood may
diffuse into red blood cells and bind to hemoglobin. Binding of oxygen to hemoglobin allows a
greater amount of oxygen to be transported in the blood. Although carbon dioxide and oxygen are the most important molecules
exchanged, other gases are also transported between the alveoli and blood. The amount of a gas that is exchanged depends on the
water solubility of the gas and the affinity of the gas for hemoglobin. Water vapor is also
excreted through the lungs, due to humidification of inspired air by the lung tissues.
Red blood cells transit the alveolar capillaries in about 3/4 of a second.
- Most gases (including carbon dioxide and nitrous
oxide) reach equilibrium with the blood before the red blood cells leave the alveolar capillaries. Gases that reach
equilibrium before the blood leaves the alveolar capillaries are perfusion limited, since the amount of the gas exchanged
depends solely on the volumetric flow rate of blood past the alveoli.
- However, carbon monoxide is stored in such high concentrations in the blood, due to
its strong binding to hemoglobin, that equilibrium is not reached before the blood leaves the alveolar capillary. Thus, the
concentration of carbon monoxide in the arterial system can be used to assess the resistance of the alveolar walls to gas
diffusion. Transport of carbon monoxide is thus termed diffusion limited.
Oxygen is normally perfusion limited, but in disease conditions it can be diffusion limited.
Defense against pathogens
The lungs are constantly exposed to airborne pathogens and dust particles. The body employs
many defenses to protect the lungs, including tonsils in the nasopharynx which traps germs, small hairs (cilia) lining the
trachea and bronchi supporting a constant stream of
mucus out of the lungs, and reflex coughing and sneezing to dislodge mucus contaminated with dust
particles or micro-organisms. The windpipe (trachea) divides into 2 airways the right and left bronchus
Diseases
- 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
(elevated by cigarette smoke) and alpha-1-antitrypsin (the activity varies due to genetics or reaction of a critical methionine
residue with toxins including cigarette smoke). 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 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 caused by the dysfunction of a
transmembrane protein responsible for the transport of chloride ions. This causes
huge amounts of mucus to clog the bronchiolites, similar to chronic bronchitis. The result is a persistent cough and reduced lung
capacity.
- 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 alveoli, which can be caused by both
viruses and bacteria. Toxins and fluids are released
from the virus causing the effective surface area of the lungs to be greatly reduced. If this happens to such a degree that the
patient cannot draw enough oxygen from his environment, 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.
See also
Additional images
References
External links
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