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The alveolus (plural:alveoli), tiny hollow sacs which are continuous with the airways, are the sites of gas exchange with the blood.



The alveoli are found in the respiratory zone of the lungs.


The alveoli are very small porous air sacs.

The alveoli have an innate tendency to collapse because of the bubble-like shape and high curvature. Phospholipids, which are called surfactants, and pores help to equalize pressures and prevent collapse.

The alveolar walls contain capillaries and a very small interstitial space. In some alveolar walls there are pores between alveoli. There are two major alveolar cell types in the alveolar wall:

  • Flat Type I cells forming the structure of a alveolar wall
  • Type II cells which secrete surfactant which lowers the surface tension and divide to produce Type I cells


The alveoli are small with very thin walls. They have a radius of 0.1mm and wall thickness of about 0.2µm.

Pulmonary gas exchange is driven by passive osmotic diffusion and does not require ATP-fueled enzyme-based transport. Substances move through the concentration gradient from a higher concentration to a lower concentration. In the alveoli, this means oxygen in the red blood cells will have a lower concentration than in the air. Conversely, carbon dioxide will have a higher concentration in the red blood cells than in the air. This causes the diffusion of oxygen into the blood, binding to haemoglobin protein molecules, and the diffusion of carbon dioxide through to the alveoli to be expelled into the air. Although carbon dioxide and oxygen are the main molecules exchanged, water vapor is also found to be excreted through the lungs.

One of the dangers of this process is that molecules with a high affinity for hemoglobin, such as carbon monoxide, may also bind to red blood cells. Carbon monoxide will readily diffuse past the alveoli in the lungs and into the blood cells. This means that if the concentration of carbon monoxide is high enough, oxygen deprivation will occur.

The lungs contain about 300 million alveoli, each wrapped in a fine mesh of capillaries. The lungs are constantly exposed to airborne pathogens and dust particles. The body employs many defenses to protect the lungs, including 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.

Alveolar gas pressures

Normal alveolar partial pressures for O2 and CO2 are 105mmHg and 40mmHg respectively. For normal air partial pressures for O2 and CO2 are 160mmHg and 0.3mmHg respectively. The alveolar oxygen pressure is lower because some oxygen leaves to the pulmonary capillaries. The alveolar carbon dioxide pressure is higher because carbon dioxide enters the alveoli from the pulmonary capillaries.

The factors that determine the values for alveolar PO2 and PCO2 are:

  • The pressure of outside air
  • The rate of alveolar ventilation
  • The rate of total body oxygen consumption and CO2 release

Hypoventilation exists when the ratio of carbon dioxide production to alveolar ventilation increases. Hyperventilation exists when the same ratio decreases.

Exchange between blood and gas

The blood that enters the pulmonary capillaries is the systemic venous blood which enter the lungs via the pulmonary arteries.

Due to differences in partial pressures across the alveolar-capillary membrane, O2 diffuses into the blood and CO2 diffuses out. Thus, the blood that returns to the heart has the same PO2 and PCO2 as the alveolar air. The more pulmonary capillaries participating in this process, the more total O2 and CO2 can be exchanged.

Matching air supply and blood supply in alveoli

To be most efficient the right proportion of alveolar ventilation and capillary perfusion should be available to each alveolus.

Homeostatic responses in the lungs minimize the mismatching of ventilation and blood flow.


  • 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 delicate lining of the alveoli is broken down, greatly reducing the effective surface area for diffusion. The gradual loss of the lungs' ability to draw oxygen into the blood deprives organs of oxygen. The heart attempts to pump more blood through the body in order to satisfy the body's need for oxygen, which in some cases may cause strain on the heart.
  • Chronic bronchitis occurs when too much 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 more a genetic condition caused by the dysfunction of the transmembrane conductance regulator, a transmembrane protein responsible for the transport of chloride ions. This causes huge amounts of mucus to clog the bronchiolites, simular to chronic bronchitis. The result is a persistent cough and reduced lung capacity.
  • Diffuse interstitial fibrosis
  • Lung cancer is a common form of cancer causing the uncontrolled growth of cells in the lung tissue. It is often difficult to prevent once started, due to the sensitivity of lung tissue to radiological exposure.
  • 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 he may need supplemental oxygen.

Last updated: 07-31-2005 00:42:45
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