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The alveoli (singular: alveolus) are very small air sacs found in the lung tissue of certain organisms. They consist of porous sacs made up of small cells, which allow fluids to diffuse through. The lungs utilise passive osmotic diffusion which does not require the assistance of 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 their air and subsequently the alveoli of the lungs. Conversely, carbon dioxide will have a higher concentration in the red blood cells[?] than in the alveoli of the lungs. 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 vapour is also found to be excreted through the lungs.

One of the dangers of this process is that molecules with a high affinity for haemoglobic 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 deprevation will occur. The alveoli of the lungs consist of millions of tiny sacs, each wrapped carefully in a network of capillaries. Often, the lungs and its alveoli sacs are the first line of defence against airborne bacteria. There are many defences which are employed to protect the lungs, including small hairs in the breathing passages, mucus lining in the lungs as well as coughing and sneezing to dislodge mucus contaminated with dust particles or micro-organisms.

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.

In asthma, the bronchioles[?], or the "bottle-neck" into this 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 are broken down greatly reducing the effective surface area for diffusion. The gradual loss of the lungs' ability to draw oxygen into the blood will deprive organs of oxygen. The heart will attempt 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 irritants are exposed to the lung tissue. In Chronic Bronchitis, the air passage 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 more 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.

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 its onset, due to the sensitivity of lung tissue to radiological exposure.

This article is incomplete, and any contributions would be welcome. Improvements in grammer, spelling or factual information presented herein are also welcome.

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