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The Structure of Alveoli and their role in Gas Exchange including the role and function of Surfactants.

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Introduction

The Structure of Alveoli and their role in Gas Exchange including the role and function of Surfactants Introduction Alveoli are found within the lungs. The trachea splits into 2 bronchi, which in turn split into multiple bronchioles. These in turn branch into finer and finer tubes, ending with the alveolar ducts, which terminate in alveolar sacs which are surrounded by clusters of alveoli. The alveolar wall is composed of two types of cell. Type I is involved in the diffusion of respiratory gases. Type II is involved in the production of surfactant. Into each alveolar sac opens a group of alveoli. Alveoli are shaped as tiny 'balloon-like' cells, to allow for efficient gas exchange and relatively easy inflation/deflation. The alveoli act as a specialised gaseous exchange surface in mammals, and therefore require constant access to the blood stream and air. This gas exchange occurs by the diffusion across type I cells and is aided by a large surface area, an extensive capillary network, a small diffusion path, and a fluid layer lining the alveoli. ...read more.

Middle

negative with respect to atmospheric pressure. The amount of net pressure required for inflation is dictated by the surface tension and radii of the alveoli. During inhalation the radii of the alveoli increase from about 0.05 mm to 0.1 mm. As surface tension exerts a slight inward pull, a slight negative pressure is set up around the adjacent capillaries, which may draw tissue fluid from them (Tissue fluid contains water, glucose, amino acids, fatty acids, glycerol, mineral salts, dissolved gases and vitamins). This impedes the exchange of gases between the alveoli and capillaries. The surface tension that would occur is due to the attraction between water molecules and tends to reduce the surface to its minimum, therefore minimising the efficiency of gas exchange. To increase the radii of the alveoli from .05 mm to 0.1 mm would require a large amount of outward pressure. One role of the surfactant in alveoli is to dramatically reduce the net amount of outward pressure required to inflate the alveoli. Type II cells (specialised epithelial cells) ...read more.

Conclusion

This keeps diffusion efficient in gas exchange. However, diffusion is not 100% efficient, i.e. not only oxygen diffuses into the blood and carbon dioxide only diffuses out. There is always some oxygen diffused out and carbon dioxide diffused in as the movement of gas particles is still random. When blood leaves the alveolus it contains the same partial pressure of oxygen and carbon dioxide as the alveolar air. Blood reaching the alveoli has a lower partial pressure of oxygen, and a higher partial pressure of carbon dioxide than the alveolar air. During this equalisation of pressures, the percentage saturation of the blood rises from about 70% to 90%. The composition of alveolar air, however, remains relatively constant throughout. However, alveolar air is very different from atmospheric air, containing over 100 times as much carbon dioxide and about 2/3 as much oxygen. The process of respiration is still inefficient because some alveoli are inevitably under-ventilated. In addition, a proportion of blood which goes to the lungs does not go through any alveolar capillaries and therefore never gets oxygenated, this leads to the result that the blood leaving the lungs is carrying its maximum capacity of oxygen. 1 ...read more.

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