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The respiratory process is made possible by the alveoli – the small tissue sacs in the lungs. These small air sacs are much like a bunch of grapes that deflate and inflate during the breathing process.  Specifically, these tissues in the lungs are the center of gaseous exchange wherein oxygen gets absorbed in the blood stream for cell growth and carbon dioxide is removed. Its unique structure not only facilitates rapid and efficient exchange of gases that are essential for survival but in destroying foreign matter that threatens the overall health of the individual.

Function of alveoli

The alveoli are very important in the gaseous exchange in the atmosphere through the blood during respiration. Gas exchange is the process wherein oxygen is absorbed and carbon dioxide is taken out in the capillaries. Oxygen is important for cell growth and it enters the body during inhalation. On the other hand, carbon dioxide is the byproduct of the body’s metabolic process and exits the blood stream through exhalation.

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Alveoli structure and location

The alveoli are specifically designed to effectively fulfill their function in gas exchange. An adult lung has around 300 to 400 million alveoli that are 200 to 300 microns small. They contain elastic fibers and collagen and are made up of epithelial cells. The alveolus wall is made up of squamous alveolar cells and is remarkably thin to facilitate in the rapid exchange of gases between the air in the lungs and capillaries. These tiny air sacs expand when a person inhales and relax when a person exhales.

The alveoli are made up of two major cells, and these are:

  • Squamous epithelial cells

These cells are also referred to as Type I cells which play a vital role in gas diffusion. The lengthy cytoplasmic extensions of these cells are spread along the wall of the alveolus and make up the thin alveolar epithelium for efficient exchange of gases.

  • Great alveolar cells

These cells are referred as Type II cells which are important in maintaining the stability of the alveoli by producing surfactant that will reduce the surface tension of water. It is also important in separating membranes so gas exchange can occur efficiently as well as in repairing damaged endothelium in the alveolus.

These tiny air sacs comprise the end branch of the respiratory tree within the lungs in which gas exchange takes place.

What happens during gas exchange in the alveoli?

Gas exchange is the diffusion of two gases in the lungs, namely, oxygen and carbon dioxide. Gases move in to high concentration gradient (partial pressure) and move out from low concentration gradient. The oxygen in the alveoli has lower concentration gradient than in the blood, so it diffuses to the red blood cells. The oxygen-rich red blood cells then travel from the pulmonary arteries to the systemic arteries. Since the oxygen in the blood has high concentration gradient, it diffuses to the body tissues with a low concentration of oxygen.

On the contrary, body tissues have high concentration of carbon dioxide, so it diffuses into the blood with low concentration of carbon dioxide. The blood that is high in carbon dioxide is pumped back to the heart and to the lungs wherein carbon dioxide is removed from the blood and exchanged with oxygen in the alveoli. That is why people breathe oxygen and exhale carbon dioxide.

Alveoli diagram (pictures)

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