The alveoli or the air sacs are the terminal parts of the respiratory system. It is the site of actual gas exchange known as internal respiration. The human body has approximately 300 million alveolus. Each of them performs specifically to allow the oxygen to enter your body and the carbon dioxide to exit. Whenever you inhale, the diameter of the individual alveolus increases to accommodate the air inside. 


Under the microscope, the alveoli look like hollow spaces. The empty spaces are the area of gas exchange. The lining is a simple squamous epithelial tissue to allow the diffusion of gas to occur. It contains collagen and elastic fibers to accommodate the constant stretching of the organ. Because of the large amount of capillaries, oxygen easily diffuses in the blood and the carbon dioxide is expelled. 


The alveoli have three types of cells. They are the Type I cells, Type II cells and the macrophages. The type I cells are also known as the squamous cells. They line the structure of the alveolar wall. The Type II are the Great alveolar cells. They are important in the secretion of pulmonary surfactants. The cells release their secretion through exocytosis. Lastly, the macrophages are important in defending the alveoli from microbial invasion. They are also known as the dust cells. 



The surfactants released by the Type II cells reduce the surface tension of water. They consist of phospholipids and protein mixture. They are important in the pulmonary compliance, the ability of the lungs and the thorax to expand and to keep the alveolar stability. Without the surfactants, the alveoli will collapse at the end of the expiration. In cases of premature delivery, the amount of surfactant produced must be enough prior to the actual childbirth to ensure survival of the newborn.


Respiration occurs in different ways in the body. It can be the external respiration involving the exchange of gases between the organism and the environment. The process occurs in the alveoli. Another type of respiration is the internal, which involves the blood and the cells. The blood delivers the oxygen while the cells expel the carbon dioxide. Lastly, the oxygen taken up by the cell is used up to generate energy. This process is called cellular respiration. 

With the tiny structures in the lungs, the alveoli serves its function in supplying the body with its needed oxygen. Because of their valuable contribution in maintaining life, it is only proper to take care of them.

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