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

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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. The secondary function of alveoli is the production of surfactant.

Figure 1

Structure

A certain amount of gaseous exchange can take place across the walls of the smaller bronchioles, however it is the alveoli which have the specialised surface for gaseous exchange. There are approximately 720 million alveoli present in the human lungs, with a total surface area of 75-95cm3. The wall of the alveolus is made up of squamous epithelium. This is composed of cells which are flattened like paving stones, alternatively called pavement epithelium. The resulting sheet of cells is thin and delicate, often less than 2.0 µm (micrometres) thick. It is found within the alveoli because it needs to act as a readily permeable protective covering for molecules and ions in solution. This lays between the capillary network that is in very close proximity to the alveoli. It offers minimum resistance to the diffusion of gases from one side of the cell to the other.
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Within the alveolus there are alveolar phagocytes, which are white corpuscles, which reside within it to help remove pathogens and remove dirt/dust that may otherwise block the gaseous exchange surface and reduce the efficiency of the alveolar cell, as mucus can't be used inside alveoli as they can in the bronchi and trachea. This is because mucus would prevent the permeable squamous epithelial cell membrane from being used efficiently for the diffusion of gases. Also within the alveoli reside special epithelial cells which produce surfactant.

Surfactant and its Function

Inflating the alveoli in the process of ...

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