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Flattened epithelial cells, also known as squamous epithelial cells, primarily facilitate diffusion and filtration due to their unique structural adaptations. The thin, flat morphology of these cells allows for substances to easily pass through them, making them ideally suited for locations in the body where rapid exchange of materials is essential. For example, in the alveoli of the lungs, these cells enable efficient gas exchange between the air and blood due to their minimal barrier thickness. Similarly, in capillaries, the flattened structure enhances the diffusion of nutrients, gases, and waste products between blood and surrounding tissues. This property is critical for maintaining homeostasis and supporting various physiological processes.

In contrast, structural support is typically the role of connective tissues, while protective functions are more characteristic of stratified epithelial cells, which have multiple layers. Storage of nutrients is not a function of epithelial cells; instead, that role is generally associated with certain types of connective tissues and other specialized cells in the body.