Acapillary is a tiny blood vessel where: oxygen and nutrients diffuse (move) from the blood to the cells near the capillary. carbon dioxide and other waste molecules produced by the cells near the capillary diffuse into the blood. explain why it is useful for the walls of capillaries to consist of a single layer of thin, flattened cells. (5pts)

It is useful for the walls of capillaries to consist of a single layer of thin, flattened cells in order to easily diffuse oxygen and nutrients easily from the blood and into the capillaries.  Not only that, but this also makes it more efficient for carbon dioxide and other waste molecules to diffuse from the capillaries and into the blood.  If capillaries were thicker than a cell, then it would be more difficult for materials to diffuse through the thick layer of cells.

That would be a working theory.