Answer
To allow effective diffusion of nutrients and oxygen to the cells and also diffusion of waste and carbon dioxide from the cells
Explanation.
-Capillaries are small vessels that are only one cell thick; which is an adaptation that allows them to undertake their function effectively.
-The function of capillaries is to allow food and oxygen to diffuse to cells while waste is diffused from cells.
-Therefore; they need to be thin walled for these functions.
-If the capillaries did not have thin walls, the gases could not diffuse through, which would be difficult to carry out their functions.
