Final answer:
Capillary walls are thin structures made of a single layer of endothelial cells, facilitating the exchange of substances with surrounding tissues.
Step-by-step explanation:
The capillary walls are thin, one-cell structures. The blood vessels such as arteries and veins are composed of three layers, namely epithelial, mesothelial and endothelial. The capillary walls consist of a single layer of endothelial cells. This single-layer structure is well-suited for the exchange of substances between the blood and surrounding tissues, allowing substances like water, oxygen, glucose, and other nutrients, as well as waste products such as carbon dioxide, to pass quickly and easily through the capillary walls.