Final answer:
Capillaries are characterized by their microscopic size, single-layered thin walls, and role in connecting arteries and veins, providing a site for substance exchange in microcirculation.
Step-by-step explanation:
The characteristics of capillaries include having thin walls, being microscopic in size, and their function to connect arteries and veins. The thin walls of capillaries consist of little more than a single layer of endothelial cells, which are well-suited for the exchange of substances between the blood and surrounding tissues. Capillaries form networks called capillary beds to facilitate this exchange. These characteristics enable capillaries to efficiently perform their role in microcirculation.