Which structures are microscopic blood vessels that are only one cell in thickness?

Definition:
        
              "Capillaries are small hair like microscopic structure through which only one blood cell can pass at a time".

Explanation:

                    Capillaries are about 8-10 um in diameter, forming a network through put the body tissues, and exchange of oxygen, nutrient and waste between the blood and body tissues occur though capillaries.

As a connection between arterial and venous system:

Capillaries conncet arterial system with venous system because it is the ultimate destination of arterial blood from the heart and the starting point for the flow of venous blood back to heart. 

D.) domain, kingdom, phylum, class, order, family, genus, species