Correct Option: A

The wall of blood vessels (arteries & veins) are made up of mainly 3 layers – the outer most tunica externa has a loose connective tissue, collagen fibres, lymph vessels and the middle tunica media has circular smooth muscle fibres, some elastic fibres and the inner most tunica interna is a single layered endothelium of polyhedral squamous cells. If the arteries reach organs they form arterioles which are thinner and mainly consist smooth muscles and when they subdivide in the tissues they make capillaries. The wall of capillaries are very thin (usually less than one micron) and have numerous minute pores and made up of only endothelium. Exchange of material takes place between blood and tissue fluid across the endothelial membrane of capillaries through active diffusion.