These vessels have a poorly defined tunica externa and the tunica media consists of only 1-2 smooth muscle cell layers. These vessels are known as resistance vessels. This is due to changes in the amount of force required to circulate blood through the cardiovascular system. These are the only vessels that permit exchange between the blood and interstitial fluids in peripheral tissues. Substrates exchanged are listed in your textbooks.
Capillaries have no tunica media nor externa.
They are endothelial tubes lined externally by a basement membrane. There are two major types of capillaries - continuous and fenestrated. Capillaries function as part of a network called capillary beds. They do not work as solitary units. The blood pressure in the venous system is substantially lower than that in arteries, and therefore the walls of veins aren t as thick as that of arteries.