Hello,
Vascular smooth muscle (VSM), which is present in ALL arteries regardless of size, has a different role depending on the kind of artery. Similar to a stretched rubber band returning to its original size, a major conduit artery like the aorta has a lot of elastic tissue inside of it, which enables it to expand during systole and then elastically recoil to shrink back to its original diameter during diastole. This function smooths out the difference between systolic and diastolic pressures during each cardiac cycle, preventing systemic blood pressure from experiencing as much variation in pressure as the Because of this, we refer to the major arteies as acting as a pressure reservoir.
The VSM becomes increasingly significant as arteries narrow, and by the time you reach the arterioles, the so-called resistance vessels, you have VSM capable of causing significant changes in artery width. This is what regulates blood pressure and blood flow to certain tissues. Both the autonomic nervous system and regional cues from the tissue being fed play a complicated role in controlling artery VSM.
Capillaries are just a single layer of endothelial cells backed by some basement membrane; they lack a VSM and elastic tissue. Due to their extreme thinness, they serve their primary purpose as exchange vessels, which allows for the quick exchange of nutrients, wastes, and gases (O2, CO2). functioning as a pressure reservoir.
Veins serve as a large volume reservoir for the cardiovascular system because they have less elastic tissue than arteries and are more compliant (they stretch easier). In fact, the venous system contains around 70% of all blood volume at any given time. Veins are therefore referred to as "capacitance vessels" for this reason. Veins also have autonomic-controlled VSM that can contract, sending more blood back to the heart so that more can be put into the arterial system—for example, during emergency situations like major blood loss or during exercise. This volume reservoir can be used as needed. Return of blood to the heart (so-called venous return) is also helped by veins' compliance--as muscles contract, they can easily squeeze the veins, to propel blood back to the heart, and their one-way valves ensure that the flow goes one way.
Thanks,
Eddie