Describe the structure of the arteries, veins, and capillaries. What properties do each vessels' wall-type bestow on the vessel? How is blood moved through each type of vessel?
Arteries and veins both have the same three layers in their vessel walls. The outer layer consists of connective tissue. In arteries, the outer layer has many fibers which give the arteries the elasticity needed to withstand the surges of blood produced by the pumping action of the heart. The middle layer consists of smooth muscle which helps the vessels dilate or constrict. Veins have a much less well developed smooth muscle layer than arteries have. Thus, the walls of veins are easily collapsible which allows the surrounding skeletal muscles to squeeze the blood in the veins forward toward the heart during body movement. This and not the pumping action of the heart is the mechanism by which blood is returned to the heart since the hydrostatic pressure produced by the heart on the blood is very low by the time the blood reaches the veins. The blood is prevented from moving backwards by a system of one-way valves in the veins. The inner layer of the vessel walls of arteries and veins is made of connective tissue lined with a simple squamous cell endothelium. As the arterioles connect with the capillaries, the two outer layers of their vessel walls as well as the connective tissue of the third layer end leaving the capillaries with vessel walls that are only a
