Blood vessels are the hollow tubes which carry blood throughout your body. There are three basic types of blood vessels. Arteries carry blood away from your heart towards the vast system of capillaries. Across the single celled walls of capillaries is where all of the nutrient and oxygen exchange between blood and the cells of your body occurs. From the capillaries blood travels through the veins to return to the heart and be filled with oxygen again. All blood vessels are covered on the inside by a layer of endothelial cells.
Most people associate red, oxygenated
blood with arteries and blue, deoxygenated
blood, with veins. Though this is true for systematic
circulation, it is exactly opposite in pulmonary
circulation. This is because arteries and veins are not defined by the type
of blood that they carry, but the direction in which they carry it. Arteries
always carry blood away from the heart, while veins always carry blood
toward the heart.
Arteries
Arteries
have a relatively large middle layer. Since the middle layer consists mostly
of muscle and elastic fibers arteries have the unique property of being able
to stretch and squeeze. This elasticity assists the heart by keeping the blood
going and pushing it through the vessels.
Veins
The
middle, muscular, layer of veins is much smaller than that of arteries because
they do not have the same need to stretch and squeeze. Since veins are so far
from the contracting forces of the heart often times the blood inside of them
has the tendency to flow backward, usually as a result of gravity. To prevent
this backward flow large veins have one way valves which keep the blood flowing
in one direction.
Capillaries
Since capillaries must allow oxygen and nutrients to pass across its walls the walls are extremely thin, in fact they are only one cell thick.
