The blood moves through the body through the blood vessels -- essentially, flexible tubes that branch out and subdivide. There are different types of blood vessels: the arteries, capillaries, and veins.
Arteries carry the oxygen-rich blood that the heart pumps to the rest of the body. The heart pumps the blood out through one main artery, the dorsal aorta. This branches out into smaller arteries, which branch out in turn. The smallest arteries are called arterioles, and connect to capillaries.
Because the arteries carry large quantities of blood that is under
high pressure from the beating of the heart, they are wide and thick.
The walls of an artery consist of three layers: a tough outer layer,
a middle layer of muscle, and a smooth inner layer through which the
blood can flow easily. The muscles in the middle layer help the heart
pump the blood, squeezing down to move the blood along. You can feel
the pulsing of the arteries as your pulse.
Blood passes from the arterioles into the capillaries. Capillaries are very narrow -- only one cell wide. They have very thin walls made of overlapping flat cells called endothelium; the walls are thin so that oxygen and carbon dioxide can pass through them easily. Inside the capillaries, the red blood cells release their oxygen, which passes through the capillary walls and into the surrounding tissue. The tissue releases its waste products, like carbon dioxide, which passes through the capillary walls and into the red blood cells.
Some organs -- the liver, spleen, and bone marrow -- contain blood vessels called sinusoids instead of capillaries. Like capillaries, sinusoids are composed of endothelium. Sinusoids are a bit larger than capillaries.
From the capillaries/sinusoids, the de-oxygenated, waste-laden blood passes into the veins for its return trip to the heart. Veins are like arteries in that they have three layers. But since the blood is not under as much pressure, the walls of veins are thinner. Veins contain one-way valves to keep the blood flowing toward the heart, even against the pull of gravity. Because the blood in veins contains so little oxygen, it appears bluish rather than bright red. That's why the veins you can see through your skin (for example, in the underside of your wrist) are blue.
-- Sue Wilson
Illustration: Redrawn from Carol Ballard, THE HEART AND CIRCULATORY SYSTEM (Austin, Texas: Steck-Vaughn Company, 1997), pg.26.