All about your circulation system

There are two complete circulation systems in your body.
One goes from the heart to the lungs and back again—the pulmonary system—while the other goes from your heart to all other parts of your body, and back to the heart.
This is the systemic system.
When your heart pumps blood out from itself, this blood is under quite high pressure. If you were to cut a large artery, the blood would spurt out a distance of four feet or so.
The vessels which carry blood away from your heart are called arteries. They have quite thick, elastic walls.
As blood is pumped into the arteries, they swell a bit. When your heart rests for a moment, the elastic walls help to maintain blood pressure.
Big arteries divide into smaller branches, and so on and on.
Finally, these little tiny arteries become what are called capillaries. These are the smallest blood vessels you have (they are so small that the blood cells go through them in single file).
To show how really little they are, consider this: if you placed 50 of these smallest capillaries side by side, their combined thickness would be about that of a human hair.
These extremely tiny little tubes are in almost every part of your body. Prick your finger with a pin and you get a drop of blood. You have broken some of the capillaries.
To get back to your heart, the blood flows through another network—the veins.
Capillaries join together to form tiny veins. These, in turn, combine to form bigger ones and so on, until finally, all the blood gets back to the heart via two very large veins—the upper and lower vena cava.
The walls of veins are much thinner that those of arteries—they don’t have nearly as much muscle. When you exercise the muscles in, say, your legs, this squeezes the veins a bit, pushing the blood along.
But the blood can only flow one way—towards the heart. This is because veins have neat little one-way valves all through them, which close to prevent any “backflow” away from the heart.
A very simple, but effective system.
The network of blood vessels is your delivery system. It enables oxygen to be carried to all parts of the body, and carbon dioxide to be carried away.
It enables nutrients to be delivered to your cells, and waste products to be carried away to your kidneys for disposal.
It carries all the things your body cells need in order to stay alive and to function. It is truly a marvelous and intricate system.
It also is a big system. If you could stretch out all of your blood vessels into a single line, it would go around the equator about four times—roughly 100,000 miles!

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