Your body contains about 10-12 pints (4.7-5.7 L) of blood. That life-giving blood is constantly on the move: it makes the entire circuit of your body three times every minute, passing through 60,000 miles of blood vessels. That's enough to circle the globe two-and-a-half times. In the course of an average lifetime, a heart pumps about a million barrels of blood. That's enough to fill three supertankers.

Your blood vessels, consisting of arteries, veins, and capillaries, range in size from arteries as wide as a garden hose to capillaries so thin that it would take 10 of them, lined up side by side, to form the thickness of a human hair. But they all have one thing in common: they are designed to move your blood as quickly and efficiently as possible. That means they need to be strong, flexible, and smooth. Even the components of your blood, like red blood cells, platelets, and white blood cells, are designed for movement. Red blood cells can actually flex so that they can flow through the finest of capillaries.

Circulation
Your blood is pumped from the right side of your heart to your lungs, where it picks up oxygen. From there, your blood returns to the left side of your heart, which then pumps this oxygen-rich blood through your arteries. The arteries branch into capillaries, which distribute the oxygen and nutrients in the blood to your cells. Your blood then enters the network of veins, which carry the now oxygen-depleted blood back to the right side of your heart. Then the blood is pumped back into your lungs to pick up oxygen again.

Most of your blood volume (about 65%) is carried in your veins. Inside the veins (and particularly in the leg veins) are valves, consisting of two flaps made of elastic tissue. These valves prevent blood from flowing backwards and keep it moving toward the heart. Blood return from the legs occurs mainly through the deep veins.

Blood flow through the veins is complex. Veins normally have low blood pressure, but flow rates through them can vary from fast (when muscles contract) to almost nothing when you stand or sit quietly. Gravity has a strong effect on flow rate as well.

More on this topic

What Are Thrombosis & Embolism? (VIDEO)
Your Blood Moves
Thrombus & Embolus
Symptoms & Risk Factors
Blood Must Flow
Consequences of Clots
Tests & Diagnosis for DVT
Prevention & Treatment

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