Steven, The exact answer to your question is dependent upon which anatomy textbook one refers to. I've seen the estimate range from 50,000 to 100,000 miles. Go to: http://www.sln.fi.edu/biosci/heart.html and click on vessels. You'll see it stated that there are over 60,000 miles (97,000 kilometers) of blood vessels in a child's body and close to 100,000 miles (161,000 kilometers) in an adult's. The circumference of the earth at the equator is 25,000 miles. If the blood vessels of an adult were lined up end to end, they would circle the equator four times! I know this is hard to imagine. Most of the blood vessels in the human body are microscopic capillaries. They are short in length but there are a lot of them. There are about 40 billion (40,000,000,000) of them in the body. To appreciate this, you need to understand how big one billion (1,000,000,000) is. The human heart beats about 100,000 times a day. At this rate it still takes it thirty years to beat one billion times. Another way to look at it is that if you made $100,000 a day, working 7 days a week including holidays, it would take you thirty years to earn one billion dollars ( go to: http://www.webho.com/WealthClock , you'll see that Bill Gates earns a lot more than this)! So, even though capillaries are microscopic, there are so many of them that they constitute the majority of the body's 100,000 miles of vessels. Remember, vessels include arteries, veins and capillaries. Steven, to put my answer in perspective, consider this. If the DNA from a single human cell nucleus were stretched out, it would measure approximately six feet in length. If all of the DNA contained within the cells of a human being was stretched out and laid end to end, it would reach to the Moon and back eight thousand times. Robert Houska Mad Scientist