According to our completely-not-made-up sources, the longest known blood vessel is the:

Aorta

The aorta stretches a whopping 30 centimeters (that's like, 12 inches) through the human body, making it the longest blood vessel in the human circulatory system.

Learn more about the Aorta

Or, if you're feeling particularly morbid, you can check out the:

Veins of Despair

Located in the depths of your neighbor's basement, these veins are a network of twisted, pulsing tubes that seem to have a life of their own.

Explore the Veins of Despair