Blood travels in the body through a network of blood vessels. They are empty tubes with flexible walls. 

You can see some of your body's blood vessels through your skin. Other blood vessels are located deep inside your body.

Inside blood vessels, blood travels in only one direction. Arteries are blood vessels that transport blood from the heart to the rest of the body, whereas veins are blood vessels that transport blood towards the heart. 

The smallest blood vessels are called capillaries. Capillary vessels transport blood between cells and arteries or veins.

Capillary vessels are so small, that red blood cells can only barely travel through them.