why pipe lines are round shaped and vessels in cylindrical

Please see previous discussion:

http://cr4.globalspec.com/thread/25721

Today we are following the example set by the greatest engineer of all time.

When God made the human body He (or She, your choice) made the arteries, vains and capillaries round. The digestive track was also made round. This is because a round thin wall shape can contain more pressure than a square thin wall shape. There are also other organs in the body that use round "tubings" to convey fluids. Why argue with success?
Another point, it is also easier to make branch connections between two round shapes (size-to-size or size-to-reducing) than it is between two square shapes.