The walls of blood vessels are composed of elastin, collagen and vascular smooth muscle (VSM).  Elastin is easy to stretch but collagen is much stiffer. VSM is NOT found in capillaries.  This is because capillaries are basically endothelial tubes.

FACTS:

  • VSM is not found in capillaries
  • arterioles have the greatest % of VSM (thus, they can contract most and have smallest C)
  • veins have higher C than arteries