There are different types of blood vessels in the body and each different type of vasculitis will usually affect specific kinds of blood vessel. It may be helpful to have an understanding of the different types of blood vessel to help understand the different types of vasculitis.
The blood vessels that carry blood from the heart to the body’s organs are called arteries. The main artery coming out of the heart is called the aorta and from this smaller arteries branch off (like roads coming off a motorway) to go to specific organs (eg the renal arteries go to the kidneys, the carotid arteries go to the head and the coronary arteries supply the heart muscle).
Inside the organ the arteries divide up into smaller and smaller arteries called arterioles and finally into the very tiny blood vessels called capillaries (imagine the tiniest veins in a leaf from a tree) where oxygen and nutrients come out of the blood to supply the organ. After this the capillaries join up to form veins to go back to the heart and then on to the lungs to be resupplied with oxygen before going back to the heart and around the body again.
Different types of vasculitis will usually affect mainly one size of blood vessel. Takayasu’s arteritis and Giant cell or Temporal arteritis are called large vessel vasculitis because they mainly affect the aorta and the biggest arteries as they branch off. Polyarteritis nodosa and Kawasaki disease are called medium vessel vasculitis because they affect the middle sized arteries as they go into the organs.
Most of the other types of vasculitis, such as Wegener’s granulomatosis (GPA), and Microscopic polyangiitis etc, are called small vessel vasculitis because they cause inflammation in the smallest blood vessels, the capillaries, as they supply the insides of the organs. Vasculitis is an auto-immune disease where the body’s immune system attacks blood vessels instead of defending them against infection.
An excellent video that explains very clearly and graphically, what vasculitis is all about, from the Khan Academy in the USA.
Also from the US, the Cleveland Clinic have produced a presentation on “What is Vasculitis and Defining Remission”.