They only discovered that blood came in different types in 1900. Blood comes in four main types - O, A, B and AB. Group O is the most common which means it's in demand the most. But blood can also be subdivided into its main components - red cells, white cells, platelets and plasma. Unfortunately red cells only last 35 days and platelets are only good for five days.

William Harvey was the first physician to discover that blood circulates round the body back in 1628. Sir Christopher Wren actually injected fluids into the circulation of animals, using an instrument invented by the aforementioned Mr. Harvey.

The first successful blood transfusion was carried out in 1665 by Dr Richard Lower, using animals as subjects. When they started performing transfusions on humans, initially with animal blood, they couldn't understand why their recipients kept dying. It wasn't until 1900 when Dr Karl Landsteiner of Vienna discovered that blood comes in four different groups. He got the Nobel Prize for his discovery. Didn't stop him dying of a blood clot though.