Richard Lower was able to perform these transfusions thanks to William Harvey who, in 1628, announced his theory of the circulation of the blood. Before Harvey, people had always realized that blood was vital to life, but they didn't know how it circulated in the body.
A vital development in blood transfusion came in 1900, when Karl Landsteiner demonstrated the different blood groups in human beings. After that, people were transfused with blood of their own group whenever possible.
Nowadays, blood transfusion saves many lives. People can be treated for shock by pumping plasma (the fluid part of the blood) into them. Large reserves of blood are necessary for open-heart operations, sometimes as much as 20 pints a patient.