Blood donation. Catheter inserted into a vein in the arm of a blood donor. The process of puncturing a vein is called venepuncture. The catheter is connected by a tube to a bag on a weighing balance. The digital display of the balance shows that a volume of 45O millilitres (ml) of blood has been collected so far. Usually about 475ml is collected. This blood will be given to patients in transfusions to replace lost blood, for example during surgery or after accidents.