Here is some information that I got from Tootsies dog carers monthly newsletter on blood donation - hope it is alright to cut and paste it
Across the UK the Pet Blood Bank has 450 donors providing 1000 units of blood products for emergencies. In simple terms 100's of dogs are still alive today thanks to the work of the Pet Blood Bank. Pet Blood Bank UK is a charitable organisation that collects, stores and supplies blood products for immediate use in an emergency. There are lots of reasons why your pet could need a transfusion - road traffic accidents or where there is blood loss, major surgery, treatment of diseases that destroy the red blood cells etc. The bloods and plasma can also be used to treat gastroenteritis, to provide immunity for example in severe viruses such as parvo, or in blood clotting disorders such as haemophilia when an affected dog is at risk of bleeding to death from a small injury. Without the Blood Bank we would go back to the dark ages of vets ringing around in an emergency to find a donor - a practice fraught with problems, not least running out of time. Donor dogs must be between 1 and 8 years old, over 25kg in weight and be fit and healthy. Dogs need to be of a calm nature as they will need to lie still. There are benefits to being a blood donor too, the dogs receive a full clinical examination, and first time donors will have a full health profile run to ensure they are healthy on the inside as well - all free of charge.
Hope it helps somebody but Tootsie is too small to donate and would probably try to take the arm off the nurse if she came anywhere near her with a syringe