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HUNTSVILLE, Ala. (WHNT) - When you see a dog at the vet, don't assume it's sick, it may be helping another dog see more days.

"An emergency clinic probably does more transfusions than we do here, we probably do a handful of transfusions a year," explained Dr. Staci Armstrong with Whitesburg Animal Hospital.

It's a longtime practice that few people know about. Most donors at Whitesburg, and many other hospitals, are pets of clinic employees.

"Cats have three blood types an dogs have a lot of different blood groups, there's at least 13 different blood types," said Dr. Armstrong.

Dogs can only donate to other dogs, cats to other cats, and so on. But like us, the donors have to be pre-screened.

"They need to be within a certain age range, typically 2-7 years old, and meet weight requirements as well," she explained. "We also screen for blood borne diseases that could be transmitted."

The system is very much like that of human blood transfusions.

Dogs usually need the most transfusions and the blood expires after a while, so vets usually seek the resources after there is a need.

There are also nationwide blood banks from which clinics can order.