Humans aren't the only ones who can donate blood.

Tracy Hendrickson

Hematologist Tracy Hendrickson wasn't even 50 years old when she sold her business and found herself a few millions dollars richer. But instead of buying a fancy house and taking it easy, she went back to work. Hendrickson used her early retirement to focus on her volunteer project, Companion Pets Blood Bank in Tulsa, the only blood bank for dogs in the entire state of Oklahoma.

"Financial freedom gave me the opportunity to retire before 50 and do what I love most," she told Tulsa People.

Hendrickson has reportedly recruited a pool of about 250 dogs—her three-year-old boxer, Mitchyll, among them—that can donate blood as often as once a month. In return for their generosity, they're given treats and all the butt scratches their hearts desire.

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According to Tulsa People, Hendrickson collects an average of 100 to 200 units per month, which she stores at local emergency clinics so vets have 24/7 access to it. Emergencies requiring blood donations run the gamut, and include flea and tick anemia, severe hookworm infections, cancer, and being hit by a car.

"I get to play hero a lot," Hendrickson, who also runs a Boxer rescue, told KJRH, "rushing around town delivering blood units (and) saving dogs' lives."

"Sometimes that blood donation is the lifeline for another dog in a crisis-type situation," she added, "so, yeah, they're ultimately a hero."

Not every dog can donate blood though, Hendrickson noted. "Dogs have to be between one and seven years old. They have to do a sit and stay, probably for about five minutes, getting loved on and lots of treats afterwards," she explained. "They also to be up-to-date on shots, on heartworm preventative and all-around good general health." It's also preferable for doggy donors to weigh about 50 pounds.

Find more information on Companion Pets Blood Bank here.