Hit by a car, butt is swollen and red, blood in the urine

Blood in the urine after a dog is hit by a car is very likely an internal injury, such as rupture and bleeding of the bladder, spleen, or other internal organs. Severe internal bleeding can cause abdominal fluid buildup and peritonitis, and without prompt treatment can be fatal, so take the dog to a veterinary clinic promptly for an ultrasound.

If the exam shows only lower urinary tract bleeding, you can give the dog appropriate anti-inflammatory, hemostatic, and bladder-mucosa repair medications such as amoxicillin clavulanate, etamsylate, carbazochrome, or sucralfate. The specific medication depends on the exam results; the owner must not self-medicate.