Common urinary tract infections from bacteria like E. coli often cause bloody cat urine needing antibiotics prescribed by the vet.

Bladder crystals or stones can tear, ulcerate and cause bleeding when passing through urethras, especially in male cats

Kidney inflammation, infection or cancer produces bloody urine. Toxins build needing emergency diagnosis.  

External physical trauma from a fight or accident damaging lower urinary tracts or internal organs makes pee bloody.

Advanced heart or liver failure can cause systemic internal bleeding leading blood to leak into urine as a complication.  

Obstructions from crystals, mucus plugs or urethral strictures block normal flow, tearing tissue causing bloody leakage.