How do blood clots form in the bladder after a foley catheter is inserted?

Catheter trauma: As the Foley goes thru the urethra and into the bladder it can cause abrasions, leading to bleeding and clotting sometimes. However, if it is being inserted for renal stones or a hemorrhagic cystitis or other renal issue, it may be unrelated to the Foley catheter. Usually Foley trauma just causes the urine to be blood tinged temporarily. Patients pulling and tugging on their Foley can do this.