We consider blood in a cat's urine to be an emergency. If it is coupled with straining in the litter box like this poor cat, especially in a male cat, you should contact your vet or the emergency centre immediately.
Blood in the urine indicates either a medical problem, like a urinary tract infection, or an anxiety problem.
Urinary tract infections occur in older cats, particularly older females with kidney disease or constipation problems.
The urethras of young male cats sometimes completely block. If your male or desexed male cat is straining and only producing a few drops of urine, and seems distressed do not hesitate to call a vet. They soon become painful and toxic, and can die.
The most common cause of blood in the urine is anxiety. Sometimes it is difficult to know why our cats are anxious but usually it is to do with another cat. Neighbourhood cats straying into your yard, too many cats in the household, a new cat, or human visitors can trigger anxiety in cats. Anxiety causes bladder inflammation and pain and a vicious cycle of anxiety - inflammation - pain - more anxiety is set up. Pain relief and allaying the anxiety soon helps these stressed cats.
Your vet will tease out the possible causes of the blood in the urine and treat accordingly.