The presence of red blood cells in the urine, officially called Hematuria, can be a symptom of a wide range of conditions, many of which are serious and others which will cause your dog pain if left untreated. Any dog displaying such symptoms should be immediately referred to a veterinarian.

As might be expected, most causes of Hematuria involve infections and conditions specifically affecting the urinary tract. Probably the most common cause of Hematuria is a Urinary Tract Infection (UTI), or bladder or kidney infection/disease, all of which can be treated with antibiotics. Physical signs of these kinds of infection include pus, blood or crystals in the urine and lower back pain. There may also be a marked increase or decrease in the volume of urine produced. Bladder Stones can also cause blood in the urine, and can often be treated using anti-inflammatory drugs and occasionally surgical removal. In non-neutered males, an infection or disease of the prostate can also result in blood in the urine. Prostate disease may also lead to enlarged testicles and frequent urination, with treatment varying widely. Because of the range of illnesses that can affect the urinary system, it is simply not possible to know which specific condition needs to be treated without a thorough veterinary examination, which will include blood work and perhaps x-rays.

As a side note, a common cause of kidney disease, which might be responsible for the blood, are tick-transmitted diseases such as Lyme disease, which will have a range of other symptoms and need to be treated as quickly as possible. Aside from illness or infection, other causes of blood in the urine include physical trauma to the abdomen area, poisoning (for example by the rat poison Warfarin, of  which one of the main symptoms is frequent and bloody urination), and a tumor growth somewhere in the abdominal region.