In normal situation urine color is yellow but in some diseases it is changed to red.Kidneys filter the blood to produce urine and urine is a mix of water, soluble minerals like sodium and metabolites like urea.
There are three major reasons for red urine:
1- Hematuria: means red blood cells in the urine due to stone in the urinary tract, crystaluria ( crystals in the urine), trauma to the urinary system, urinary parasitic infection ,prostate diseases, tumors
2- Hemoglobinuria: means hemoglobin- a part of red blood cells- in urine. In this situation some RBCs in vascular system have been exploded. Some poisons, blood parasites, phosphorous deficiency , immune mediated hemolytic anemia, infections, genetic illnesses and coagulopathy-defect in coagulation- can lead to hemoglobinuria.
3- Myoglobinuria: means myoglobin in urine. Myoglobin is a part of muscles which causes the color of them and carries oxygen for muscle cells. With increasing the age the amount of myoglobin is raised ,for this reason the color of muscles in old pets are dark red.Vitamin E or selenium deficiency, convulsion, severe exercise or trauma to the muscles may lead to myoglobinuria.