my rat is peeing blood..but not just randomly

Question
I'm keeping a rat for my friend and she didnt leave me with a cage so I was keeping the rat in the bathtub...i know what youre thinkng...ohhh god its going to freeze to death or something else will happen but I put towels on the bottom and had all sorts of stuff for him to play with...all safe of course. I accidentally left the door open and having three dogs I noticed my bigger one wasnt in the living room begging for me to play with him. I instinctly went to the bathroom and the dog was nowhere but the rat has slobber on his side so I assume my dog tried to eat it...not good, so I rinsed him off and gently towel dried him. after him sitting on my shoulder all night while I typed, I put him in the tub again to go to bed. As i was setting him down I watched for a minute and he started peeing diluted blood. It wasnt solid thick blood but the urine had spots of blood in it. Could I just be freaking out or should I take him to the vet? He was also in a petting zoo today so would that have anything to do with it?

Answer
While it seems that your intentions are good and you are trying to take good care of him, this doesn't seem like the best place to keep a rat. Why didn't your friend leave you the cage? Anyway you need to make sure he can easily reach food and water, maybe keep an area heater in there and SHUT THE DOOR. I know it was an accident but that has got to be your top priority. If your dog tried to pick him up he could be bleeding because of damaged internal organs. If not, its possible that he has a urinary tract infection. Either way he needs to see the vet immediately.