Any blood loss, needs treating immediately ,it is a frightening situation for both dog and owner, remember to keep calm, reassure your dog, and look for the site of the bleeding. if the blood is on the floor it is a good idea to start with the paws, as a rule I look for foreign bodies in the pads, cuts to the pads and ripped claws. 

My dog is bleeding what do I do?

Don't panic, be calm and methodical in your search for the site of the injury, remember to re-assure your dog, muzzle him/her if necessary, once you have found the wound, check to see if you can see anything immediately visible in the wound, if you do (do not try to remove it you do not know how much damage has been done internally) place pads around the object and apply firm pressure around it (never on top of it) seek veterinary help immediately. 

For wounds with no foreign bodies, place a pad over the wound and apply firm pressure, if the blood continues to come through apply more pads, never remove the original ones as to do so can impede the clotting process, apply a bandage over the pads keeping the pressure firm but not too tight Never use a tourniquet unless you have been trained to do so.( I always check for the pulse after applying any bandage to make sure the pressure is not stopping the blood flow to other parts of the body).

Seek veterinary help immediately.

Remember blood loss can be dangerous, be ready and able to cope with worsening of the situation, Shock or unconsciousness can follow serious blood loss always check that the dog is Breathing normally if not you may have to commence the A.B.C procedure (Airway, Breathing, Circulation).
Always have any injury or wound looked at immediately by your veterinarian.

First Aid is not meant to replace veterinary attention.