What happens when a female dog is in heat?

First she'll have some bleeding from her vagina. Then she will need to be securely shut either in your home or in a secure roofed enclosure that dogs cannot access. Bear in mind that a dog scenting a bitch in heat will climb an impossible fence She'll be agitated and want to get out, but don't let her. If you have any doubt your bitch is on heat you'll need to see a vet to have assessed her state of health.