Do dogs swell up when going into heat?

Usually, yes. This is an act called "tying". This occurs because blood flows to a tissue found near the center of the males appendage. While it can be rather painful for both parties involved, you should never try forcefullyseparatingtwo dogs once they've tied as it can seriously injure the dogs. Tieing usually lasts anywhere from 10 minutes to a full hour. If the male or female panics and tries to rip themselves apart, you need to see if you can calm them down by sitting and talking to them quietly. Don't get too close, though, as males are notorious for getting aggressive during mating.