In The Last of Us Part 2, no one is safe. One minute you’re out for a relaxing horseback ride in the snow, the next you’re getting your face ripped off by an infected manbeast. It’s enough to make anyone a little tense. And it seems Naughty Dog has added some new technology to the engine to really sell how amped the characters in this universe are. Everyone, it seems, has a heartbeat.

There’s no heartbeat monitor in The Last of Us Part 2. It’s not a gameplay mechanic where you have to keep a steady heartbeat and there’s no stamina meter to worry about. But characters’ heart rates do alter the game in significant ways.

Anthony Newman, the game’s co-director, explained this new technology in an interview with Polygon.

“Just every facet of the game updated some new level,” said Newman. “One of them is the audio where, I’m not sure if you noticed it, but as Ellie sprints around and then she settles, she’ll kind of [mimics heavy breathing] catch her breath.”

This is a cool touch, but heavy breathing after a sprint isn’t all that unique in video games. Apparently it goes even deeper, though.

“What’s happening behind the scenes is that [Ellie] has a heart rate that is oscillating up and down,” Newman said. “It goes up when you sprint, it goes up when you melee, it goes up when you take damage, and goes up into the presence of enemies. And that modulates the bucket of breathing sounds that she’s able to use.”

In other words, any action you take will have an impact on the sounds that Ellie is making. Even just being near enemies will cause her to breathe a little harder, making the scene even more tense and realistic.

But she’s not alone. Every character in The Last of Us Part 2 is driven by the same tech, including human enemies and, yes, even the infected.

“It’s been incredible because I found myself able to play cat and mouse with Clickers better than ever before because I can kind of understand them by the noises that they’re making,” said Newman. “And the humans will do the same thing with breathing and sprinting and stuff.”

The Last of Us Part 2 got some attention by giving names to all of the enemies in the game (which causes them to accurately call out to one another in times of danger or stress). “So not only do our human enemies have names,” said Newman. “They have hearts!”