How do you write an essay about a thief that stole a car and when he sees the police road block ahead his heart beats fast?

You don't.

That's not an essay, in which you discuss something, or argue a point of view. That's a story, in which you imagine what happened, describe it, invite your reader to feel part of the action.

Imagine you're there, and you're the thief. How are you feeling? What do you do? What happens? How does it all end?

That's what you write about!

The only alternative, if it really is an essay that's required, is that this is in response to a question on Human Biology, in which you are required to write about the physiological responses to the stressful situation, of what this does to his heart rate and pulse, his breathing, to his body's production of adrenaline, and its effects, and so on. But it has to be either this or the story.