When someone is startled, their heart rate increases. They didn’t run or perform any physical activity, so why did their heart rate increase?
Humans, and most other animals, under the proper circumstances (fear), are readied by their bodies to make a decision. Either to stay and fight, or to make a run for it (commonly known as the fight or flight response). In order to ready itself, the bodies’ cells must have a way of communicating with one another. This is where the process of cell signaling comes into play.
There are three types of cell signaling: local signaling, long-distance signaling, and nerve cell signaling (formally known as pecarine, endocrine, and synaptic signaling, respectively). In pecarine signaling, a cell releases a specialized signaling molecule called a ligand, into intracellular space. These ligands are targeted at cells in the immediate vicinity. In endocrine signaling, an endocrine cell releases a hormone into the bloodstream to reach other cells throughout the body (synaptic signaling is not as important in the fight or flight response).
In a fight or flight situation, endocrine cells release the hormone adrenaline into the bloodstream. Adrenaline increases heart rate and communicates other messages to cells in different parts of the body to perform actions that will help the body fight or fly.