Factors our Heart rate

Doctors use a stethoscope or take our pulse to determine our heartbeat. Obviously the rate of our heartbeat can be an indication of our physical health. 

The question is "What does it show?". What factors can affect the rate of the heartbeat? 

 Project description:
In this project you will perform experiments to study the factors that may affect the rate of heartbeat. For every factor that you choose to study, perform experiments and record your observation in a data table that will later be used to draw a graph. 
 Details of this project
More details or support for this project are available at the members section of ScienceProject.com's web site. Material needed for experiment or a science kit about this title may be available at MiniScience.com.