If you need to quickly monitor your heartbeat rate and do not have any additional equipment with you, but your smartphone than Heart Rate & Pulse Monitor will come to your rescue

This app can measure your heart rate by detecting changes in blood volume below the skin's surface .

Here's how it works: Every time your heart beats, the amount of blood that reaches the capillaries in your fingers and face swells and then recedes. Because blood absorbs light, apps are able to capture this ebb and flow by using the flash of your phone's camera to illuminate the skin and create a reflection.