Measuring heart beats

Here are Group A counting their heart beats.

In science we have been learning about the human heart. We know that it is made of muscle and works hard all our lives! It's important to keep it healthy so it can pump blood full of oxygen to all the cells in our body.

When we measure our heart beat it is called a 'pulse.' It is measured in beats per minute. For our experiment we measured our pulse when sitting, walking and then after running around very fast. We found that when we ran fast our hearts beat faster. Why does the heart do that? What is happening when that happens?