Why does my heart rate jump when I walk up stairs?

Im 13, I am a tad bit chubby but I have a better than average stamina and can do things other fit people can do. But how come my heart rate goes up when I walk or run up stairs?

Why does my heart rate jump when I walk up stairs?
The more your muscles work the more oxygen they need. Blood delivers oxygen to every cell in the body. So when you climb stairs, your muscles work harder and so your heart pumps faster to deliver more oxygen to the muscles. The more weight you have, too, the harder the muscles have to work to lift you up the stairs, the more oxygen they need, and the faster your heart pumps.