If your sibling dressed up in a scary costume to give you a fright, your eyes would see this, and chemical and electrical messages in the brain would send impulses to the heart, to cause it to beat faster. This would deliver more blood and oxygen to the cells of the arms and legs, so you could run away.

And when you are at home on a Saturday night, relaxing and watching your favourite TV show, the opposite thing happens: the heart doesn’t have to work as hard and so it relaxes and returns to its resting speed, with most blood going to the gut rather than the limbs, again caused by chemical and electrical messages.