system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why is it when im exhausted and collapse on my bed my heart races wildly for a few seconds?
gpt
When you're standing, your heart has to do more than just pump blood to your feet; it has to pump with enough pressure to bring the blood BACK UP the legs too, and also pump blood up to the brain (both fighting gravity). The heart has a few functional things it can change, but here we'll just talk about how fast it beats, and how much blood it pumps per heartbeat. Obviously, both of those factors are working pretty hard when standing (which also makes giraffes pretty amazing animals), and when switching suddenly to lying down that gravity is instantly not forcing the heart to work as hard. Since it takes a few seconds to adjust how the heart is working, it will still be pumping a LOT of blood with suddenly almost zero resistance. In order to continue pumping correctly (until the heart adjusts), the heart has to pump faster to keep up.