system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come there are some automated body functions that we can "override" and others that we can't?

For example, we can will ourselves breathe/blink faster, or choose to hold our breath. But at the same time, we can't will a faster or slower heart rate or digestion when it might be advantageous to do so. What is  the difference in the muscles involved or brain regions associated with  these automated functions?
gpt
For stuff like breath and blinking, those are controlled by muscles that receive signals from our brain to contract and relax. This is why we can override those actions.

For heart rate, it is controlled by pacemaker cells that are independent of the brain. Another example is when the doctor taps on your knee and your leg kicks, you can’t stop it. The signal never actually reaches your brain, just to your spinal chord and back.

Edit: clarification

Edit: you can indirectly control your heart rate by influencing it with other factors (movement and breathing). But you cannot only change your bpm through sheer willpower.