Medical question about heart rates

I notice that, when taking my pulse, my heart rate temporarily slows down when I breathe out, and then speeds up again when I breathe in. Does anyone know why this is?

P.S. - I’m not in the least worried about this and am not looking for medical advice. I’m just curious about the mechanism which causes this. Thanks.

When you inhale, you decrease the pressure in the chest (to get the air in), and that allows more blood to return to the heart which then makes the heart speed up.

When you exhale, you increase the pressure in the chest (to push the air out) and less blood flows into the heart which makes the heart slow down.

This is technically called a “sinus arrhythmia” and is considered a “normal variant”. Not a problem at all. I usually see it in people with better physical fitness.

Wikipedia has a more neurologically related explanation: Sinus arrhythmia - Wikipedia