The headline is broadly defensible, but the qualifications matter. Effect sizes vary by population, the strongest claims rest on shorter trials, and credible voices push back on how it's typically framed.
The brain registers that because there are a set of neurons in the heart called the sinoatrial node. It sends that information to the brain. That information is registered by the brain and the brain sends a message back to the heart to speed the heart up.
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Native comments, Twitter mentions, and Reddit threads about this claim — surfaced together so the conversation isn't fragmented across platforms.
Bookmarking — the dossier-vs-overview split is the right call. Most of the time I want overview; sometimes I want receipts.
Would love a "what would change this verdict" RSS feed. Sign me up if it exists.
Put differently, inhaling speeds your heart rate up. Now, the converse is also true. When you exhale, your lungs deflate. Your diaphragm moves up, and as a consequence, there's slightly less space for the heart. So, the heart shrinks a little bit, not a ton, but it's enough such that whatever blood is in the heart moves through more quickly per unit volume. That faster movement is sensed by neurons within the heart, sends a signal to the brain, and the brain activates those neurons within nucleus ambiguus, and very quickly sends a signal to the sinoatrial node to slow your heart rate down. Put differently, exhales slow your heart rate down, and they do so by way of vagal control over the sinoatrial node.