Why does exhaling slow down heart rate?

Exhaling slows down the heart rate primarily due to the activation of the parasympathetic nervous system via the vagus nerve. When you exhale, the diaphragm moves up, reducing the space in the thoracic cavity, which causes the heart to become slightly smaller. This decrease in heart volume causes blood to move more quickly through a smaller space, and this change is detected by the nervous system. In response, a signal is sent via the vagus nerve to slow down the heart rate. This process is part of what's known as respiratory sinus arrhythmia, a natural increase and decrease in heart rate that occurs with breathing 1.