There are just 2 scenarios involved here. Either the heart rate must be increased or decreased.

The heart rate must go up

 The heart rate must go up because the blood pressure is low, there’s a shortage of oxygen, an excess of CO₂ or the pH is too low – some of these go together e.g. O₂, CO₂ and pH interaction during exercise, stress, etc.

The hormone that increases heart rate is noradrenaline. This can be released by the brain’s medulla via the sympathetic neurons when baroreceptors which sense low blood pressure or chemoreceptors which sense excess or a lack of certain chemicals send signals via sensory neurons.

Noradrenaline then binds to the heart’s sino-atrial node which results in…