The body has several feedback mechanisms to maintain adequate blood flow and blood pressure.
If blood pressure drops, the heart beats faster in an attempt to raise it. This is reflex tachycardia.
This can happen because of reduced blood volume: when someone bleeds, or loses fluid due to excessive sweating or diarrhea, blood volume drops. In response, the feedback mechanism raises heart rate to increase cardiac output.
Or unexpected changes in blood flow, such as orthostatic hypotension. When a person suddenly stands up, gravity causes about a pint of blood to accumulate in the blood vessels of the legs and lower limbs. As a result, the amount of blood that returns to the heart and is pumped by the heart decreases and blood pressure drops.
Normally, the body responds quickly to a drop-in blood pressure: the heart beats faster and more forcefully to increase blood output.
Because this tachycardia occurs under hemodynamic conditions, it is called reflex tachycardia.
Keyword: reflex tachycardia