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Reflex tachycardia is a form of tachycardia, rapid beating of the heart, is an increased heart rate in response to the stimulus of decreased blood pressure which is transmitted via the cardiac nerves.
There are a number of causes. These include:
Decreases in blood volume
In situations where blood volume decreases, eg due to bleeding or dehydration due to excessive sweating or diarrhea, the heart rate increases to improve cardiac output and the oxygen supply to tissues
Decreases in blood flow=
The most common cause of this is orthostatic hypotension, as a person stands suddenly, gravity causes blood to pool in the veins of the legs and lower body. As a result, blood flow falls, the amount of circulating blood is reduced, and blood pressure falls. In a reflex response the heart beats faster to increase blood flow.
Nervous system physiology: neurophysiology - reflex