However, it will take 100msec for the whole of both atria to depolarize. The heart must relax before it can contract in response to a second stimulus, so it cannot go into tetanus. Under certain circumstances the long refractory period can make an irregularity of the heartbeat get worse. If the ventricle is stimulated during diastole an extrasystole will be produced that will make the heart be refractory when the next action potential from the sinoatrial node reaches the ventricle and the normal beat will be lost. They must rely on external heat sources.
All species have a preferred body temperature range at which they optimally function.
According to Chart 1, warming the sinus venous accelerated the heart and cooling down the sinus venous slowed it. Poikilothermic hearts differ from mammalian hearts but they still react the same to extreme manipulation of temperature increases and decreases. When temperature of the human body increases, peripheral blood vessels dilate and cardiac output increases, this in effect can cause heat stroke. On the other hand, earlier studies have found that cold weather increases blood pressure in humans and can influence other sources correlated to heart attack threat. Sympathetic drugs increased rate of depolarization therefore increasing frequency of heartbeats.