if a person loses a lot of blood,for example in an accident,the pulse may become very weak.why?
Less blood means that there lessvolume around to be transported. For the heart that means that the heart receives less volume when it relaxes in order to refill the the heart chamber (diastole). This in turn, means that there is less blood to pump during each time the heart expels the blood into the circulation (systole). Additionally, there is a twist in the physiology of the heart: The force that the heartmuscle can generate actually depends on the amount of filling of the heart chamber during diastole (this is called the Frank-Starling mechanism). So not only is there less blood to expel, it can also only be expelled with a weaker force. For less severe blood loss, our blood vessels can make up for that by getting narrower, but in really severe blood loss, this mechanism becomes exhausted and we definitely see a weaker pulse (or, which is the same, a decreasing blood pressure).
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