The process of exsanguination (death from loss of blood) causes the heart rate to drastically increase in order to effectively pump what blood you have all over the body and most importantly to the brain. At some point, the amount of blood making it to the brain is not enough and you pass out. During this whole process, you become cold and dizzy because not enough blood is circulating around the body. You can feel extremely heavy/weak and, indeed, get short of breath with little feats of exertion.
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