The great physician Dr. Starling states, heart rate times stroke volume equals cardiac output. There must be an adequate heart rate in order to meet metabolic demand. If the heart rate is to low, the body cannot maintain metabolic demand. Therefore we must speed up the rate of contractions.
A patient who has low blood volume will require fluid resuscitation. The ventricles must have fluid to fill the myocardium so that there may be cardiac output. The greater the amount of fluid and stretching of the muscle, the greater the contraction. The heart will improve cardiac output immediately. Do not delay care to administrate any additional therapies that are not required ASAP..