Mrs. Johnson is brought to the emergency room after being involved in an auto accident. She is haemorrhaging and has a rapid, thready pulse, but her blood pressure is still within normal limits....

Mrs. Johnson is brought to the emergency room after being involved in an auto accident. She is haemorrhaging and has a rapid, thready pulse, but her blood pressure is still within normal limits. Describe the compensatory mechanisms that act to maintain her blood pressure in the face of blood loss.