Mrs. Rees is brought to the emergency room after being involved in an auto accident. She is hemorrhaging and has a rapid pulse that can barely be felt, but her blood pressure is still within normal...

Mrs. Rees is brought to the emergency room after being involved in an auto accident. She is hemorrhaging and has a rapid pulse that can barely be felt, but her blood pressure is still within normal limits. Describe the compensatory mechanisms that are maintaining her blood pressure in the face of blood loss.