A 47 year old teacher goes to her physician complaining of dizziness, fainting, and shortness of breath. She appears pale and chronically ill. Her heart rate is 70/min supine and 68/min sitting and standing. Her blood pressure is 120/74 supine, 98/54 sitting, and 84/48 standing. Heart sounds and lung sounds are normal. Her abdominal exam and her general neurologic exam are both normal. Routine lab exams are normal. Identify any abnormal data in the description of this patient? Several studies were performed to evaluate the patient’s circulatory control mechanisms. She was placed on a tilt table which allows rapid changes in the patient’s orientation. Her blood pressure while supine was 102/60. She was then tilted to a 45º heads-up position and her blood pressure fell to 70/35. There was no change in her heart rate when tilted. What possible mechanisms could account for this result?