A 45-year-old man with spinal cord injury reports feeling di…

A 45-year-old man with spinal cord injury reports feeling dizzy and lightheaded when standing up quickly from a chair. On examination, his blood pressure drops from 140/85 mmHg while supine to 110/70 mmHg after standing for 2 minutes, and his heart rate increases slightly. Which of the following is the most likely explanation?