The way I understand it gravity causes blood to pool in your legs whenever you stand. The body compensates for this by increasing your heart rate and constricting blood vessels, thereby ensuring that enough blood returns to your brain. But in people with postural hypotension, this compensating mechanism fails and blood pressure falls, leading to symptoms of dizziness, lightheadedness, blurred vision and even fainting.
When staying in bed for long periods of time increases the volume of blood and the increase in volume increases blood pressure. However, people with postural hypotension due to neurological disorders usually have higher blood pressure when lying down.
There is a significant drop in blood pressure and that can be problematic and your husband should be evaluated regarding his high blood pressure and postural hypotension (low bp).
Hope this helps, and if you have any further questions or comments you are welcome to respond. Thanks for the question, take care.
Hope this helps