Why does postural orthostatic hypotension occur upon standing for the first time in a young adult who has been supine in skeletal traction for 3 weeks?

The patient's blood pressure (vasomotor) system is "untuned"

Explanation:

There is likelihood for venous pooling to occur as a result of improper constriction of the peripheral blood vessels. Furthermore, the medications that the patient may have been receiving may have effect on the blood pressure, thereby causing a reduction in blood pressure. There is also a possibility that the patient has not taken enough fluid and may not have been on intravenous fluids. These are likely factors that can cause postural orthostatic hypotension in a patient standing for the first time in 3 weeks.