In contrast to younger patients, older patients are more prone to a decrease in blood pressure upon standing because:

• A. the aging process results in an overall increase in blood volume.
• B. the body is less able to adapt the BP to rapid postural changes.
• C. their red blood cells are destroyed at a faster than normal rate.
• D. any change in position causes blood to be shunted to the brain.

Answer: (B)

When you stand, gravity pulls blood toward the legs, reducing return to the heart. Normally the body quickly compensates by activating baroreceptors that raise heart rate and constrict vessels to keep blood pressure stable. In older adults, these reflexes are slower and less effective, and venous pooling is greater, so the adjustment to rapid postural changes is incomplete. The result is a drop in blood pressure upon standing, with dizziness or fainting risk.

The other ideas don’t fit: aging doesn’t increase overall blood volume, so standing wouldn’t inherently lower BP for that reason; red blood cell turnover isn’t the issue here; and standing doesn’t shunt blood to the brain—it's the failure to maintain enough blood flow back to the heart that causes the problem.