The answer to this question would be: Increased sensitivity of the renin--angiotensin--aldosterone system
In geriatric patient, some factor that can be found could be stiffering/sclerosis of the arteries, increased resistance of the peripheral vascular, and decreased renal blood flow. Atherosclerosis that happens in the body can stiffen and decrease the diameter of blood vessel, increasing its resistance. It also reduce blood flow.
