Final answer:
Regulation of plasma osmolarity is reduced in the elderly due to a decreased thirst response and potential decline in renal function, impairing the body's ability to maintain stable osmolarity through water intake and concentration.
Step-by-step explanation:
Regulation of plasma osmolarity is reduced in the elderly predominantly due to a decreased thirst response. As individuals age, their sensation of thirst becomes less acute, leading to decreased water intake. Moreover, aging can be associated with a decline in renal function and an impaired ability to concentrate urine, exacerbating the impact of reduced fluid intake. This decreases the body's ability to maintain a stable plasma osmolarity, given that water intake is a crucial component of this homeostatic mechanism.
Regulation of plasma osmolarity involves the hormone ADH (antidiuretic hormone or vasopressin), which is released from the posterior pituitary in response to increased plasma osmolarity detected by osmoreceptors in the hypothalamus. ADH aids the kidneys in reabsorbing water, thereby regulating urine volume and concentration. However, in the elderly, the efficiency of this system may decline, further contributing to the difficulty in maintaining plasma osmolarity.