MODULE SUPPLEMENT: ENDOCRINE SYSTEM
Posterior Pituitary

So far we've focused on the hypothalamic-pituitary axes. However, the posterior pituitary is also important. The posterior pituitary is composed mainly of glial-like cells call pituicytes. Nerve tracts originate in the supraoptic (=>Antidiuretic Hormone [ADH] or Vasopressin) and paraventricular (=>Oxytocin) nuclei of hypothalamus.

There may be some reduced amounts of hormones stored with age. What is found is that, in older adults as compared to younger adults: basal levels of Antidiuretic hormone (ADH/Vasopressin) increase with age; more ADH is released after osmoreceptor stimulation; and less ADH is released after baroreceptor stimulation. Because of these changes in the responsiveness to certain stimuli, older adults may be more predisposed to the Syndrome of Inappropriate Anti-Diuretic Hormone secretion (SIADH). (See Concentrating and Diluting Functions; Water Homeostasis--Note: this link will open in a new browser window.)

At the same time, however, renal responsiveness to ADH declines. The clinical implications of this are discussed in greater detail in the renal module (see Changes in Renal Homeostatic Controls--Note: this link will open in a new browser window), but in summary, this decrease in responsiveness at the renal level predisposes older adults to dehydration because of decreased ability to retain water. Thus, there are two somewhat contrasting effects that place the older adult at risk for both SIADH and dehydration.