14.7: Antidiuretic Hormone (ADH)
- Page ID
- 84110
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Source and Control of Secretion
Antidiuretic hormone (ADH) is released by neurons that originate in the hypothalamus and extend through the infundibulum to the posterior pituitary (Figure 14.1, Figure 14.2). ADH secretion is stimulated by an increase in osmotic pressure or a decrease in blood pressure. Conversely, it is inhibited by decreased osmotic pressure, increased blood pressure, and alcohol.
Effects
Antidiuretic hormone provides a communication link in the negative feedback mechanisms that maintain homeostatic levels of body osmotic pressure and blood pressure. When more ADH is secreted, more water is allowed to pass from the filtrate in the kidney collecting ducts back into the blood. The reabsorbed water helps prevent increases in body osmotic pressure and, by helping maintain a substantial blood volume, prevents lowering of blood pressure. The constriction of blood vessels caused by ADH also helps sustain adequate blood pressure.
Conversely, a decrease in ADH secretion causes more water to escape in the urine, increasing body osmotic pressure or lowering blood volume and blood pressure.
Age Changes
Though there is wide variation among individuals, aging results in an average increase in blood levels of ADH, particularly when body osmotic pressures are high. Since the extra ADH stimulates more water reabsorption by the kidneys, this age-related increase seems to compensate partially for the corresponding decline in the ability of the kidneys to retain water when needed. The ability to increase ADH secretion in response to low blood pressure seems to remain intact or decrease slightly. There is no apparent age change in the ability to decrease ADH secretion to eliminate more water in the urine. Therefore, the contributions to homeostasis made by ADH do not decline significantly, and one of them seems to improve in many individuals.