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Maintenance of the tonicity of extracellular fluids within a very narrow range is crucial for proper cellular function. Normal blood tonicity is maintained over a 10-fold variation in water intake by a coordinated interaction among vasopressin, thirst, and the renal system. Intravascular volume, on the other hand, is controlled mainly by the renin–angiotensin–aldosterone system and the natriuretic peptide family.
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To maintain water balance, vasopressin secretion stimulates water reabsorption by the kidney, thereby reducing future water losses; and thirst stimulates water ingestion, which restores past water losses. The 2 systems work in parallel to efficiently regulate the tonicity of the extracellular fluid (Fig. 518-1). However, when both vasopressin secretion and thirst are compromised, life-threatening abnormalities in plasma osmolality can occur.
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REGULATION OF PLASMA TONICITY: VASOPRESSIN SECRETION, METABOLISM, AND ACTION
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Vasopressin (also termed antidiuretic hormone [ADH]) is a 9-amino acid peptide synthesized in hypothalamic paraventricular and supraoptic magnocellular neurons and stored in the posterior pituitary, or neurohypophysis, from where it is released into the systemic circulation (Fig. 518-2) to regulate plasma osmolality.
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Normal blood osmolality ranges between 280 and 290 mOsm/kg H2O. An osmosensor located outside the blood-brain barrier near the anterior hypothalamus can detect as little as a 1% to 2% change in blood osmolality. When osmolality increases above a threshold of 283 mOsm/kg, vasopressin is released from the posterior pituitary at a concentration proportional to plasma osmolality, up to a maximum of 20 pg/mL when blood osmolality reaches 320 mOsm/kg.
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Vasopressin is also secreted in response to a substantial decrease of at least 8% in intravascular volume or pressure. By the time ...