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  • ACTHAdrenocorticotropic hormone
  • ADHAntidiuretic hormone
  • AIDS Acquired immunodeficiency syndrome
  • AVPArginine vasopressin
  • CSW Cerebral salt wasting
  • DDAVP Desamino, d-8 arginine vasopressin
  • PAVP Plasma vasopressin concentration
  • pOsm Plasma osmolality
  • SIADH Syndrome of inappropriate antidiuretic hormone secretion
  • V1-3 Vasopressin receptor, types 1-3

Vasopressin is the water-retaining hormone in all mammals and along with thirst is the primary regulator of osmolality. Pressure and volume are primarily regulated by changes in sodium balance mediated via renin, angiotensin, and aldosterone. The relative importance of vasopressin in regulation of osmolality versus regulation of pressure is reflected in the sensitivity to changes in osmolality versus changes in pressure/volume. Figure 5–1 illustrates the exquisite sensitivity of the osmostat to as little as a 1% change in osmolality. The regulation of vasopressin secretion by baroreceptors, however, involves many concurrent and synergistic sympathetic inputs, and decrease in volume or pressure of 10% to 15% is necessary before there is a measurable increase in plasma vasopressin.

Figure 5–1

Vasopressin secretion in response to percentage increases in osmolality or decreases in pressure or volume.

(Redrawn, with permission, from Robertson GL, Berl T. Water metabolism. In: Brenner BM, Rector FC, Jr, eds. The Kidney. vol. 1. 3rd ed. Philadelphia, PA: WB Saunders; 1986: 385.)

The exquisite sensitivity in the relation of plasma osmolality to urine osmolality and urine volume is illustrated in Figure 5–2. While the normal range of plasma osmolality encompasses a range of approximately 10 mOsm/L, for any individual, the set-point is much more narrow. Small changes in osmolality produce a corresponding and linear change in plasma vasopressin. As illustrated in Figure 5–2, the normal range of plasma osmolality and plasma vasopressin produces a corresponding linear increase in urine osmolality from maximally dilute to maximally concentrated. This entire range from maximally dilute urine to maximally concentrated urine is accomplished by a narrow range of plasma vasopressin (from approximately 1 to 5 pg/mL). In unusual circumstances, plasma osmolality can rise higher than the normal range, and there is a corresponding increase in plasma vasopressin, but urine osmolality plateaus at the concentration of the renal inner medulla. When urine in the collecting duct is iso-osmotic with the urine in the inner medulla, maximum urine concentration is achieved. While the relation of plasma osmolality, plasma vasopressin, and urine osmolality are linear, the relation of these to urine volume is not linear. Rather, there is a logarithmic relationship between urine volume and urine osmolality. The total urine volume required to excrete a fixed quantity of urine osmolytes changes relatively little until plasma vasopressin and urine concentration are nearly absent, then urine volume increases dramatically from a few liters per day to 18 to 20 L/d.

Figure 5–2

Normal physiologic relationship between plasma osmolality (pOsm; mOsm/kg H2O), plasma vasopressin (PAVP; pg/mL), ...

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