Review of Medical Physiology - William F. Ganong 2002

Formation and Excretion of Urine
Regulation of Composition and Volume of Extracellular Fluid
Regulation of Osmotic Pressure

Regulation of osmotic pressure is achieved primarily through thirst mechanisms and vasopressin secretion. The total Osmotic Pressure of Body Fluids is directly proportional to the sum of the total content of Na+ and K+ divided by the total fluid volume. Consequently, the osmotic pressure of body fluids changes As a result of an imbalance between the content of these essential electrolytes and The amount of fluid ingested or lost (see Chapter 1). When the effective osmotic pressure of Blood Plasma rises, vasopressin secretion increases, which stimulates thirst mechanisms. Water is retained in the body, diluting the hypertonic blood plasma, and fluid intake is increased whenever possible (Fig. 39-1). Conversely, if blood plasma becomes hypotonic, vasopressin secretion drops and The excretion of excess fluid is enhanced. In this way, the osmotic pressure of body fluids is maintained within a relatively narrow normal range. Normally, the osmotic pressure of blood plasma is 280-295 mOsm/kg H2O; at a plasma osmotic pressure of 285 mOsm/kg, vasopressin secretion is maximally suppressed, and at higher values, it gradually increases (see Fig. 14-14). The function of these regulatory mechanisms and the pathological conditions arising from their impairment are discussed in detail in Chapters 14 and 38.



Last update: 10/08/2026

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