Which one of the following is the major determinant of plasma osmolality ?
Correct Answer: Serum sodium
Description: Sodium homeostasis Most of the body's sodium is located in the ECF, where it is by far the most abundant cation. Accordingly, total body sodium is the principal determinant of ECF volume. Sodium intake varies widely between individuals, ranging between 50 and 250 mmol/24 hrs. The kidneys can compensate for these wide variations in sodium intake by increasing excretion of sodium when there is sodium overload, and retaining sodium in the presence of sodium depletion, to maintain normal ECF volume and plasma volume. Functional anatomy and physiology The functional unit for renal excretion is the nephron (Fig. 14.2). Blood undergoes ultrafiltration in the glomerulus, generating a fluid that is free from cells and protein and which resembles plasma in its electrolyte composition. This is delivered into the renal tubules, where reabsorption of water and various electrolytes occurs. (More detail on the structure and function of the glomerulus is given in Ch. 15.) The glomerular filtration rate (GFR) is approximately 125 mL/min (equivalent to 180 L/24 hrs) in a normal adult. Over 99% of the filtered fluid is reabsorbed into the blood in the peritubular capillaries during its passage through successive segments of the nephron, largely as a result of tubular reabsorption of sodium. The processes mediating sodium reabsorption, and the factors that regulate it, are key sodium plays main role in plasma osmolarity to understanding clinical disturbances and pharmacological interventions of sodium and fluid balance. The nephron can be divided into at least four different functional segments in terms of sodium reabsorption (Fig. 14.3). Ref Davidson 23rd edition pg 350
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