Figure 14

A, Relationship between net sodium balance and extracellular fluid (ECF) volume. Sodium balance is intimately linked to volume balance because of powerful mechanisms that tightly regulate plasma and ECF osmolality. Sodium and its accompanying anions constitute the major contributors to ECF osmolality. The integration of sodium intake and losses establishes the net amount of sodium in the body, which is compartmentalized primarily in the ECF volume. The quotient of these two parameters (sodium and volume) determines the sodium concentration and, thus, the osmolality. Osmolality is subject to very tight regulation by vasopressin and other mechanisms. In particular, vasopressin is a very powerful regulator of plasma osmolality; however, it achieves this regulation primarily by regulating the relative solute-free water retention or excretion by the kidney [13-15]. The important point is that the osmolality is rapidly regulated by adjusting the ECF volume to the total solute present. Corrections of excesses in extracellular fluid volume involve more complex interactions that regulate the sodium excretion rate.

B, Relationship between the ECF volume and blood volume. Under normal conditions a consistent relationship exists between the total ECF volume and blood volume. This relationship is consistent as long as the plasma protein concentration and, thus, the colloid osmotic pressure are regulated appropriately and the microvasculature maintains its integrity in limiting protein leak into the interstitial compartment. The shaded area represents the normal operating range [13]. A chronic increase in the total quantity of sodium chloride in the body leads to a chronic increase in ECF volume, part of which is proportionately distributed to the blood volume compartment. When accumulation is excessive, disproportionate distribution to the interstitium may lead to edema. Chronic increases in blood volume increase mean circulatory pressure (see Fig. 1-3) and lead to an increase in arterial pressure. Therefore, the mechanisms regulating sodium balance are primarily responsible for the chronic regulation of arterial pressure. (Panel B adapted from Guyton and Hall [13].)

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