The kidneys retain 1 L of water in response to elevated AVP levels, returning osmolality and [Na˜] to normal, but increasing ECF volume. Conversely, the kidneys excrete 1 L of water in response to decreased AVP levels, reducing ECF volume.

The impact of changes in Na+ balance on ECFV can be illustrated as follows: (1) Adding NaCl (without water) to the ECF increases the [Na+] and osmolality. (2) This increase in ECF osmolality stimulates the secretion of AVP from the posterior pituitary, which then acts on the kidneys to conserve water. (3) Decreased renal excretion of water, along with water ingestion, restores plasma osmolality and plasma [Na+] to normal. However, the ECFV is now increased by 1 L. (4) Removing NaCl (without water) from the ECF decreases plasma [Na+] and plasma osmolality. (5) The decrease in ECF osmolality inhibits AVP secretion. As a result, the kidneys excrete water. (6) Increased renal excretion of water returns the plasma [Na+] and plasma osmolality to normal. However, ECFV is now decreased by 1 L. This demonstrates how changes in Na+ balance affect ECFV due to the efficiency of the AVP system in maintaining normal body fluid osmolality.

To maintain normal renal function, several conditions must be met: (1) Glomerular filtration rate (GFR) and proximal tubule reabsorption determine the delivery of tubular fluid to Henle's loop. (2) Reabsorption of NaCl by the nephron segments, particularly the thick ascending limb of Henle's loop, must be normal. (3) The presence of a hyperosmotic medullary interstitium is crucial, which is maintained through NaCl reabsorption by Henle's loop and effective accumulation of urea. Adequate dietary protein intake is necessary for urea accumulation. (4) Maximum levels of AVP must be present, and the collecting duct must respond normally to AVP.

The salts of Na+ are the major solutes of extracellular fluid (ECF) and play a significant role in determining ECF osmolality. Although alterations in Na+ balance are commonly assumed to disturb ECF osmolality, under normal circumstances, this is not the case. The AVP and thirst systems maintain body fluid osmolality within a narrow range. Adding or removing NaCl from the ECF changes the volume of this compartment but not the [Na+] (compare initial and final conditions in Fig. 35.9). For example, adding NaCl to the ECF (without water) increases the [Na+] and osmolality of the compartment. In response, AVP secretion and thirst are stimulated, resulting in water ingestion and reduced renal water loss. This restores plasma osmolality (and serum [Na+]) to their initial values, but the ECF volume is increased. The opposite occurs when NaCl is lost from the ECF. Monitoring changes in ECF volume (ECFV) can be done by measuring body weight, as 1 L of ECF equals 1 kg of body weight.
In response to elevated AVP levels In response to decreased AVP levelsthe kidneys retain 1 L of water the kidneys excrete 1 L of waterreturning osmolality and [Na˜] returning osmolality and [Na˜]to normal, but increasing ECF volume.
to normal, but reducing ECF volume.
• Fig. 35.9 Impact of changes in Na+ balance on ECFV. (1) Addition of NaCl (without water) to the ECF increases the [Na+] and osmolality. (2) The increase in ECF osmolality stimulates secretion of AVP from the posterior pituitary, which than acts on the kidneys to conserve water. (3) Decreased renal excretion of water together with water ingestion restore plasma osmolality and plasma [Na+] to normal. However, the ECFV is now increased by 1 L. (4) Removal of NaCl (without water) from the ECF decreases plasma [Na+] and plasma osmolality. (5) The decrease in ECF osmolality inhibits AVP secretion. In response to the decrease in plasma AVP the kidneys excrete water. (6) Increased renal excretion of water returns the plasma [Na+] and plasma osmolality to normal. However, ECFV is now decreased by 1 L. As illustrated, changes in Na+ balance alter ECFV because of the efficiency of the AVP system to maintain normal body fluid osmolality. (Adapted from Koeppen BM, Stanton BA. Renal Physiology. 5th ed. Philadelphia: Elsevier; 2013.) thick ascending limb of Henle’s loop. Delivery of tubular fluid to Henle’s loop depends on GFR and proximal tubule reabsorption.
2.
Reabsorption of NaCl by the nephron segments must be normal; again, the most important segment is the thick ascending limb of Henle’s loop.
3.
A hyperosmotic medullary interstitium must be present. The interstitial fluid osmolality is maintained by NaCl reabsorption by Henle’s loop (conditions 1 and 2) and by effective accumulation of urea. Urea accumulation in turn depends on adequate dietary protein intake.
4.
Maximum levels of AVP must be present and the collecting duct must respond normally to AVP.
Control of Extracellular Fluid Volume and Regulation of Renal NaCl Excretion
The major solutes of ECF are the salts of Na+ (see also the major determinant of ECF osmolality, alterations in Na+ balance are commonly assumed to disturb ECF osmolality. However, under normal circumstances this is not the case because the AVP and thirst systems maintain body fluid osmolality within a very narrow range (discussed earlier). As illustrated in
Fig. 35.9 , adding or removing NaCl from ECF changes this body fluid compartment’s volume and not the [Na+] (compare initial condition and final conditions). For example, addition of NaCl to the ECF (without water) increases the [Na+] and osmolality of this compartment. (ICF osmolality also increases because of osmotic equilibration with the ECF.) In response, AVP secretion and thirst are stimulated, and as a result water is ingested and renal water loss is reduced. This restores plasma osmolality (and serum [Na+]) to their initial values, but the volume of the ECF is now increased. The opposite occurs when NaCl is lost from the ECF. Changes in ECF volume (ECFV) can be monitored by measuring body weight, because 1 L of ECF equals 1 kg of ). Of these, NaCl is the most abundant. Because NaCl is body weight.