GFR increases mainly due to a decrease in sympathetic nerve activity. Sympathetic fibers control the diameter of the afferent and efferent arterioles of the glomerulus. When sympathetic nerve activity decreases, the arterioles dilate, leading to an increase in renal plasma flow (RPF). The afferent arterioles are affected more, causing an increase in hydrostatic pressure within the glomerular capillary and subsequently increasing GFR. Natriuretic peptides also contribute to this response by dilating the afferent arterioles and constricting the efferent arterioles. Therefore, increased levels of natriuretic peptides during expansion of extracellular fluid volume (ECFV) further increase GFR. With the increase in GFR, the filtered load of Na+ also increases.

In the proximal tubule and loop of Henle, the reabsorption of Na+ decreases. Several mechanisms contribute to this reduction. Decreased sympathetic nerve activity, which stimulates proximal tubule Na+ reabsorption, leads to decreased Na+ reabsorption. Additionally, the reduced levels of angiotensin II, which directly stimulates Na+ reabsorption in the proximal tubule, also contribute to the decrease in Na+ reabsorption. Alterations in the Starling forces across the proximal tubule further affect NaCl reabsorption. The elevated hydrostatic pressure in the glomerular capillaries during ECFV expansion increases the hydrostatic pressure within the peritubular capillaries, while the decrease in filtration fraction reduces peritubular oncotic pressure. These changes in Starling forces reduce the absorption of solute and water from the lateral intercellular space, leading to a decrease in tubular reabsorption of NaCl. As a result, more NaCl is delivered to the loop of Henle. The reduced nerve activity, low angiotensin II levels, and low aldosterone levels during ECFV expansion decrease NaCl reabsorption by the thick ascending limb of the loop of Henle. Consequently, the fraction of the filtered load delivered to the distal tubule increases.
Fig. 35.12 1. GFR increases. GFR increases mainly as a result of the decrease in sympathetic nerve activity. Sympathetic fibers innervate the afferent and efferent arterioles of the glomerulus and control their diameter. Decreased sympathetic nerve activity leads to arteriolar dilation and an increase in renal plasma flow (RPF). Because the effect appears to be greater on the afferent arterioles, the hydrostatic pressure within the glomerular capillary is increased, thereby increasing the GFR. Because RPF increases to a greater degree than GFR, the filtration fraction (GFR/RPF) decreases. Natriuretic peptides also increase GFR by dilating the afferent arterioles and constricting the efferent arterioles. Thus the increased natriuretic peptide levels that occur during ECFV expansion contribute to this response. With the increase in GFR, the filtered load of Na+ increases.
2. The reabsorption of Na+ decreases in the proximal tubule and loop of Henle. Several mechanisms reduce Na+ reabsorption by the proximal tubule. Because activation of the sympathetic nerve fibers that innervate this nephron segment stimulates proximal tubule Na+ reabsorption, the decreased sympathetic nerve activity that results from ECFV expansion decreases Na+ reabsorption. In addition, angiotensin II directly stimulates Na+ reabsorption by the proximal tubule. Because angiotensin II levels are also reduced under this condition, proximal tubule Na+ reabsorption decreases as a result. Starling forces across the proximal tubule also change. The elevated hydrostatic pressure within the glomerular capillaries in ECFV expansion also tends to increase the hydrostatic pressure within the peritubular capillaries. In addition the decrease in filtration fraction reduces the peritubular oncotic pressure. These alterations in the capillary Starling forces (i.e., hydrostatic and oncotic) reduce the absorption of solute (e.g., NaCl) and water from the lateral intercellular space and thus reduce tubular reabsorption of NaCl (see for a complete description of this mechanism). Both the increase in the filtered load and the decrease in NaCl reabsorption by the proximal tubule result in delivery of more NaCl to the loop of Henle. Because activation of sympathetic nerves and angiotensin II and aldosterone stimulate NaCl reabsorption by the thick ascending limb of the loop of Henle, the reduced nerve activity and low angiotensin II and aldosterone levels that occur with ECFV expansion reduce NaCl reabsorption by the thick ascending limb. Thus the fraction of the filtered load delivered to the distal tubule is increased.