The importance of Starling forces in regulating solute and water reabsorption by the proximal tubule is underscored by the phenomenon of glomerulotubular (G-T) balance. Spontaneous changes in GFR markedly alter the filtered amount of Na+ (filtered Na+= GFR × [Na+] in the filtered fluid). Without rapid adjustments in Na+ reabsorption to counter the changes in filtration of Na+ , urinary excretion of Na+ would fluctuate widely and disturb the Na+ balance of the body and thus alter ECFV and blood pressure (see for more details). However, spontaneous changes in GFR do not alter Na+ excretion in urine or Na+ balance when ECFV is normal because of the phenomenon of G-T balance. When body Na+ balance is normal (i.e., ECFV is normal), G-T balance refers to the fact that reabsorption of Na+ and water increases in proportion to the increase in GFR and filtered amount of Na+ . Thus a constant fraction of the filtered Na+ and water is reabsorbed from the proximal tubule despite variations in GFR. The net result of G-T balance is to reduce the impact of changes in GFR on the amount of Na+ and water excreted in urine when ECFV is normal.
Two mechanisms are responsible for G-T balance. One is related to the oncotic and hydrostatic pressure differences between the peritubular capillaries and the lateral intercellular space (i.e., Starling forces). For example, an increase in the GFR (at constant RPF) raises the protein concentration in glomerular capillary plasma above normal. This protein-rich plasma leaves the glomerular capillaries, flows through the efferent arterioles, and enters the peritubular capillaries. The increased πpc augments the movement of 1.
The four major segments of the nephron (proximal tubule, Henle’s loop, distal tubule, and collecting duct) determine the composition and volume of urine by the processes of selective reabsorption of solutes and water and secretion of some solutes.
2.
Tubular reabsorption of substances filtered by the glomerulus allows the kidneys to retain substances that are essential and regulate their levels in plasma by altering the degree to which they are reabsorbed. Reabsorption of Na+ , Cl− , other anions, and organic anions and cations together with water constitutes the major function of the nephron. Approximately 25,200 mEq of Na+ and 179 L of water are reabsorbed each day. Proximal tubule cells reabsorb 67% of the glomerular ultrafiltrate, and cells of Henle’s loop reabsorb about 25% of the NaCl that was filtered and about 15% of the water that was filtered. The distal segments of the nephron (distal tubule and collecting duct system) have a more limited reabsorptive capacity. However, although the proximal tubule reabsorbs
Brown D, et al. New insights into the dynamic regulation of water and acid-base balance by renal epithelial cells. Am J Physiol Cell Physiol. 2012;302:C1421-C1433.
Dantzler WH, et al. Urine-concentrating mechanism in the inner medulla: function of the thin limbs of the loops of Henle. Clin J Am Soc Nephrol. 2014;9:1781-1789.
solute and fluid from the lateral intercellular space into the peritubular capillaries. This action increases net solute and water reabsorption by the proximal tubule.
The second mechanism responsible for G-T balance is initiated by an increase in the filtered amount of glucose and amino acids. As discussed earlier, reabsorption of Na+ in the first half of the proximal tubule is coupled to that of glucose and amino acids. The rate of Na+ reabsorption therefore partially depends on the filtered amount of glucose and amino acids. As the GFR and filtered amount of glucose and amino acids increase, reabsorption of Na+ and water also rises.