Another mechanism that prevents changes in the amount of Na+ filtered by the glomerulus is the tubuloglomerular feedback mechanism, which is activated when the GFR increases. This mechanism returns the GFR and Na+ filtration to normal levels, ensuring that spontaneous changes in GFR only result in increased Na+ filtration for a short period of time. The mechanisms responsible for G-T balance maintain constant urinary Na+ excretion, thereby maintaining Na+ homeostasis, ECFV, and blood pressure until the GFR returns to normal.

The distal tubule and collecting duct play a significant role in the final adjustments to urine composition and volume, as well as in the regulation by hormones and other factors. Approximately 67% of the filtered solutes and water undergo these final adjustments primarily in the distal tubule and collecting duct.

Substances are secreted from the blood into the tubular fluid as a means of excreting metabolic byproducts and eliminating exogenous organic anions and cations (such as drugs) and toxins from the body. Many organic anions and cations are bound to plasma proteins and cannot be ultrafiltered, making secretion their main route of excretion in urine.

The reabsorption of NaCl by the kidneys is regulated by various hormones (including angiotensin II, aldosterone, AVP, natriuretic peptides [ANP, BNP, and urodilatin], uroguanylin, and guanylin), sympathetic nerves, dopamine, and Starling forces. Water reabsorption is primarily regulated by AVP.

References:
- Divers J, Freedman BI. Genetics in kidney disease in 2013: susceptibility genes for renal and urological disorders. Nat Rev Nephrol. 2014;10:69-70.
- Hoenig MP, Zeidel ML. Homeostasis, the milieu interieur, and the wisdom of the nephron. Clin J Am Soc Nephrol. 2014;9: 1272-1281.
- Knepper MA, et al. Molecular physiology of water balance. N Engl J Med. 2015;372:1349-1358.
- Kortenoeven ML, et al. Vasopressin regulation of sodium transport in the distal nephron and collecting duct. Am J Physiol Renal Physiol. 2015;309:F280-F299.
- McCormick JA, Ellison DH. Distal convoluted tubule. Compr Physiol. 2015;5:45-98.
- Mount DB. Thick ascending limb of the loop of Henle. Clin J Am Soc Nephrol. 2014;9:1974-1986.
- Palmer BF. Regulation of potassium homeostasis. Clin J Am Soc Nephrol. 2015;10:1050-1060.
- Palmer LG, Schnermann J. Integrated control of Na transport along the nephron. Clin J Am Soc Nephrol. 2015;10:676-687.
- Pearce D, et al. Collecting duct principal cell transport processes and their regulation. Clin J Am Soc Nephrol. 2015;10:135-146.
- Pelis RM, Wright SH. Renal transport of organic anions and cations. Compr Physiol. 2011;1:1795-1835.
- Pluznick JL, Caplan MJ. Chemical and physical sensors in the regulation of renal function. Clin J Am Soc Nephrol. 2015;10:1626-1635.
- Roy A, et al. Collecting duct intercalated cell function and regulation. Clin J Am Soc Nephrol. 2015;10:305-324.
In addition to G-T balance, another mechanism minimizes changes in the filtered amount of Na+ . As discussed in , an increase in GFR (and thus in the amount of Na+ filtered by the glomerulus) activates the tubuloglomerular feedback mechanism. This action returns the GFR and filtration of Na+ to normal values. Thus spontaneous changes in GFR (e.g., caused by changes in posture and blood pressure) increase the amount of Na+ filtered for only a few minutes. The mechanisms that underlie G-T balance maintain urinary Na+ excretion constant and thereby maintain Na+ homeostasis (and ECFV and blood pressure) until the GFR returns to normal.
the largest fraction of the filtered solutes and water (i.e., 67%), final adjustments in the composition and volume of urine and most of the regulation by hormones and other factors occur primarily in the distal tubule and collecting duct.
3.
Secretion of substances from the blood into tubular fluid is a means for excreting various byproducts of metabolism, and it also serves to eliminate exogenous organic anions and cations (e.g., drugs) and toxins from the body. Many organic anions and cations are bound to plasma proteins and are therefore unavailable for ultrafiltration. Thus secretion is their major route of excretion in urine.
4.
Various hormones (including angiotensin II, aldosterone, AVP, natriuretic peptides [ANP, BNP, and urodilatin], uroguanylin, and guanylin), sympathetic nerves, dopamine, and Starling forces regulate reabsorption of NaCl by the kidneys. AVP is the major hormone that regulates water reabsorption.
Divers J, Freedman BI. Genetics in kidney disease in 2013: susceptibility genes for renal and urological disorders. Nat Rev Nephrol. 2014;10:69-70.
Hoenig MP, Zeidel ML. Homeostasis, the milieu interieur, and the wisdom of the nephron. Clin J Am Soc Nephrol. 2014;9: 1272-1281.
Knepper MA, et al. Molecular physiology of water balance. N Engl J Med. 2015;372:1349-1358.
Kortenoeven ML, et al. Vasopressin regulation of sodium transport in the distal nephron and collecting duct. Am J Physiol Renal Physiol. 2015;309:F280-F299.
McCormick JA, Ellison DH. Distal convoluted tubule. Compr Physiol. 2015;5:45-98.
Mount DB. Thick ascending limb of the loop of Henle. Clin J Am Soc Nephrol. 2014;9:1974-1986.
Palmer BF. Regulation of potassium homeostasis. Clin J Am Soc Nephrol. 2015;10:1050-1060.
Palmer LG, Schnermann J. Integrated control of Na transport along the nephron. Clin J Am Soc Nephrol. 2015;10:676-687.
Pearce D, et al. Collecting duct principal cell transport processes and their regulation. Clin J Am Soc Nephrol. 2015;10:135-146.
Pelis RM, Wright SH. Renal transport of organic anions and cations. Compr Physiol. 2011;1:1795-1835.
Pluznick JL, Caplan MJ. Chemical and physical sensors in the regulation of renal function. Clin J Am Soc Nephrol. 2015;10:1626-1635.
Roy A, et al. Collecting duct intercalated cell function and regulation. Clin J Am Soc Nephrol. 2015;10:305-324.