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.
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