WaterL/day1801.5178.599.2Na+ mEq/day25,20015025,05099.4K+ mEq/day72010062086.1Ca++ mEq/day5401053098.2HCO3− mEq/day43202431899.9+ Cl− mEq/day18,00015017,85099.2Glucosemmol/day8000800100.0Ureag/day56282850.0 aThe filtered amount of any substance is calculated by multiplying the concentration of that substance in the ultrafiltrate by the glomerular filtration rate (GFR); for example, the filtered load of Na+ is calculated as [Na+]ultrafiltrate (140mEq/L) × GFR (180L/day) = 25,200mEq/day.
The composition and volume of urine can vary widely in the healthy state. These values represent average ranges. Normal water excretion typically ranges between 0.5 and 1.5L/day.Data from Valtin HV. Renal Physiology. 2nd ed. Boston: Little, Brown; 1983.
Na+ is reabsorbed by different mechanisms in the first and the second halves of the proximal tubule. In the first half of the proximal tubule, Na+ is reabsorbed primarily with bicarbonate (HCO3 −) and a number of other solutes (e.g., glucose, amino acids, Pi, lactate). In contrast, in the second half, Na+ is reabsorbed mainly with Cl− . This disparity is mediated by differences in the Na+ transport systems in the first and second halves of the proximal tubule and by differences in the composition of tubular fluid at these
Bilirubin50–130mEq/L20–70mEq/L30–50mEq/L5–12mEq/L2–18mEq/L50–130mEq/L20–40mEq/L200–400mmol/L6–20mmol/L5.0–7.0500–800mOsm/kgH2O0000000 sites. In absolute terms the first half of the proximal tubule reabsorbs significantly more Na+ than the second half.
In the first half of the proximal tubule, Na+ uptake into the cell is coupled with either H+ or organic solutes, includ ing glucose (Fig. 34.1 ). Specific transport proteins mediate entry of Na+ into the cell across the apical membrane. For example, the Na+/H+ antiporter, NHE3, (see
Fig. 34.1A ) couples entry of Na+ with extrusion of H+ from the cell. H+ secretion results in reabsorption of sodium bicarbonate (NaHCO3) (see ). Na+ also enters proximal tubule cells via several symporter mechanisms, including Na+/glucose (SGLT2), Na+/amino acid, Na+/Pi, and Na+/ lactate (see
Fig. 34.1B ). The glucose and other organic solutes that enter the cell with Na+ leave the cell across the basolateral membrane via passive transport mechanisms. Any Na+ that enters the cell across the apical membrane leaves the cell and enters the blood via Na+,K+-ATPase. Thus reabsorption of Na+ in the first half of the proximal tubule is coupled to that of HCO3 − and a number of organic molecules, and this generates a negative transepithelial voltage across the proximal tubule that provides the driving force for the paracellular reabsorption of Cl− . Reabsorption of many organic molecules, including glucose and lactate, is so avid they are almost completely removed from the tubular fluid in the first half of the proximal tubule (