[excerpt starts]

The average amount of water excreted in urine per day is 1.5 liters. Sodium (Na+) is excreted at a rate of 25,200 milliequivalents (mEq) per day. Potassium (K+) is excreted at a rate of 720 mEq per day. Calcium (Ca++) is excreted at a rate of 540 mEq per day. Bicarbonate (HCO3−) is excreted at a rate of 432 mEq per day. Chloride (Cl−) is excreted at a rate of 18,000 mEq per day. Glucose is excreted at a rate of 8000 millimoles (mmol) per day. Urea is excreted at a rate of 5628 grams (g) per day. These values are based on the glomerular filtration rate (GFR) and the concentration of each substance in the ultrafiltrate.

The composition and volume of urine can vary within a healthy range. The values mentioned above represent the average ranges. Normal water excretion typically falls between 0.5 and 1.5 liters per day.

Sodium (Na+) is reabsorbed differently in the first and second halves of the proximal tubule. In the first half, it is primarily reabsorbed along with bicarbonate (HCO3−) and other solutes such as glucose, amino acids, phosphate (Pi), and lactate. In contrast, in the second half, sodium is mainly reabsorbed with chloride (Cl−). This difference is due to variances in the Na+ transport systems and the composition of tubular fluid.

The first half of the proximal tubule reabsorbs significantly more sodium than the second half. In the first half, Na+ uptake into the cell is coupled with either hydrogen ions (H+) or organic solutes like glucose. Specific transport proteins facilitate the entry of Na+ into the cell across the apical membrane. For example, the Na+/H+ antiporter (NHE3) allows Na+ entry while H+ is extruded from the cell. H+ secretion results in the reabsorption of sodium bicarbonate (NaHCO3). Na+ also enters proximal tubule cells through various symporter mechanisms, including Na+/glucose (SGLT2), Na+/amino acid, Na+/phosphate (Pi), and Na+/lactate. Glucose and other organic solutes that enter the cell with Na+ leave the cell through the basolateral membrane via passive transport mechanisms. Any Na+ that enters the cell through the apical membrane exits the cell and enters the blood through Na+,K+-ATPase. As a result, the reabsorption of Na+ in the first half of the proximal tubule is linked to HCO3− and several organic molecules, creating a negative transepithelial voltage across the proximal tubule, which facilitates the paracellular reabsorption of Cl−. Many organic molecules, including glucose and lactate, are almost completely removed from the tubular fluid in the first half of the proximal tubule.

[excerpt ends]
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 (