Glomerular filtration results in the production of large quantities (180 L/day) of an ultrafiltrate of plasma. If this ultrafiltrate were excreted unmodified, the following quantities would be lost in the urine each day: 180 L of water; 25,200 mEq of Na+; 19,800 mEq of Cl−; 4320 mEq of HCO3−; and 14,400 mg of glucose. Each of these losses represents more than 10-fold the amount present in the entire ECF. Fortunately, reabsorptive mechanisms in the epithelial cells lining the renal tubule return these substances to the circulation and to the ECF. In addition, secretion mechanisms in the epithelial cells remove certain substances from the peritubular capillary blood and add it to urine.
Measurement of Reabsorption and Secretion
The processes of filtration, reabsorption, and secretion are illustrated in Figure 6-12. A glomerular capillary is shown with its afferent and efferent arterioles. The initial part of the nephron (Bowman’s space and the beginning of the proximal convoluted tubule) is shown, lined with epithelial cells. Nearby is a peritubular capillary, which emerges from the efferent arteriole and supplies blood to the nephron.
Figure 6–12 Processes of filtration, reabsorption, and secretion in a nephron. The sum of the three processes is excretion.
Filtration. An interstitial-type fluid is filtered across the glomerular capillary into Bowman’s space. The amount of a substance filtered into Bowman’s space per unit time is called the filtered load. The fluid in Bowman’s space and in the lumen of the nephron is called tubular fluid or luminal fluid.
Reabsorption. Water and many solutes (e.g., Na+, Cl−, HCO3−, glucose, amino acids, urea, Ca2+, Mg2+, phosphate, lactate, and citrate) are reabsorbed from the glomerular filtrate into the peritubular capillary blood. The mechanisms for reabsorption involve transporters in the membranes of the renal epithelial cells. As emphasized, if reabsorption did not occur, most of these constituents of ECF would be rapidly lost in the urine.
Secretion. A few substances (e.g., organic acids, organic bases, K+) are secreted from peritubular capillary blood into tubular fluid. Thus, in addition to filtration, secretion provides a mechanism for excreting substances in the urine. As with reabsorption, the secretion mechanisms involve transporters in the membranes of the epithelial cells lining the nephron.
Excretion. Excretion or excretion rate refers to the amount of a substance excreted per unit time. Excretion is the net result, or sum, of the processes of filtration, reabsorption, and secretion. The excretion rate can be compared with the filtered load to determine whether a substance has been reabsorbed or secreted.
The following equations are used to calculate filtered load, excretion rate, and reabsorption or secretion rate:
In words, the difference between the filtered load and the excretion rate is the rate of net reabsorption or net secretion. If the filtered load is greater than the excretion rate, there has been net reabsorption of the substance. If the filtered load is less than the excretion rate, there has been net secretion of the substance. This type of calculation is shown in Figure 6-13; one example is given for a substance that is reabsorbed, and another example is given for a substance that is secreted.
Figure 6–13 Examples of substances that are reabsorbed or secreted. A, Example of net reabsorption of Na+. Na+ is filtered and reabsorbed by the renal epithelial cells; Na+ excretion is the difference between filtered load and reabsorption rate. B, Example of net secretion of PAH (Para-aminohippuric acid). PAH is filtered and secreted by renal epithelial cells; PAH excretion is the sum of filtered load plus secretion rate. Calculations are shown for filtered load, reabsorption or secretion rate, and excretion rate (mEq/day). GFR, Glomerular filtration rate; PNa+, plasma concentration of Na+; PPAH, plasma concentration of PAH; UNa+, urine concentration of Na+; UPAH, urine concentration of PAH.
Figure 6-13A illustrates the renal handling of Na+, a solute that is freely filtered and subsequently reabsorbed. In this example, the filtered load of Na+ is 25,200 mEq/day (GFR × [P]Na+), and the excretion rate of Na+ is 100 mEq/day (× [U]Na+). Because the filtered load of Na+ is higher than the excretion rate, there must have been net reabsorption of Na+. The kidney reabsorbs 25,100 mEq/day, which is 99.4% of the filtered load (25,100 mEq/25,200 mEq).
Figure 6-13B illustrates the renal handling of PAH, a solute that is filtered and subsequently secreted. In this example, the filtered load of PAH is 18 g/day (GFR × [P]PAH), and the excretion rate of PAH is 54 g/day ( × [U]PAH). Because the filtered load of PAH is less than the excretion rate, there must have been net secretion of PAH, amounting to 36 g/day (excretion rate − filtered load). In this example, the secretion rate of PAH is twice that of the original filtered load.
Glucose—Example of Reabsorption
Glucose is filtered across glomerular capillaries and reabsorbed by the epithelial cells of the proximal convoluted tubule. Glucose reabsorption is a two-step process involving Na+-glucose cotransport across the luminal membrane and facilitated glucose transport across the peritubular membrane. Because there are a limited number of glucose transporters, the mechanism is saturable; that is, it has a transport maximum, or Tm.
Cellular Mechanism for Glucose Reabsorption
Figure 6-14 shows the cellular mechanism for glucose reabsorption in the early proximal tubule. The luminal membrane of the epithelial cells faces the tubular fluid (lumen) and contains the Na+-glucose cotransporter. The peritubular membrane or basolateral membrane of the cells faces the peritubular capillary blood and contains the Na+-K+ ATPase and the facilitated glucose transporter. The following steps are involved in reabsorbing glucose from tubular fluid into peritubular capillary blood:
Figure 6–14 Cellular mechanism of glucose reabsorption in the early proximal tubule.
1. Glucose moves from tubular fluid into the cell on the Na+-glucose cotransporter (called SGLT) in the luminal membrane. Two Na+ ions and one glucose bind to the cotransport protein, the protein rotates in the membrane, and Na+ and glucose are released into the ICF. In this step, glucose is transported against an electrochemical gradient; the energy for this uphill transport of glucose comes from thedownhill movement of Na+.
2. The Na+ gradient is maintained by the Na+-K+ ATPase in the peritubular membrane. Because ATP is used directly to energize the Na+-K+ ATPase and indirectly to maintain the Na+ gradient, Na+-glucose cotransport is called secondary active transport.
3. Glucose is transported from the cell into peritubular capillary blood by facilitated diffusion. In this step, glucose is moving down its electrochemical gradient and no energy is required. The proteins involved in facilitated diffusion of glucose are called GLUT 1 and GLUT 2, which belong to a larger family of glucose carriers.
Glucose Titration Curve and Tm
A glucose titration curve depicts the relationship between plasma glucose concentration and glucose reabsorption (Fig. 6-15). For comparison, the filtered load of glucose and the excretion rate of glucose are plotted on the same graph. The glucose titration curve is obtained experimentally by infusing glucose and measuring its rate of reabsorption as the plasma concentration is increased. The titration curve is best understood by examining each relationship separately and then by considering all three relationships together.
Figure 6–15 Glucose titration curve. Glucose filtration, reabsorption, and excretion are shown as a function of plasma glucose concentration. Hatched areas are the splay. Tm, Tubular transport maximum.
Filtered load. Glucose is freely filtered across glomerular capillaries, and the filtered load is the product of GFR and plasma glucose concentration (filtered load = GFR × [P]x). Thus, as the plasma glucose concentration is increased, the filtered load increases linearly.
Reabsorption. At plasma glucose concentrations less than 200 mg/dL, all of the filtered glucose can be reabsorbed because Na+-glucose cotransporters are plentiful. In this range, the curve for reabsorption is identical to that for filtration; that is, reabsorption equals filtration. The number of carriers is limited, however. At plasma concentrations above 200 mg/dL, the reabsorption curve bends because some of the filtered glucose is not reabsorbed. At plasma concentrations above 350 mg/dL, the carriers are completely saturated and reabsorption levels off at its maximal value, Tm.
Excretion. To understand the curve for excretion, compare those for filtration and reabsorption. Below plasma glucose concentrations of 200 mg/dL, all of the filtered glucose is reabsorbed and none is excreted. At plasma glucose concentrations above 200 mg/dL, the carriers are nearing the saturation point. Most of the filtered glucose is reabsorbed, but some is not; the glucose that is not reabsorbed is excreted. The plasma concentration at which glucose is first excreted in the urine is called threshold, which occurs at a lower plasma concentration than does Tm. Above 350 mg/dL, Tm is reached and the carriers are fully saturated. The curve for excretion now increases linearly as a function of plasma glucose concentration, paralleling that for filtration.
The Tm for glucose is approached gradually, rather than sharply (see Fig. 6-15), a phenomenon called splay. Splay is that portion of the titration curve where reabsorption is approaching saturation, but it is notfully saturated. Because of splay, glucose is excreted in the urine (i.e., at threshold) before reabsorption levels off at the Tm value.
There are two explanations for splay. The first explanation is based on a low affinity of the Na+-glucose cotransporter. Thus, near Tm, if glucose detaches from its carrier, it will be excreted into the urine because there are few remaining binding sites where it may reattach. The second explanation for splay is based on the heterogeneity of nephrons. Tm for the whole kidney reflects the average Tm of all nephrons, yet all nephrons do not have exactly the same Tm. Some nephrons will reach Tm at lower plasma concentration than others, and glucose will be excreted in the urine before the average Tm is reached.
At normal plasma glucose concentrations (70 to 100 mg/dL), all of the filtered glucose is reabsorbed and none is excreted. Under some circumstances, however, glucosuria (excretion or spilling of glucose in the urine) occurs. The causes of glucosuria can be understood by referring again to the glucose titration curve. (1) In uncontrolled diabetes mellitus, lack of insulin causes the plasma concentration of glucose to increase to abnormally high levels. In this condition, the filtered load of glucose exceeds the reabsorptive capacity (i.e., plasma glucose concentration is above the Tm), and glucose is excreted in the urine. (2) During pregnancy, GFR is increased, which increases the filtered load of glucose to the extent that it may exceed the reabsorptive capacity. (3) Several congenital abnormalities of the Na+-glucose cotransporter are associated with decreases in Tm, causing glucose to be excreted in the urine at lower than normal plasma concentrations (Box 6-1).
BOX 6–1 Clinical Physiology: Glucosuria
DESCRIPTION OF CASE. A woman sees her physician because of excessive thirst and urination. During the previous week, she urinated hourly during the day and four or five times each night. Her physician tests her urine using a dipstick and detects glucose. She is asked to fast overnight and to report the following morning for a glucose tolerance test. After drinking a glucose solution, her blood glucose concentration increases from 200 to 800 mg/dL. Urine is collected at timed intervals throughout the test to measure urine volume and glucose concentration. The woman’s glomerular filtration rate (GFR) is estimated to be 120 mL/min from her endogenous creatinine clearance. When the reabsorption rate of glucose is calculated (filtered load of glucose − excretion rate of glucose), it is found to be constant, at 375 mg/min. The physician concludes that the cause of the woman’s glucosuria is type I diabetes mellitus (rather than a defect in the renal glucose transport mechanism).
EXPLANATION OF CASE. There are two possible explanations for this woman’s glucosuria: (1) a defect in the renal transport mechanism for glucose or (2) an increased filtered load of glucose that exceeds the reabsorptive capacity of the proximal tubule. To determine which explanation is correct, the maximal reabsorption rate for glucose (Tm) is determined by measuring the reabsorption rate as the plasma glucose concentration is increased. A value of Tm of 375 mg/min is found, which is considered normal. Thus, the physician concludes that the basis for the woman’s glucosuria is an abnormally elevated blood glucose concentration due to insufficient secretion of insulin from the pancreas. If the glucosuria had been caused by a renal defect, the Tm would be lower than normal.
Excessive urination is caused by the presence of nonreabsorbed glucose in tubular fluid. The glucose acts as an osmotic diuretic, holds water, and increases urine production. The woman’s excessive thirst is partially explained by the excessive urine production. In addition, the high blood glucose concentration increases her blood osmolarity and stimulates the thirst center.
TREATMENT. The woman is treated with regular injections of insulin.
Urea—Example of Passive Reabsorption
Urea is transported in most segments of the nephron (Fig. 6-16). In contrast to glucose, which is reabsorbed by carrier-mediated mechanisms, urea is reabsorbed or secreted by diffusion (simple diffusion and facilitated diffusion). The rate of reabsorption or secretion is determined by the concentration difference for urea between tubular fluid and blood and by the permeability of the epithelial cells to urea. When there is a large concentration difference and the permeability is high, urea reabsorption is high; when there is a small concentration difference and/or the permeability is low, urea reabsorption is low.
Figure 6–16 Urea handling in the nephron. Arrows show locations of urea reabsorption or secretion; numbers are percentages of the filtered load remaining at various points along the nephron. UT1, urea transporter 1. ADH, Antidiuretic hormone.
Urea is freely filtered across the glomerular capillaries, and the concentration in the initial filtrate is identical to that in blood (i.e., initially, there is no concentration difference or driving force for urea reabsorption). However, as water is reabsorbed along the nephron, the urea concentration in tubular fluid increases, creating a driving force for passive urea reabsorption. Therefore, urea reabsorption generally follows the same pattern as water reabsorption—the greater the water reabsorption, the greater the urea reabsorption and the lower the urea excretion.
In the proximal tubule, 50% of the filtered urea is reabsorbed by simple diffusion. As water is reabsorbed in the proximal tubule, urea lags slightly behind, causing the urea concentration in the tubular lumen to become slightly higher than the urea concentration in blood; this concentration difference then drives passive urea reabsorption. At the end of the proximal tubule, 50% of the filtered urea has been reabsorbed; thus, 50% remains in the lumen. In the thin descending limb of Henle’s loop, urea is secreted. By mechanisms that will be described later, there is a high concentration of urea in the interstitial fluid of the inner medulla. The thin descending limb of Henle’s loop passes through the inner medulla, and urea diffuses from high concentration in the interstitial fluid into the lumen of the nephron. More urea is secreted into the thin descending limbs than was reabsorbed in the proximal tubule; thus, at the bend of the loop of Henle, 110% of the filtered load of urea is present. The thick ascending limb of Henle, distal tubule, and cortical and outer medullary collecting ducts are impermeable to urea, so no urea transport occurs in these segments. However, in the presence of antidiuretic hormone (ADH), water is reabsorbed in the late distal tubule and the cortical and outer medullary collecting ducts—consequently, in these segments, urea is “left behind” and the urea concentration of the tubular fluid becomes quite high. In the inner medullary collecting ducts, there is a specific transporter for the facilitated diffusion of urea (urea transporter 1, UT1), which is activated by ADH. Thus, in the presence of ADH, urea is reabsorbed by UT1, moving down its concentration gradient from the lumen into the interstitial fluid of the inner medulla. In the presence of ADH, approximately 70% of the filtered urea is reabsorbed by UT1, leaving 40% of the filtered urea to be excreted in the urine. The urea that is reabsorbed into the inner medulla contributes to the corticopapillary osmotic gradient in a process called urea recycling,which is discussed in later sections.
Para-Aminohippuric Acid—Example of Secretion
PAH has been introduced as the substance used to measure RPF. PAH is an organic acid that is both filtered across glomerular capillaries and secreted from peritubular capillary blood into tubular fluid. As with glucose, PAH filtration, secretion, and excretion can be plotted simultaneously (Fig. 6-17). (For PAH, secretion is plotted instead of reabsorption.)
Figure 6–17 PAH titration curve. PAH (para-aminohippuric acid) filtration, secretion, and excretion are shown as a function of plasma PAH concentration. Tm, Tubular transport maximum.
Filtered load. Ten percent of the PAH in blood is bound to plasma proteins, and only the unbound portion is filterable across glomerular capillaries. The filtered load of PAH increases linearly as theunbound concentration of PAH increases (filtered load = GFR × [P]x).
Secretion. The transporters for PAH (and other organic anions) are located in the peritubular membranes of proximal tubule cells. These carriers have a finite capacity to bind and transport PAH across the cell, from blood to lumen. At low concentrations of PAH, many carriers are available and secretion increases linearly as the plasma concentration increases. When the PAH concentration increases to a level where the carriers are saturated, Tm is reached. After this point, no matter how much the PAH concentration increases, there can be no further increase in the secretion rate. The PAH transporter also is responsible for secretion of drugs such as penicillin and is inhibited by probenecid.
Incidentally, just as there is secretion of organic acids such as PAH, there are parallel secretory mechanisms for organic bases (e.g., quinine, morphine) in the proximal tubule. These secretory mechanisms for organic acids and bases are relevant in the discussion of non-ionic diffusion that follows.
Excretion. For a secreted substance such as PAH, excretion is the sum of filtration and secretion. At low PAH concentrations (below Tm), excretion increases steeply with increases in plasma PAH concentration because both filtration and secretion are increasing. At PAH concentrations above Tm, excretion increases less steeply (and parallels the curve for filtration) because only the filtration component increases as concentration increases; secretion is already saturated.
Weak Acids and Bases—Non-Ionic Diffusion
Many of the substances secreted by the proximal tubule are weak acids (e.g., PAH, salicylic acid) or weak bases (e.g., quinine, morphine). Weak acids and bases exist in two forms, charged and uncharged, and the relative amount of each form depends on pH (see Chapter 7). Weak acids exist in an acid form, HA, and a conjugate base form, A−. At low pH, the HA form, which is uncharged, predominates. At high pH, the A− form, which is charged,predominates. For weak bases, the base form is B and the conjugate acid is BH+. At low pH, the BH+ form, which is charged, predominates. At high pH, the B form, which is uncharged, predominates. With respect to the renal excretion of weak acids and bases, the relevant points are (1) the relative amounts of the charged and uncharged species depend on urine pH, and (2) only the uncharged (i.e., “non-ionic”) species can diffuse across the cells.
To illustrate the role of non-ionic diffusion in the renal excretion of weak acids and bases, consider the excretion of a weak acid, salicylic acid (HA) and its conjugate base, salicylate (A−). For the remainder of this discussion, both forms are called “salicylate.” Like PAH, salicylate is filtered across the glomerular capillaries and secreted by an organic acid secretory mechanism in the proximal tubule. As a result of these two processes, the urinary concentration of salicylate becomes much higher than the blood concentration and a concentration gradient across the cells is established. In the urine, salicylate exists in both HA and A− forms. The HA form, being uncharged, can diffuse across the cells, from urine to blood, down this concentration gradient; the A− form, being charged, cannot diffuse. At acidic urine pH, HA predominates, there is more “back-diffusion” from urine into blood, and the excretion (and clearance) of salicylate is decreased. At alkaline urine pH, A− predominates, there is less “back-diffusion” from urine to blood, and the excretion (and clearance) of salicylate is increased. This relationship is illustrated in Figure 6-18, which shows that the clearance of a weak acid is highest at alkaline urine pH and lowest at acidic urine pH. The principle of non-ionic diffusion is the basis for treating aspirin (salicylate) overdose by alkalinizing the urine—at alkaline urine pH, relatively more salicylate is in the A− form, which does not diffuse back into blood and is excreted in the urine.
Figure 6–18 Nonionic diffusion. Clearance of a weak acid and a weak base as a function of urine pH. C, Clearance of weak acid or base; GFR, glomerular filtration rate.
The effect of nonionic diffusion on the excretion of weak bases is the mirror image of its effect on weak acids (see Fig. 6-18). The weak base is both filtered and secreted, which results in a urine concentration that is higher than the blood concentration. In the urine, the weak base exists in both BH+ and B forms. The B form, being uncharged, can diffuse across the cells, from urine to blood, down this concentration gradient; the BH+ form, being charged, cannot diffuse. At alkaline urine pH, B predominates, there is more “back-diffusion” from urine into blood, and the excretion (and clearance) of the weak base is decreased. At acidic urine pH, BH+ predominates, there is less “back-diffusion” from urine to blood, and the excretion (and clearance) of the weak base is increased.