Tubular Reabsorption and Tubular Secretion

The volume and solute contents of the final urine that enters the renal pelvis are quite different from those of the glomerular filtrate. Clearly, almost all the filtered volume must be reabsorbed; otherwise, with a filtration rate of 180 L/day, we would urinate ourselves into dehydration very quickly. As the filtrate flows from Bowman’s capsule through the various portions of the tubule, its composition is altered, mostly by removing material (tubular reabsorption) but also by adding material (tubular secretion). As described earlier, the tubule is, at all points, intimately associated with peritubular capillaries in the cortex or cappillary-like vessels in the medulla, a relationship that permits rapid transfer of materials between the capillary plasma and the lumen of the tubule via the interstitial space.
The most common relationships among these basic renal processes, glomerular filtration, tubular reabsorption, and tubular secretion, are shown in the hypothetical. Plasma, containing 3 low-molecular-weight substances (X, Y, and Z), enters the glomerular capillaries, and approximately 20% of the plasma is filtered into Bowman’s capsule. The filtrate contains substances X, Y, and Z in the same concentrations as the plasma (ie, each one is freely filtered). The filtrate enters the proximal convoluted tubule and begins its flow through the rest of the tubule. Simultaneously, the remaining 80% of the plasma, with its substances X, Y, and Z in the same concentrations as they had when entering the kidney, leaves the glomerular capillaries via the efferent arterioles and enters the peritubular capillaries.
Suppose the cells of the tubular epithelium can secrete all the peritubularcapillary substance X into the tubular lumen but cannot reabsorb substance X. Thus, by the combination of filtration and tubular secretion, all the plasma that originally entered the renal artery is cleared of substance X, which leaves the body via the urine. Now suppose the tubule can reabsorb some of substance Y. The amount of substance Y reabsorbed is small, so most of the filtered substance Y escapes from the body in the urine. In contrast, let substance Z be reabsorbed fully. Therefore, no substance Z is lost from the body. Hence, the processes of filtration and reabsorption have canceled each other, and the net result is as though substance Z had never entered the kidney at all.
As we will see, most of the tubular transport consists of reabsorption rather than tubular secretion. An idea of the magnitude and importance of tubular reabsorption can be gained from Table 1–2, which summarizes data for a few plasma components that undergo reabsorption. are at least 3 important generalizations to be drawn from this table:
1)Because of the huge GFR, the quantities filtered per day are enormous, generally larger than the amounts of the substances in the body. For example, the body contains about 40 L of water, but the volume of water filtered each day may be as large as 180 L. If reabsorption of water ceased but filtration continued, the total plasma water would be urinated within 30 min.
2)Reabsorption of waste products, such as urea, is incomplete, so that large fractions of their filtered amounts are excreted in the urine, like substance Y in our hypothetical example.
3)Reabsorption of most “useful” plasma components (eg, water, electrolytes, and glucose) varies from essentially complete, so that urine concentrations should normally be undetectable (eg, glucose), to almost complete (eg, water and most electrolytes), so that the amounts excreted in the urine represent only very small fractions of the filtered amounts.

Average values for several substances handled by filtration and reabsorption

Renal manipulation of 3 hypothetical substances, X,Y, and Z. Substance X is filtered and secreted but not reabsorbed. Substance Z is filtered but is completely reabsorbed.

For each plasma substance, a particular combination of filtration, reabsorption, and secretion applies. The relative proportions of these processes then determine the amount excreted. A critical point is that the rates at which the relevant processes proceed for many of these substances are subject to physiological control. By triggering changes in the rates of filtration, reabsorption, or secretion when the body content of a substance goes above or below normal, these mechanisms can regulate excretion to keep the body in balance. For example, consider what happens when a person drinks a large quantity of water: Within 1–2 h, all the excess water has been excreted in the urine, partly as the result of an increase in GFR but mainly as the result of decreased tubular reabsorption of water. The body is kept in balance for water by increasing excretion. By keeping the body in balance, the kidney is the effector organ of a reflex that maintains body water concentration within very narrow limits.