PAH is filtered and secreted by renal epithelial cells PAH excretion is the sum of filtered load plus secretion rate. B, Example of net secretion of PAH ( Para-aminohippuric acid). Na + is filtered and reabsorbed by the renal epithelial cells Na + excretion is the difference between filtered load and reabsorption rate. 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.įigure 6–13 Examples of substances that are reabsorbed or secreted. If the filtered load is less than the excretion rate, there has been net secretion of the substance. If the filtered load is greater than the excretion rate, there has been net reabsorption of the substance. In words, the difference between the filtered load and the excretion rate is the rate of net reabsorption or net secretion. The following equations are used to calculate filtered load, excretion rate, and reabsorption or secretion rate: The excretion rate can be compared with the filtered load to determine whether a substance has been reabsorbed or secreted. Excretion is the net result, or sum, of the processes of filtration, reabsorption, and secretion. As with reabsorption, the secretion mechanisms involve transporters in the membranes of the epithelial cells lining the nephron.Įxcretion. Excretion or excretion rate refers to the amount of a substance excreted per unit time. Thus, in addition to filtration, secretion provides a mechanism for excreting substances in the urine. Secretion. A few substances (e.g., organic acids, organic bases, K +) are secreted from peritubular capillary blood into tubular fluid. As emphasized, if reabsorption did not occur, most of these constituents of ECF would be rapidly lost in the urine. The mechanisms for reabsorption involve transporters in the membranes of the renal epithelial cells. Reabsorption. Water and many solutes (e.g., Na +, Cl −, HCO3 −, glucose, amino acids, urea, Ca 2 +, Mg 2 +, phosphate, lactate, and citrate) are reabsorbed from the glomerular filtrate into the peritubular capillary blood.
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. Nearby is a peritubular capillary, which emerges from the efferent arteriole and supplies blood to the nephron.įigure 6–12 Processes of filtration, reabsorption, and secretion in a nephron. The sum of the three processes is excretion.įiltration. An interstitial-type fluid is filtered across the glomerular capillary into Bowman’s space. The initial part of the nephron (Bowman’s space and the beginning of the proximal convoluted tubule) is shown, lined with epithelial cells.
A glomerular capillary is shown with its afferent and efferent arterioles.
The processes of filtration, reabsorption, and secretion are illustrated in Figure 6-12. Measurement of Reabsorption and Secretion In addition, secretion mechanisms in the epithelial cells remove certain substances from the peritubular capillary blood and add it to urine. Fortunately, reabsorptive mechanisms in the epithelial cells lining the renal tubule return these substances to the circulation and to the ECF. Each of these losses represents more than 10-fold the amount present in the entire ECF. 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. Glomerular filtration results in the production of large quantities (180 L/day) of an ultrafiltrate of plasma.