The Renal Tubule Reabsorbs Filtered Substances
The bulk of the ultrafiltrate formed in the glomerulus must be reabsorbed by the renal tubules rather than excreted in the urine. To understand the importance of tubule reabsorption of filtered substances, consider the IO-kg beagle that forms 53.3 L of glomerular filtrate each day.
The ultrafiltrate contains virtually the same concentration of salts and glucose as plasma; therefore, without tubular reabsorption, the urinary loss of sodium, chloride, potassium, bicarbonate, and glucose alone would total more than 500 g of solute. In the absence of tubular reabsorption, the beagle would need to replace these chemicals constantly throughout the day by eating more than a pound of salts and drinking more than 50 L of water at the same rate as the urinary loss to maintain fluid and salt balance.Fortunately, the renal tubule efficiently retrieves these and other constituents of the ultrafiltrate. Figure 42-1 illustrates the percentages of various filtered substances that remain in the tubule fluid at different points along the tubule. One hundred percent of the filtered glucose is reabsorbed by the proximal tubule; by the time the final urine is formed in the terminal collecting duct, approximately 99% of the filtered water and sodium has been retrieved.
FIGURE 42-1 Illustration of the percentage of filtered substances ((Ux∕Px) ? IOOZ(UinohnZPinulin)I remaining in the tubule fluid in various tubule segments. Creatinine is secreted by the proximal tubule and is excreted at a greater rate than the reference substance, inulin. CD, Collecting duct; DT, distal tubule; LH, loop of Henle; PT, proximal tubule. (Modified from Sullivan LP, Grantham JJ, editors: Physiology of the kidney, ed 2, Philadelphia, 1982, Lea & Febiger.)
FIGURE 42-2 Transmission electron micrograph of cross section of rat proximal tubule.The brush border (BB) of the apical plasma membrane extends from the epithelial cells into the tubule lumen (L), where it is bathed by the tubule fluid. On the basal side of the cell is the interstitial space (IS) and the peritubular capillary (C). (Magnification ?3400.)