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Reabsorption and Secretion by the Renal Tubules
As the glomerular filtrate enters the renal tubules, it flows sequentially through the successive parts of the tubule—the proximal tubule, the loop of Henle, the distal tubule, the collecting tubule, and, finally, thecollecting duct—before it is excreted as urine.
Along this course, some substances are selectively reabsorbed from the tubules back into the blood, whereas others are secreted from the blood into the tubular lumen. Eventually, the urine that is formed and all the substances in the urine represent the sum of three basic renal processes—glomerular filtration, tubular reabsorption, and tubular secretion—as follows:
Urinary excretion = Glomerular filtration - Tubular reabsorption + Tubular secretion
For many substances, reabsorption plays a much more important role than does secretion in determining the final urinary excretion rate. However, secre-tion accounts for significant amounts of potassium ions, hydrogen ions, and a few other substances that appear in the urine.
Table 27–1 shows the renal handling of several substances that are all freely fil-tered in the kidneys and reabsorbed at variable rates.
The rate at which each of these substances is filtered is calculated as
Filtration = Glomerular filtration rate x Plasma concentration
This calculation assumes that the substance is freely filtered and not bound to plasma proteins. For example, if plasma glucose concentration is 1 g/L, the amount of glucose filtered each day is about 180 L/day x 1 g/L, or 180 g/day. Because virtually none of the filtered glucose is normally excreted, the rate of glucose reabsorption is also 180 g/day.
From Table 27–1, two things are immediately apparent. First, the processes of glomerular filtration and tubular reabsorption are quantitatively very large rel-ative to urinary excretion for many substances. This means that a small change in glomerular filtration or tubular reabsorption can potentially cause a relatively large change in urinary excretion. For example, a 10 per cent decrease in tubular reabsorption, from 178.5 to 160.7 L/day, would increase urine volume from 1.5 to 19.3 L/day (almost a 13-fold increase) if the glomerular filtration rate (GFR) remained constant. In reality, however, changes in tubular reabsorption and glomerular filtration are closely coordinated, so that large fluctuations in urinary excretion are avoided.
Second, unlike glomerular filtration, which is relatively nonselective (that is, essentially all solutes in the plasma are filtered except the plasma proteins or substances bound to them), tubular reabsorption is highly selective. Some sub-stances, such as glucose and amino acids, are almost completely reabsorbed from the tubules, so that the urinary excretion rate is essentially zero.
Many of the ions in the plasma, such as sodium, chloride, and bicarbonate, are also highly reabsorbed, but their rates of reabsorption and urinary excretion are variable, depending on the needs of the body. Certain waste products, such as urea and creati-nine, conversely, are poorly reabsorbed from the tubules and excreted in relatively large amounts.
Therefore, by controlling the rate at which they reabsorb different substances, the kidneys regulate the excretion of solutes independently of one another, a capability that is essential for precise control of the composition of body fluids.
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