Renal Tubular Reabsorption and Secretion
Renal Tubular Reabsorption and Secretion
As the glomerular filtrate enters the renal tubules, it flows sequentially through the successive parts of the tubule-the proximal tubule, loop of Henle, distal tubule, collecting tubule, and collecting 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:
Urinary excretion equals Glomerular filtration minus Tubular reabsorption plus Tubular secretion. For many substances, tubular reabsorption plays a much more important role than secretion in determining the final urinary excretion rate. However, tubular secretion accounts for significant amounts of potassium ions, hydrogen ions, and a few other substances that appear in the urine.
TUBULAR REABSORPTION IS QUANTITATIVELY LARGE AND HIGHLY SELECTIVE
TUBULAR REABSORPTION IS QUANTITATIVELY LARGE AND HIGHLY SELECTIVE
Table twenty-eight dash one shows the renal handling of several substances that are all freely filtered in the kidneys and reabsorbed at variable rates. The rate at which each of these substances is filtered is calculated as follows:
Filtration equals Glomerular filtration rate times Plasma concentration
This calculation assumes that the substance is freely filtered and not bound to plasma proteins. For example, if plasma glucose concentration is one gram per liter, the amount of glucose filtered each day is about one hundred eighty liters per day times one gram per liter, or one hundred eighty grams per day. Because virtually none of the filtered glucose is normally excreted, the rate of glucose reabsorption is also one hundred eighty grams per day.
From Table twenty-eight dash one, two things are immediately apparent. First, the processes of glomerular filtration and tubular reabsorption are quantitatively large relative to urinary excretion for many substances. Thus, a small change in glomerular filtration or tubular reabsorption can potentially cause a relatively large change in urinary excretion. For example, a ten percent decrease in tubular reabsorption, from one hundred seventy-eight point five to one hundred sixty point seven liters per day, would increase urine volume from one point five to nineteen point three liters per day (almost a thirteen-fold increase) if the glomerular filtration rate remained constant. In reality, 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 (essentially all solutes in the plasma are filtered except the plasma proteins or substances bound to them), tubular reabsorption is highly selective. Some substances, such as glucose and amino acids, are almost completely reabsorbed from the tubules, so the urinary excretion rate is essentially zero. Many 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. Waste products, such as urea and creatinine, conversely, are poorly reabsorbed from the tubules and are excreted in relatively large amounts.
Therefore, by controlling their reabsorption of different substances, the kidneys regulate excretion of solutes independently of one another, a capability that is essential for precise control of the body fluid composition. In this chapter, we discuss the mechanisms that allow the kidneys to selectively reabsorb or secrete different substances at variable rates.