Tubular reabsorption: large quantitatively and highly selective

2021-04-30 10:36 PM

For many substances, tubular reabsorption plays a much more important role than excretion in determining the rate of final excretion in the urine.

 

When the dialysis solution enters the tubule, it flows through the sections of the proximal tubule, the Henle loop, the distal tubule, the connecting tubule, and finally the collecting tubule, before urinary excretion. During this process, some substances are reabsorbed back into the bloodstream, while others are excreted from the bloodstream into the lumen.

In the end, urine is formed and all substances in the urine are the synthesis of three basic kidney processes by glomerular filtration, renal tubular secretion and reabsorption:

Urinary excretion = glomerular filtration - tubular reuptake + renal tubular secretion

For many substances, tubular reabsorption plays a much more important role than excretion in determining the rate of final excretion in the urine. However, excretion explains significant amounts of potassium ions, hydrogen ions, and a few other substances that appear in the urine.

The table shows the processing of substances that are completely filtered by the kidneys and reabsorbed at different levels. The filtration capacity of the kidneys for several substances is calculated:

Filtration = (glomerular filtration rate) x (plasma concentration)

This formulation is intended for substances that are purely kidney-filtered and are not bound to plasma proteins. For example, if the plasma glucose concentration is 1 g / L, the amount of filtered glucose per day is about 180 L / day × 1 g / L, or 180 g / day. Because normally almost no glucose is excreted. The rate of glucose reabsorption is also 180 g / day.

Board. Rate of filtration, excretion, renal reabsorption of substances.

From the table, you can immediately see 2 things.

First, the glomerular filtration and tubular reabsorption is very large compared with the excretion of many substances in the urine.

This means that only a small change in glomerular filtration or tubular reabsorption can cause a large change in urinary excretion.

For example, a 10% decrease in tubular reabsorption from 187.5 to 160.7 L / day would increase urine output from 1.5 to 19.3 L / day (a nearly 13-fold increase) if glomerular filtration flow (GFR) was not. change. However, in practice, changes in tubular reuptake and glomerular filtration are closely coordinated, so large changes in urinary excretion can be avoided.

Second, unlike glomerular filtration (all plasma solvents are filtered except for plasma proteins and their binding substances), tubular reabsorption is selective. high filtration. Some substances, such as glucose and amino acids, are reabsorbed almost completely in the renal tubules, resulting in almost zero urinary excretion. Many of the ions in the blood plasma, such as sodium, chlorine, and bicarbonate are also reabsorbed in high amounts, but their rate of reabsorption and excretion in the urine changes, depending on the body's needs. In contrast, the excess products are very few reabsorbed in the renal tubules and eliminated in relatively large amounts.

Therefore, by controlling the reabsorption of various substances, the kidneys regulate the excretion of solutes independently, which is essential for the correct regulation of body fluids.

 

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