Substances through the urinary system: filtration, re-absorption and excretion
Certain nutrients, such as amino acids and glucose, are completely reabsorbed from the renal tubules and do not appear in the urine although large amounts are filtered by glomerular capillaries.
In general, tubular reabsorption is quantitatively more important than tubular excretion to form urine, but excretion plays an important role in determining the amount of potassium and hydrogen ions and Others are excreted in the urine. Most substances that must be eliminated from the blood, especially the final products of metabolisms such as urea, creatinine, uric acid and urate, are poorly reabsorbed and thus excreted in large amounts through urine. Some foreign substances and drugs are also poorly reabsorbed but are excreted from the bloodstream into the renal tubules, resulting in a high rate of excretion. In contrast, electrolytes, such as sodium ions, chloride ions and bicarbonate ions, are so highly reabsorbed that only a small amount appears in the urine. Certain nutrients, such as amino acids and glucose,
Each process - glomerular filtration, tubular reabsorption and tubular secretion - is tailored to the needs of the body. For example, when there is excess sodium in the body, the rate at which the filtered sodium is normally increased and a smaller portion of the filtered sodium is reabsorbed, causing increased urinary sodium excretion.
For most substances, the rate of filtration and reabsorption is very large compared to the rate of excretion. Consequently, even small changes in infiltration or reabsorption can lead to relatively large changes in renal excretion. For example, an increase in glomerular filtration rate (GFR) of only 10 per cent (from 180 to 198 L / day) would increase urine output 13 times (from 1.5 to 19.5 L / day) if the unchanged renal tubular reabsorption. In fact, changes in glomerular filtration and tubular reabsorption often work in concert to produce the necessary changes in renal excretion.
Why are large amounts of dissolved substances filtered and then reabsorbed by the kidneys?
One might question the wisdom of purifying such large amounts of water and solutes and then reabsorbing most of these substances. One advantage of the high GFR is that it allows the kidneys to rapidly remove waste products from the body that depend mainly on glomerular filtration for their excretion. Most waste products are poorly reabsorbed by ducts and, therefore, relies on high GFR for effective removal from the body.
The second advantage of the high GFR is that it allows the kidneys to filter and process all the body fluids several times per day. Because total plasma volume is only about 3 litres, while GFR is about 180 L / day, the entire plasma can be filtered and processed about 60 times per day. This high GFR allows the kidneys to accurately and quickly control the volume and composition of body fluids.