Urine formation: Reabsorbed glomerular filtration
Urine formation begins when large quantities of virtually protein-free fluid are filtered from the glomerular capillaries into the Bowman's sac.
The rate of excretion of different substances in the urine represents the totality of the three renal processes, shown in Figure 26-9: (1) glomerular filtration, (2) reabsorption of magnetic substances the renal tubules into the bloodstream, and (3) excrete substances from the blood into the renal tubules. Expressed mathematically:
Rate of urinary excretion = Rate of filtration - Rate of re-absorption + rate of excretion
Urine formation begins when large quantities of virtually protein-free fluid are filtered from the glomerular capillaries into the Bowman's sac. Most of the substances in plasma, except for proteins, are freely filtered, so their concentration in glomerular filtrate in Bowman's follicle is almost the same as in plasma.
When the fluid is filtered out of the Bowman's sac and passed through the tubes, it is transformed by reabsorption of water and specific solutes back into the bloodstream or by secreting other substances from the peritoneal capillaries into the tube.
The figure shows the renal processing of the four hypothetical substances. The substance shown in Table A is freely filtered by glomerular capillaries but neither is reabsorbed nor secreted. Therefore, the rate of its excretion is equal to the rate at which it is filtered. Certain waste products in the body, such as creatinine, are disposed of in this way by the kidneys, allowing them to excrete all that has essentially been filtered out.
Figure. The basic process of the kidneys determines the composition of the urine
The rate of a substance's urinary excretion is equal to the rate at which it is filtered minus the rate of reabsorption of that substance plus the rate of excretion of that substance from the peritoneal capillary blood into the renal tubule.
In Table B, the substance is freely filtered but is also partly reabsorbed from the renal tubules back into the bloodstream.
Therefore, the rate of urinary excretion is less than the glomerular capillary filtration rate. In this case, the rate of excretion is calculated by the filtration rate minus the rate of reabsorption. This pattern is typical for many of the body's electrolytes such as sodium and chloride ions.
In Table C, the substance is freely filtered in glomerular capillaries but is not excreted in the urine because all the filtered substance is reabsorbed from the tube back into the bloodstream. This pattern occurs for certain nutrients in the blood, such as amino acids and glucose, allowing them to be preserved in body fluids.
The substance in table D is freely filtered in glomerular capillaries and is not reabsorbed, but an additional amount of this substance is secreted from the peritoneal capillary blood into the renal tubules. This pattern usually occurs for organic acids and bases, allowing them to be rapidly eliminated from the bloodstream and excreted in large quantities in the urine. The rate of excretion, in this case, is calculated by the rate of filtration plus the rate of tubular secretion.
For each substance in plasma, a specific combination of filtration, reabsorption, and excretion occurs.
The rate at which this substance is excreted in the urine depends on the relative rate of the three basic kidney processes.
Figure. The kidneys process four hypothetical substances.
A - substance is freely filtered but is not reabsorbed.
B - free filtered substance, but some of the filtered substance is reabsorbed into the bloodstream.
C - the substance is freely filtered but not excreted in the urine because all the filtered substance is reabsorbed from the renal tubules into the bloodstream.
D - the substance is freely filtered and is not reabsorbed but is secreted from the peritoneal capillary blood into the renal tubule.