Flow Through The Kidney - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 320

TABLE 12.1 Average Reabsorption Values for Various Metabolically Important Compounds

Compound Filtered Load Quantity Excreted Percent Reabsorbed

Water 180 L/day 1.5 L/day 99%

Glucose 180 g/day 0 g/day 100%

Lipids 1080 g/day 3.6 mg/day 99.99%

Sodium 630 g/day 3.3 g/day 99.5%

Bicarbonate 110 g/day 8.5 g/day 92.3%

Urea 55 g/day 35 g/day 36%

conclusions about tubule reabsorption ( Table 12.1 ). Metabolically important molecules
(water, ions, and organic molecules) are completely reabsorbed so that there is no need to
constantly ingest (or produce) these molecules. Waste products are not fully reabsorbed so
that they can be removed from the body. It is important to note that the reabsorption of
most organic compounds (e.g., glucose) is not regulated and is typically very high.
Therefore, under normal conditions, none of these compounds is found within the urine.
For these compounds, it can be considered that the kidneys do not exist, because the kid-
neys have no effect on their plasma concentration. However, the reabsorption of most
non-organic metabolically important molecules (e.g., water and ions) is tightly regulated
but is also very high under normal conditions. You can prove this by drinking a few
64-oz double-big sodas from your local gas station in under 10 minutes. It is guaranteed
that you will be visiting the bathroom soon to remove the excess water from your system.
However, if you eat a few candy bars with a lot of sugars in them, the amount of sugar in
your urine does not increase, but is stored for later use.
There are two main processes that account for the reabsorption of compounds into the
peritubular capillaries. Some substances can be reabsorbed by diffusion and others involve
some receptor mediated transport. Diffusion typically occurs across the tight junctions of
the tubule epithelial cells, whereas the receptor mediated transport occurs through the epi-
thelial cells themselves. For example, the reabsorption of urea occurs by diffusion.
However, because the composition of the glomerular filtrate is the same as the plasma
composition, there should be no concentration gradient driving force for the movement of
urea. Early on within the proximal tubule system, water is removed from the filtrate (via
receptor-mediated transport). With the removal of water, the effective concentration of
urea increases within the tubule lumen, and therefore a concentration gradient is formed
between the nephron and the peritubular capillaries. Urea can then diffuse down its con-
centration gradient from the filtrate into the plasma at a distal location along the nephron.
The reabsorption of the majority of lipid soluble compounds occurs in this manner, and is
therefore dependent on the early reabsorption of water, to effectively increase the molecu-
lar concentration within the nephron.
For a material to be reabsorbed via receptor-mediated transport, the molecule must first
diffuse to the wall of the nephron tubule. The molecule must then cross the luminal wall
into the tubule epithelial cells. The molecule could then diffuse across the tubule epithelial