Biomedical Engineering Reference
In-Depth Information
One way to quantify the glomerulus function is through the glomerular filtration rate.
This in essence is the amount of fluid that passes from the glomerular capillaries into
Bowman's space per unit time. The glomerular filtration rate is determined by the net fil-
tration pressure, the permeability of the glomerular capillary endothelial cell membrane,
and the area available for filtration. In an average person, the glomerular filtration rate is
nearly 125 mL/min or 180 L/day. This is nearly 50 times more fluid being filtered out of
the glomerular capillaries than across all other capillaries within the systemic circulation.
Remember that in an average adult, the total blood volume is 5 L and that the plasma
composes approximately 60% of this, or 3 L. That suggests that the kidneys filter the entire
plasma volume approximately 60 times per day. This allows the kidneys to very tightly
regulate the composition of plasma constituents.
As discussed above, the glomerular filtration rate is not constant (e.g., the positive feed-
back from the juxtaglomerular system). In fact, neural and hormonal input on the kidneys
can affect the filtration rate. It is important to also recall that the glomerulus is located in
between two arterioles, the afferent arteriole and the efferent arteriole, which both can reg-
ulate the blood flow through the glomerulus. Constriction of the afferent arteriole reduces
the hydrostatic pressure of the glomerulus and thus reduces the glomerular filtration rate.
This occurs because there is an increase in the resistance to flow through the afferent arte-
riole into the glomerular capillaries. Constriction of the efferent arteriole increases the
hydrostatic pressure of the glomerulus and this increases the glomerular filtration rate.
This occurs because there is an increase in the resistance to flow through the efferent arte-
riole out of the glomerulus. A third case is a constriction of both the afferent arteriole and
efferent arteriole, which tends to have no effect on the glomerular filtration rate. This
occurs because of the opposing effects of the increase in flow resistance. Dilation of the
two arterioles has the reverse effect of those described here.
It is possible to quantify the amount of a plasma molecule that enters the nephron each
day. This quantity is termed the filtered load and is obtained by multiplying the plasma
concentration of a molecule by the glomerular filtration rate. For instance, the plasma con-
centration of sodium is approximately 3.5 g per liter of blood. This makes the filtered load
for sodium equal to 3.5 g/L
630 g of sodium filtered per day. By compar-
ing this value to the concentration of the same molecule within the urine, one can deter-
mine if that molecule experience a net reabsorption or a net secretion within the nephron.
If the urine concentration of a molecule is lower than the filtered load, this suggests that
there was a net reabsorption of the molecule into the peritubular capillaries. However, if
the urine concentration of a molecule is higher than the filtered load, this suggests that
there was a net secretion of the molecule into the nephron.
180 L/day
3
5
12.3 TUBULE REABSORPTION/SECRETION
Tubule reabsorption is the process by which molecules from the glomerular filtrate are
returned back to the plasma. This occurs along the entire nephron unit. Metabolically
important molecules are nearly completely reabsorbed, whereas wastes are reabsorbed to
some extent, with the majority of waste molecules making it into the urine. By looking at
the relative percent reabsorbed of various molecules,
it
is possible to draw some
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