Biomedical Engineering Reference
In-Depth Information
cell to the basolateral cell membrane. The molecule then crosses this cell membrane into
the peritubular capillaries. It is not necessary for the molecule to be actively transported
across both of the cell membranes, and typically the molecule would move down its con-
centration gradient when crossing one of the barriers. For example, sodium can diffuse
into the tubule epithelial cells, but it is then actively transported across the epithelial cells
basolateral membrane to enter the bloodstream. If the transport of a molecule is active
across at least one barrier, then it falls into the category of receptor-mediated transport.
It is interesting to note that the reabsorption of many molecules is coupled to sodium
movement across the tubule epithelial cells. This type of movement is mediated by a co-
transporter, which in this case utilizes the energy derived from the movement of sodium
in the direction of its electrochemical gradient to drive the movement of another molecule
(for example, glucose, many organic compounds, and some inorganic ions) against its elec-
trochemical gradient. The activity of co-transporters is classified by the amount of mole-
cules that can be transported in a unit time. Under most normal conditions, the maximum
rate of transport is never reached by the co-transporters. However, if the nephron concen-
tration of a particular compound becomes so large that all of the binding sites on every
transporter are occupied, then the maximum transportation rate is reached (i.e., the trans-
porters are saturated) and this compound may enter the urine. As an example, under dia-
betic conditions, it is possible for the glomerular filtrate glucose concentration to exceed
the maximum transportation rate, and then glucose enters the urine and is excreted. There
is actually an old legend that before the age of modern medicine, diabetes mellitus would
be diagnosed by determining how “sweet” a patient's urine was. How much truth is in
this legend is up for debate, but regardless, diabetic patients can excrete glucose in urine,
whereas under normal conditions, all of the glucose within the glomerular filtrate is reab-
sorbed into the plasma.
Tubule secretion is the process by which molecules from the peritubular capillaries
move into the tubule lumen. Similar to reabsorption, secretion can occur via diffusion or
receptor-mediated transport. You may be wondering why the peritubular capillaries
secrete compounds into the lumen. There are a variety of reasons, based on the particular
compound. Many toxins or foreign compounds are secreted to be fully removed from the
plasma. Hydrogen ions are secreted to regulate the pH of the blood. Secretion mediated
by diffusion occurs similarly to the diffusion associated with tubule reabsorption, except
that it occurs in the opposite direction. Interestingly, the receptor-mediated secretion of
molecules is typically coupled to sodium reabsorption. Therefore, the electrochemical gra-
dient of sodium drives the movement of other compounds against their electrochemical
gradients. These types of transports are typically called anti-porters because the two mole-
cules move in opposite directions. Hydrogen ion secretion makes use of an anti-porter
coupled to sodium.
It is important to note before we discuss specific examples of reabsorption and secre-
tion, what components of the nephron perform what functions during urine formation.
The primary function of the proximal tubule is to reabsorb large quantities of water and
other solutes within the glomerular filtrate. This helps to form the concentration gradient
which will be used in later segments of the nephron to drive the reabsorption and/or
secretion of particular compounds. The Loop of Henle also functions to reabsorb large
quantities of solutes and small quantities of water. The proximal tubule system is also
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