Flow Through The Kidney - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 322

responsible for the secretion of the majority of compounds, except for potassium. This
early movement of solutes and water within the proximal tubule and the Loop of Henle
are by bulk processes, where the major goal of these processes is to get the plasma/urine
solute concentration close to its acceptable level. The distal convoluted tubule and the col-
lecting duct system are primarily responsible for fine-tuning the concentrations of the
solutes and determining the final excreted concentration and the final plasma concentra-
tion. Therefore, it should be intuitive that the majority of the mechanisms that exert control
on the nephron (e.g., hormones) and affect urine concentration act on the distal convoluted
tubule and the collecting duct. As a summary, the amount of a compound that is excreted
can be calculated by measuring the amount filtered, secreted, and reabsorbed, as follows:

Amount Excreted Amount Filtered Amount Secreted Amount Reabsorbed

5 1 2

12.4 SODIUM BALANCE/WATER BALANCE

As discussed in the previous section, the majority of sodium is reabsorbed throughout
the entire tubule system. Recall that the concentration of sodium is high in both the tubule
lumen and the interstitial fluid surrounding the nephron. The movement of sodium initi-
ates from the passive diffusion of sodium, through a sodium channel, from the tubule
lumen, into the epithelial cells lining the tubule. To maintain the electrochemical neutrality
of luminal epithelial cells, potassium efflux (from the epithelial cell into the tubule lumen)
is coupled to the process of sodium movement. This movement of potassium is by bulk
diffusion as well. The movement of sodium out of the epithelial cell into the interstitial
fluid is achieved by an active mechanism: the sodium-potassium ATPase pump. This
pump moves three sodium ions out of the epithelial cell and two potassium ions into the
epithelial cell for every ATP that is hydrolyzed into ADP. Sodium then moves into the
peritubular capillaries.
Sodium reabsorption is regulated via the amount and type of channels present on the
luminal surface of the epithelial cell. The entry of sodium into the epithelial cells that com-
pose the cortical collecting duct is by diffusion through sodium channels. Along the proxi-
mal collecting tubule, the movement of sodium into the epithelial cells is typically by co-
transport with organic molecules such as glucose or by counter-transport with hydrogen
ions. Therefore, the movement of sodium down its electrochemical gradient provides the
energy for the reabsorption of glucose and the secretion of hydrogen ions. The transport of
sodium across the basolateral membrane, via the sodium-potassium ATPase pump, is typi-
cally constant along the entire length of the tubule lumen. The quantity of these pumps
present can be regulated by other mechanisms to determine the amount of sodium reab-
sorbed into the peritubular capillaries, although this does not normally occur.
We will now move our discussion to the control of sodium reabsorption. In essence, no
sodium is secreted into the nephron. Therefore, the total amount of sodium that is excreted
is equal to the amount reabsorbed subtracted from the amount filtered. Changes in the
glomerular filtration rate or the reabsorption rate of sodium will affect the excretion rate
of sodium, but how are changes in sodium concentration sensed in the body? Currently,