Control of Plasma Volume

The kidneys must excrete an isotonic fluid in order for the osmotic balance of the
extracellular fluid to be maintained. This means that water itself cannot simply be
removed on its own, and must be excreted with other solutes to
maintain this osmolarity. The water reabsorption at the kidneys is
controlled by, mainly, the movement of sodium (and subsequently
chloride) ions out of the nephron, such that water will follow.
67% of the reabsorption happens at the PCT and 25% at the loop
of Henle. The rest is reabsorbed in a tightly regulated manner in
the remaining nephron.
? Before talking about the specifics regarding sodium
reabsorption, it is worth mentioning the role the GFR plays in
sodium reabsorption. When the GFR changes, there will be a
subsequent change in the concentration of sodium in the filtrate
and the PCT will still reabsorb 67% of this. This means that the
remaining sodium can be reabsorbed along the rest of the nephron. ?
Proximal Convoluted Tubule
The PCT is mainly involved in the reabsorption of sodium ions, chloride ions, water, and
glucose from the tubular lumen. It can be divided into early (S1) and late (S2 and S3)
proximal convoluted tubule which have different apical
transporters.
· The early PCT has a Na+-K+-ATPase on the basolateral
surface, removing sodium ions from the intracellular
fluid. The sodium ion electrochemical gradient
produced as a consequence allows sodium to move into
the cell, and is couple to H+ ion movement via the NHE
and glucose symporters. Water and chloride ions will
subsequently diffuse into from the nephron lumen into
the extracellular fluid at the cellular junctions
(paracellular route), to maintain the electrochemical, isosmotic balance.
· The distal PCT most of the organic molecules and HCO3- ions have been removed.
There is the Na+-K+-ATPase on the basolateral surface still and sodium reabsorption
is now mainly coupled to H+ ions via the NHE. Chloride ions move across actively
now in exchange for other negatively charged compounds, maintaining
electrochemical neutrality in the ECF.
The solutes