‫ نصير جواد المختار‬.‫د‬
Physiology
Lecture IX: Regulation of Na+ and Water Reabsorption
Na+ are dominant EC cation, thus they exert about (90-95%)
osmotic pressure in ECF (in combination with Cl - anion). After Na+
freely filtered at glomerulus, its reabsorption which is active occur in all
tubular segment except descending limb of loop of Henle, while HOH
reabsorption is by diffusion and dependent on Na+ reabsorption.
In proximal tubules by the rate of Na + move out of cell to fluid and
achieved by Na+/K+ ATPase pumps in the basolateral membrane of cells.
This active transport of Na+ out of cell diminish intracellular Na +
concentration compared to luminal concentration. This in turn establish
Na+ concentration gradient, thus Na+ diffuses from tubule through brush
border to cell.
The continuous active transport of Na+ out of cell creates electrical
potential (-70 mv) which is an additional factor favours Na + diffusion
from tubular lumen to cell.
Cl- will be cotransported with Na+ by difference in electrical charge
when the concentration of solute decrease in lumen and increase in
interstitium. This resulted in concentration difference which promote
water osmosis in same direction.
In Loop of henle:
Reabsorption occur in ascending limb by active/ passive transport
but is impermeable to HOH. Therefore Na+ concentration falls to a low
level within tubular fluid.
In distal convoluted tubule CCT, CD is highly variable with rate
controlled by concentration of aldosterone.
In presence of large amount of aldosterone all Na+ reabsorbed
while in its absence large proportional of Na+ entered distal tubule isn’t
reabsorbed passed to urine.
Regulation of Aldosterone Secretion:
Aldosterone secretion is regulated by rennin angiotensin system
and major comp. is juxtaglomerular apparatus composed of 3 types of
cells:
1. JG Cells:
Located in media of afferent arteriols and they synthesize, store
and release rennin enzyme. They serve as baroreceptor and respond to
increase in trans????? Pressure gradient between afferent arteriole and
interstitium innervated by sympathetic nerve. They serve as volume
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‫ نصير جواد المختار‬.‫د‬
Physiology
receptor which monitor renal perfusion pressure and stimulated by
hypovolumia decreased renal perfusion pressure.
2. Macula Densa Cells:
Located at transition between thick ascending segment of loop of
Henle and distal tubule and function as chemoreceptor stimulated by
decreased Na+/Cl- load (not innervated).
3. Messangial Cell:
Rennin is secreted to blood stream where it combines with rennin
substrate- angiotensenogin synthesized in liver to form (AnI). The latter
converted in lung by pulmonary endothelial cells to (AnII) by angiotensin
converting enzyme.
(AnII) has following functions:
I. Aldosterone synthesis and secretion.
II. Potent vasoconstrictor of renal arteriole at low plasma
concentration.
How kidney changes aldosterone circulating amount?
Increase Na+ intake lead to increase EVF volume, lead to increase
arterial pressure. Juxtaglomerular apparatus receive sympathetic
innervations when baroreceptor stimulated by increase atrial pressure.
They ?????? information to brain that in form JGA via renal nerve to:
Decrease rennin output → decrease AnI → decrease AnII → back
aldosterone released → decrease NA+ reabsorption and more Na+
excreted.
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