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Physiology
of the Renal Diluting
and
Concentrating Mechanisms
FIGURE 1-1
Principles of normal water balance.
In most steady-state situations, human water intake matches water losses through
all sources. Water intake is determined by thirst (see Fig. 1-12) and by
cultural and social behaviors. Water intake is finely balanced by the need to
maintain physiologic serum osmolality between 285 to 290 mOsm/kg. Both water
that is drunk and that is generated through metabolism are distributed in the
extracellular and intracellular compartments that are in constant equilibrium.
Total body water equals approximately 60% of total body weight in young men,
about 50% in young women, and less in older persons. Infants’ total body water
is between 65% and 75%. In a 70-kg man, in temperate conditions, total body
water equals 42 L, 65% of which (22 L) is in the intracellular compartment and
35% (19 L) in the extracellular compartment. Assuming normal glomerular
filtration rate to be about 125 mL/min, the total volume of blood filtered by
the kidney is about 180 L/24 hr. Only about 1 to 1.5 L is excreted as urine,
however, on account of the complex interplay of the urine concentrating and
diluting mechanism and the effect of antidiuretic hormone to different segments
of the nephron, as depicted in the following figures.
Determinants
of the renal concentrating mechanism. Human kidneys have two populations of
nephrons, superficial and juxtamedullary. This anatomic arrangement has
important bearing on the formation of urine by the countercurrent mechanism. The
unique anatomy of the nephron
[1] lays
the groundwork for a complex yet logical physiologic arrangement that
facilitates the urine concentration and dilution mechanism, leading to the
formation of either concentrated or dilute urine, as appropriate to the person’s
needs and dictated by the plasma osmolality. After two thirds of the filtered
load (180 L/d) is isotonically reabsorbed in the proximal convoluted
tubule, water is handled by three interrelated processes:
1)
the delivery of fluid to the diluting segments;
2) the
separation of solute and water (H2O) in the diluting segment; and
3) variable
reabsorption of water in the collecting duct. These processes participate in the
renal concentrating mechanism
1. Delivery of sodium chloride (NaCl)
to the diluting segments of the nephron (thick ascending limb of the loop of
Henle and the distal convoluted tubule) is determined by glomerular filtration
rate (GFR) and proximal tubule function.
2. Generation of medullary
interstitial hypertonicity, is determined by normal functioning of the thick
ascending limb of the loop of Henle, urea delivery from the medullary collecting
duct, and medullary blood flow.
3. Collecting duct permeability
is determined by the presence of antidiuretic hormone (ADH) and normal
anatomy of the collecting system, leading to the formation of a
concentrated urine
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