Page 50 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Renal Physiology 39

Descending thin limb Interstitium Ascending thick limb

300 300 300 400 400 200 300 300 200 350 350 150
300 300 300 400 400 200 300 300 200 350 350 150
300 300 300 400 400 200 300 300 200 350 350 150
300 300 300 400 400 200 300 300 300 350 350 150
300 300 300 400 400 200 400 400 400 500 500 300
300 300 300 400 400 200 400 400 400 500 500 300
300 300 300 400 400 200 400 400 400 500 500 300
300 300 300 400 400 200 400 400 400 500 500 300
1 2 3 4

300 300 150 325 325 125 300 300 125 312 312 112
300 300 150 325 325 125 325 325 225 375 375 175
350 350 300 425 425 225 325 325 225 375 375 175
350 350 300 425 425 225 425 425 225 425 425 225
350 350 300 425 425 225 425 425 225 425 425 225
350 350 300 425 425 225 425 425 400 513 513 313
550 500 500 600 600 400 425 425 400 513 513 313
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Descending thin limb Interstitium Ascending thick limb

312 312 112 312 312
375 375 175 375 375
375 375 175 375 375
425 425 225 425 425
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513 513 313 513 513
513 513 313 513 513
700 700 500 700 700
Collecting duct
Figure 2-12 Stepwise operation of the countercurrent multiplier mechanism of urinary concentration.
Numbers refer to osmolalities (mOsm per kg H 2 O) of tubular fluid and interstitium. (Reprinted with
permission from Valtin H. Renal function: mechanisms preserving fluid and solute balance in health, 2nd ed.
Boston: Little, Brown, 1983: 166.)

Thus, water reabsorption in the cortical collecting duct ROLE OF THE VASA RECTA
under the influence of ADH is important in reducing If the water removed from the medullary collecting duct
the fluid load delivered to the medullary collecting duct. in the presence of ADH were allowed to remain in the
In the absence of ADH, the collecting duct is imperme- medullary interstitium, the hyperosmotic gradient would
able to water. The fluid entering this portion of the neph- dissipate rapidly. However, this does not occur because of
ron has an osmolality of approximately 100 mOsm/kg.
the countercurrent exchange function of the vasa recta.
Under these conditions, additional sodium chloride with-
Plasma in the vasa recta entering the medulla from the
out water is removed from the tubular fluid during its
cortex encounters an increasingly hyperosmotic medul-
course through thecorticalcollectingduct and inner med-
lary interstitium. As a result, water is removed from the
ullary collecting duct so that the final urine osmolality can
vessels and solutes (e.g., sodium chloride and urea) enter
be as low as 50 mOsm/kg. However, the outer medullary
the vessels. After passing the hairpin turn of the loop, the
collecting duct is impermeable to sodium.
vasa recta climb back toward the renal cortex. Now they
Even in the absence of ADH, urine osmolality may be
encounter a medullary interstitium of progressively
greater than 50 mOsm/kg if the animal is dehydrated. decreasing osmolality so that water enters the vessels
The GFR is decreased by dehydration, and there is an and solutes are removed. In this way, water is removed
increase in the proximal tubular reabsorption of sodium from and solutes are recycled back into the medullary
chloride and water. Less tubular fluid reaches the distal interstitium, thus preventing dissipation of the osmotic
nephron, and urine osmolality can approach 400 gradient. This process is known as countercurrent
mOsm/kg. 50
exchange. That the vasa recta can effectively remove water