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l o o p of Henle outer(OM) and inner(IM) medullary a r c h i t e c t u r e , and p a r t i c u l a r l y the dichotomic merging of c o l l e c t i n g ducts were incorporated into a d i f f e r e n t i a l equation model of the renal counterflow system

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CÜMPUTATION OF RENAL OSMOTIC WORK IM A " P A S S I V E "

CENTRAL CORE MODEL INCORPORATING MEDULLARY GEO- METRY AND PELVIC UREA REFLUX. L o r y , P . , C i l g , A . and H o r s t e r ,M. * Physiologisches Inst.Univ.München*, Petten- / koferstr.12 and Math.Inst.TU München,Arcisstr.21»München 2

Medullary countercurrent parameters,i.e.,solute and solvent fluxes and permeabilities in vivo and in v i t r o , i n c l u d i n g urea r e f l u x from pelvic u r i n e ( l ) , l o o p of Henle outer(OM) and inner(IM) medullary a r c h i t e c t u r e , and p a r t i c u l a r l y the dichotomic merging of c o l l e c t i n g ducts were incorporated into a d i f f e r e n t i a l equation model of the renal counterflow system. The computation(multiple shooting method(2))of r e - nal osmotic work allowed predictions about concentration p r o f i l e s in a l l tubular medullary structures.

Results. The results produced by t h i s model agree with the measured values along the nephron and in p e l v i c urine.

C o r t i c o - m e d u l l . j e t . OM-IM j e t . t i p of p a p i l l a DLH 140 18.0 3.0 498 62 1 1.0 552 175 13.7 ALU 30 83.0 10.8 456 169 12.6 552 175 13.7 CD 23 121.0 21.0 143 707 131.0 286 587 261.0 IS 140 9*0 - 455 144 - 513 245 Numbers indicate:NaCl (mmol«l"1); urea (mmol«I"1); TP/P i n u l i n . Abbrev.:DLH-desc.loop of Henle;ALH-asc.loop of Hen- l e ; C D - c o l l e c t . d u c t ; I S - i n t e r s t i t i u m . P e l v i c urine was 331, 463, 301, the osmotic a c t i v i t y was 1065 mosm.l"1.

Conclusions.Principal c h a r a c t e r i s t i c s of the model a r e : ( i ) Urea recycles through the loop of Henle to OM segments and water abstraction enhances urea concentration in OMCD, as predicted(3). (ii)CD a r c h i t e c t u r e (exponential reduction of tubes) i s d e c i s i v e f o r maintaining OMCD urea concentration against low IS urea up to CD p e l v i c entry, even i f IMCD u- rea permeability c o e f f i c i e n t is unfavorably high, ( i i i ) Additional entry of urea' by d i f f u s i o n from pelvis into IM IS,thereby enhancing urea trapping in IM. ( i v ) The proper- t i e s ( i i ) a n d ( i i i ) are more e f f e c t i v e in supplying urea to IM than IMCD urea d i f f u s i o n alone.

(1) Schmidt-Nielsen,B. et al.,Kidney Int.18:419,1980; (2) S t o e r , J . , B u l i r s c h , R . Intr.numer.analysis»Sprinoer 1980; (3) Kokko,0.,Rector,F.C.,Kidney Intern. 2:214,19721

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