Determination of glomerular intracapillary and transcapillary pressure gradients from sieving data II. A physiological study in the normal dog

The two theoretical models proposed previously to calculate the intracapillary and transcapillary glomerular pressure gradients from the sieving of macromolecules such as PVP have been used to analyse in 22 normotensive dogs the sieving curve relating the sieving coefficients, Φ, to molecular size (Φ: glomerular clearance of PVP fractions/GFR). Neither the “localc₂” model-filtrate unmixed at the outer face of the capillaries walls—nor the constantc₂ model-filtrate well mixed—allowed to obtain realistic values for the hemodynamical parameters. Indeed with the localc₂ model, the best fit between calculated and experimental sieving curves could be obtained only by reversing the intracapillary pressure gradient; conversely the constantc₂ model obliged to decrease the intracapillary pressure so abruptly along the capillaries, that retrofiltration took place in the distal parts of the vessels. This difficulty has been overcome by combining the two models; the so-called “hybrid model” considers that the filtrate is well mixed in the vicinity of the urinary pole only. The following results were obtained: 1. PGC_a and PGC_e (intracapillary pressures at the afferent and efferent extremities of the capillaries) equal to 49.7±1.03 and 41.8±1.00 mm Hg respectively. 2. Pressure equilibrium is generally reached at the efferent extremity of the vessels. 3. The slope of PGC\(\left( {\frac{{PGC_a - PGC_e }}{{PGC_a }}} \right)\) varies inversely to F.F. (filtration fraction). 4. The model, however, does not allow to rule out the possibility of retrofiltration.